Processes for the oxidation of [ 11c]ch 4 or [ 13c]ch 4 using iodine-based catalysts

Abstract

The invention relates to the production of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 comprising reacting [11C]CH4 or [13C]CH4 with an iodine(III) catalyst. The 11C labelled products can be used to prepare radiotracer compounds containing an [11C]CH3 group for use in PET imaging.

Description

[0001]PROCESSES FOR THE OXIDATION OF [11C]CH4OR [13C]CH4USING IODINE-BASED CATALYSTS FIELD The present invention relates to the partial oxidation of [11C]CH4 or [13C]CH4 using iodine-based catalysts, which allow the reaction to be performed rapidly at low temperatures and pressures. The chemistry can be used to form [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2, which can be used as methylating agents to form11C radiotracer compounds for use in PET imaging, or [13C]CH3OSO2CF3, or [13C]CH3N(SO2CF3)2, which can be useful for introducing13C into compounds and then following processes by13C NMR. BACKGROUND Positron emission tomography (PET) is a powerful medical imaging technique that is applied in many fields, including oncology, neurology, cardiology and others. PET uses radiotracer compounds are tagged with positron-emitting radionuclides, most commonly carbon-11 (11C). The radiotracer compound is administered to the patient and the radionuclide (e.g. carbon-11) emits a positron which travels through tissue and collides with an electron, releasing a pair of gamma rays that are detected by detectors in the PET system. The most common primary precursor to11C radiotracer compound is carbon-11 labelled CO2 (11CO2), which is typically produced in a cyclotron. The11CO2 is then reduced to form a secondary precursor which can be used to introduce11C into a radiotracer compound. Usually, the11CO2 is first reduced to11CH4, most commonly using a Ni / H2 catalyst system, and the11CH4 is transformed into a secondary precursor which is then used to form the radiotracer compound. A common secondary precursor is11CH3OTf (OTf = CF3OSO2-, commonly known as “triflate”), which is a powerful methylating agent that can be used to introduce a11CH3 group into a radiotracer.11CH3OTf is typically produced from11CH4 by reacting the11CH4 with iodine (I2) at very high temperatures of around 700 °C to form11CH3I, followed by reaction with a triflate source such as AgOTf at elevated tempreatures of around 200 °C to form11CH3OTf. This is an energy intensive and time consuming process, which is particularly disadvantageous given the short half life of carbon-11 (t1 / 2 = 20.33 minutes), which means that a lot of11C decays during the process of manufacturing the11CH3OTf. This adds to the overall cost and inefficiency of the process. Since11C emits damaging radiation, the synthesis of radiotracer compounds typically needs to be undertaken by automated or remote controlled synthesis equipment housed inside lead-shielded fumehoods. There is therefore a need for simple synthetic procedures that can be undertaken in this manner. Also, it is important for the reactions to be rapid so that the radiotracer can be synthesized and purified ideally within 2-3 half lives of11C, i.e. within 40-60 minutes. As a result of the above drawbacks with existing methods of preparing11C labelled precursor compounds such as11CH3OTf, it would be highly beneficial for there to be a rapid, one-step process that can be used to form11CH3OTf and other useful precursors to radiotracer compounds from11CH4, which can be performed under mild conditions. That would reduce11C losses, increase the energy efficiency and lower the cost of the process. Ideally, the reaction would be catalytic rather than stoichiometric, in order to further improve efficiency and reduce cost. Recent work on methane oxidation has not yielded an efficient process that can be used to produce compounds such as methyl triflate from methane rapidly under mild conditions in a catalytic manner. It has been shown that inexpensive, highly electrophilic main-group (p-block) compounds TlX3 and PbX4 (X = trifluoroacetate, CF3CO2–) can cleanly oxidise CH4 (forming CH3X and HX) while being reduced to TlX and PbX2 respectively (see below). The reaction requires a high pressure of methane (around 35 bar) and elevated temperatures (around 180 °C), which are relatively harsh conditions that add to the cost of the process. Under these conditions, however, O2 (E0: O2 / H2O = +1.23 V vs SHE) is unable to regenerate Tl3+ / Pb4+from Tl+ / Pb2+(E0Mn+ / M(n-2)+> +1.23 V) and thus reactivity is stoichiometric (see Science, 2014, 343, 1232-1237). Main-group mediated catalytic oxidation of CH4. The reaction is stoichiometric in M (M = Tl or Pb). Conditions: CF3CO2H solvent, 180 °C, 35 bar CH4. Hypervalent iodine species have been used to oxidise hydrocarbons including methane and ethane to alcohol esters. For example, Gunnoe and coworkers (J. Am. Chem. Soc.2014,136, 8393-8401) have shown that iodate (IO3-) salts can be used to convert methane to methyl trifluoroacetate (MeOC(O)CF3; MeTFA) in the presence of trifluoroacetic acid (HTFA) and chloride ions. Similarly, iodine (III) species such as ICl3and I(TFA)3can be used to convert methane to MeTFA in HTFA in the presence of chloride ions. However, these reactions only result in any appreciable yield of methyl esters at high temperatures of at least around 180 °C and high pressures of the order to 240-6900 kPa. Also, these reactions are stoichiometric, not catalytic, with respect to the iodine species. There is a need in the art for processes that address the drawbacks discussed above and which enable the production of11C-labelled radiotracer precursor compounds, such as11CH3OTf, directly from11CH4in a rapid process which takes place under mild conditions. Additionally, it would be useful to be able to form13C-labelled compounds in a similarly facile way. This would allow13C labelled compounds to be produced, which could be useful for example for facilitating the tracking of biological processes by13C NMR spectroscopy. SUMMARY OF THE INVENTION The present invention provides a process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 comprising reacting [11C]CH4 or [13C]CH4 with an iodine(III) compound, wherein the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3 or -N(SO2CF3)2 group. Preferably, the iodine(III) compound is I(OSO2CF3)3 or I[N(SO2CF3)2]3. The process may comprise passing [11C]CH4 or [13C]CH4 gas over the iodine(III) compound in the solid state. The process may comprise adding the [11C]CH4 or [13C]CH4 to a solution of the iodine(III) compound in acid in a reaction vessel, wherein the acid is an acid that has a pKa of less than 3 on the aqueous scale or wherein the acid is an acid that has a Hammett acidity function (H0) of less than -2, providing that the acid is not H2SO4 / SO3. Said process may further comprise adding NO2 to the reaction vessel. The present invention also provides a process for the preparation of a radiotracer compound containing an [11C]CH3 group comprising performing the process for the preparation of [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 described herein and reacting [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 produced by said process with a radiotracer precursor compound, thereby forming the radiotracer compound. Preferred features of the processes of the invention are described below and defined in the appended claims. BRIEF DESCRIPTION OF THE FIGURES Figure 1 depicts the % conversion of CH4as a function of temperature in a solution of I(OTf)3in HOTf in an NMR tube with no stirring, wherein the CH4was added at 1 bar. Figure 2 is the gas phase IR spectrum of the headspace of a reaction between I2in trifluoromethanesulphonic acid, and NO2. Figure 3 is the gas IR spectrum of the NO2used in the Examples. DETAILED DESCRIPTION The present invention solves the problems described above by providing a simple, one-step, energy efficient processes for the production of11C-labelled radiotracer precursor compounds [11C]CH3OSO2CF3 and [11C]CH3N(SO2CF3)2 directly from [11C]CH4 under mild conditions. The processes may also be used to make the13C analogues, [13C]CH3OSO2CF3 and [13C]CH3N(SO2CF3)2, which may be useful for example for introducing [13C]CH3 groups into molecules and thus facilitating following reactions by13C NMR spectroscopy. Specifically, the present invention provides a single-step process that uses an inexpensive catalyst based on iodine in order to oxidise methane to the desired product. The reaction proceeds cleanly and selectively in high yield, avoiding the need for complex separation techniques to isolate the product. The process operates under industrially-attractive conditions (low temperatures such as 50-100°C and low pressures, even atmospheric pressure). This is a significant milestone that offers clear advantages over existing synthesis routes for radiotracer precursors such as [11C]CH3OTf, which typically require more steps and harsher operating conditions. The provision of a route of converting [11C]CH4 to a radiotracer precursor compound such as [11C]CH3OTf in a single step without having to use high temperatures and pressures and without using an expensive catalyst provides a remarkable step forward in this field. Crucially, the reaction takes place quickly and can therefore be performed whilst minimising losses of11C. The present invention is based on the surprising discovery that certain iodine (III) compounds in acidic media can be used to convert [11C]CH4 to [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 without overoxidation to CO2 at dramatically milder conditions than known processes described above which utilise Pb or Tl catalysts or other hypervalent iodine compounds in the presence of chloride ions. The reaction can also be performed at much milder conditions than the reactions described above that involve reacting [11C]CH4 with I2 at elevated temperature and then performing a further reaction to form [11C]CH3OTf.. In particular, the processes of the present invention can be used to oxidise methane cleanly and in high yield at pressures as low as 1 bar and at temperatures as low as room temperature, which is a highly unexpected, groundbreaking result in view of the well-known difficulty of activating alkanes such as methane at low temperatures and pressures. The process can be performed straightforwardly in a stoichiometric manner, but attractively, the present invention also provides a process which is catalytic with respect to iodine, by using NO2, an abundant waste gas, as the redox mediator. In doing so, the process effectively takes two waste gas streams (CH4and NO2) and uses them to synthesise a methyl ester. Further still, the NO produced by reduction of the NO2during the process can be re-oxidised by O2to regenerate NO2in a facile manner, making the waste gas NO2a catalytic redox mediator and making O2 the terminal oxidant. The overall catalytic process is summarised below, for the case where the iodine (III) catalyst is I(OTf)3 (OTf = triflate = CF3SO3-), which is dissolved in triflic acid (CF3SO3H (“HOTf”)). Definitions pKa refers to the -log10 of the acid dissociation constant (Ka) of an acid (i.e. pKa = -log10Ka). Ka is defined as: wherein HA is the acid, which dissociates into H+and A- ions, and the square brackets denote the concentration of each species. pKa as defined herein refers to pKa on the aqueous scale, meaning the pKa value measured in water at 25 °C. The pKa of most acids on the aqueous scale can be found in standard reference texts and so does not need to be measured in the majority of cases. If needed, pKa can be measured by potentiometric titration by titrating a sample of the relevant substance with acid, such as 0.1M HCl, and monitoring the titration with a pH electrode. The pKa value can be determined from the equivalence point of the titration curve, i.e. the point at which the slope of the titration curve is at its greatest (i.e. where the inflection point occurs and the line changes from upward curvature to downward curvature). At this point the pH = pKa. Such methods of determining pKa are standard and well-known in the art. The Hammett acidity function (H0) is a measure of the acidity of a substance. H0is a standard measure of acidity and H0values for many acids can be found in standard reference texts, such that H0does not need to be measured in the majority of cases. If needed, it can be calculated using the formula: wherein “log” is the common logarithm (log10), B is a weak base indicator, such as p-nitrochlorobenzene, BH+is the conjugate acid of B and pKBH+ = -log10(K) for the dissociation of BH+where K is the dissociation constant of BH+at 25 °C. represents the ionisation ratio of the indicator and this can be measured directly using UV- visible spectroscopy (i.e. by a spectrophotometric method) at 25 °C. Those skilled in the art will be familiar with measurement of such parameters by UV-visible spectroscopy. Such methods are widely described in the literature, for example in J. Am. Chem. Soc., 1971, 93, 5083–5087. The term “wherein the acid is an acid that has a pKa of less than 3 on the aqueous pKa scale or the acid is an acid that has a Hammett acidity function (H0) of less than -2” (and likewise for other disclosed values of pKa and H0) means that the acid is either (i) an acid that has a pKa of less than 3 on the aqueous pKa scale, as defined above, or (ii) an acid that has a Hammett acidity function (H0) of less than -2. The acid may be an acid that has both a pKa of less than 3 on the aqueous scale and a H0value of less than -2 (and likewise for other disclosed values of pKa and H0), or it may be an acid that satisfies one or other of these requirements but not both. The term “alkyl” refers to a straight- or branched-chain alkyl group containing only single bonds between carbon atoms and in which the carbon atoms are saturated with hydrogen atoms. Examples of alkyl groups include methyl (Me), ethyl (Et), n-propyl, isopropyl, butyl, isobutyl, sec-butyl, tert- butyl (tBu), pentyl, isopentyl, tert-pentyl, hexyl, isohexyl, and groups that in light of the ordinary skill in the art and the teachings provided herein would be considered equivalent to any one of the foregoing examples. The term “substituted” means that the specified group or moiety bears one or more substituents. The term "unsubstituted" means that the specified group bears no substituents. The term “optionally substituted” means that the specified group is unsubstituted or substituted by one or more substituents. Where the term “substituted” is used to describe a structural system, the substitution is meant to occur at any valency-allowed position on the system. In cases where a specified moiety or group is not expressly noted as being optionally substituted or substituted with any specified substituent, it is understood that such a moiety or group is intended to be unsubstituted. The term “perfluorinated” means that the carbon atoms in the relevant moiety are substituted with a fluorine atom at every available valency, in line with its standard meaning in the art. “H2SO4 / SO3” refers to sulfuric acid (H2SO4) that contains dissolved SO3. This substance is commonly known as oleum, or fuming sulfuric acid, and is usually produced by dissolving SO3 in concentrated H2SO4. Typically, in oleum, the H2SO4 contains dissolved SO3 in an amount of 1-65% w / w. A “sulfonic acid” is an acid that contains a -SO3H moiety. A “carboxylic acid” is an acid that contains a -CO2H moiety. A “perfluorinated C1-8 alkyl sulfonic acid” comprises a perfluorinated C1-8 alkyl group attached to a sulfonic acid (SO3H) moiety. A “perfluorinated C1-8 alkyl carboxylic acid” comprises a perfluorinated C1-8 alkyl group attached to a carboxylic acid (CO2H) moiety. An “iodine(III) compound” is a compound in which iodine is present in the +3 oxidation state. “OTf” is an abbreviation for “triflate”, which is the common name for a trifluoromethanesulfonate (CF3SO3-) moiety. It is the conjugate base of triflic acid (trifluoromethanesulfonic acid, CF3SO3H, also known as HOTf). “Tf” is an abbreviation for a CF3SO2 moiety. [11C]CH4 is methane (CH4) that is isotopically enriched with11C. The [11C]CH4 may not be isotopically pure and may contain some [12C]CH4. As those skilled in the art will appreciate, it is practically impossible to obtain the theoretical maximum specific radioactivity for a sample of a11C compound due to unavoidable isoptopic dilution by naturally occurring12C. [13C]CH4methane (CH4) that is isotopically enriched with13C. The [13C]CH4may not be isotopically pure and may contain some [12C]CH4. [11C]CH3OSO2CF3and [13C]CH3OSO2CF3are CH3OSO2CF3in which the carbon atom in the CH3group is11C or13C, respectively, and likewise for [11C]CH3N(SO2CF3)2and [13C]CH3N(SO2CF3)2. A “radiotracer compound” is a compound that is tagged with a11C atom, which can be used as a probe in PET imaging. Those skilled in the art will be familiar with a number of compounds that are commonly used for this purpose. A number of specific examples are provided below. When a value is defined as being in the range from X to Y, the range includes the endpoints X and Y. Process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N 2 or [13C]CH3N(SO2CF3)2 As explained above, the invention provides a process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 comprising reacting [11C]CH4 or [13C]CH4 with an iodine(III) compound, wherein the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3 or -N(SO2CF3)2 group. Naturally, it will be understood that when the process is used to prepare [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3, the iodine in the iodine(III) compound needs to be bonded to at least one -OSO2CF3group, and when the process is used to prepare [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2, the iodine in the iodine(III) compound needs to be bonded to at least one -N(SO2CF3)2 group. Preferably, the process comprises reacting [11C]CH4with the iodine(III) compound, in order to produce [11C]CH3OSO2CF3or [11C]CH3N(SO2CF3)2, which can be used to prepare radiotracer compounds for PET imaging applications. More preferably, the process comprises reacting [11C]CH4with the iodine(III) compound, preferably I(OSO2CF3)3, thereby forming [11C]CH3OSO2CF3. When [11C]CH4or [13C]CH4is added to the iodine(III) compound, the methane is oxidised, thereby forming CH3OSO2CF3or CH3N(SO2CF3)2, in which the carbon atom in the CH3group is11C or13C. At the same time, the iodine is reduced from I(III) to I2. As explained further below, the I2can be oxidised back to an iodine(III) compound by NO2, which causes the NO2to be reduced to NO, thus enabling the reaction to be performed catalytically by regenerating the iodine(III) compound. Further still, the NO produced when NO2is reduced can be reoxidised to NO2using O2, meaning that O2acts as the terminal oxidant and catalytic amounts of NO2can be used. This contributes to the environmentally beneficial nature of the process, since the only reagents used up during the reaction are methane and O2. Performing the reaction in the solid state The process may be performed by passing [11C]CH4or [13C]CH4gas over the iodine(III) compound in the solid state. The reaction may proceed at room temperature, i.e. a temperature in the range from about 20oC to about 30oC, but proceeds faster at elevated temperatures. It is therefore preferable to perform the reaction at a temperature in the range from about 50oC to about 150oC, more preferably at a temperature in the range from about 75oC to about 125oC. There is no particular upper limit on the pressure that may be used for the reaction of [11C]CH4 or [13C]CH4 gas with the iodine(III) compound in the solid state, and this will largely be dictated by the nature of the reaction vessel used. The iodine(III) compound is present in a reaction vessel and the [11C]CH4 or [13C]CH4 gas may be added to the reaction vessel at any pressure of about 0.5 bar or above, such as a pressure in the range from about 0.5 bar to about 200 bar. However, a significant advantage of the process of the invention is that the reaction proceeds well at low pressures, which makes the process cheaper and simpler to operate and does not require specialist equipment that can withstand high pressures. Therefore, preferably, the [11C]CH4 or [13C]CH4 is added at a pressure of about 10 bar or less (e.g. at a pressure in the range from about 0.5 bar to about 10 bar), preferably 5 bar or less (e.g. at a pressure in the range from about 0.5 bar to about 5 bar), and more preferably at a pressure in the range from about 0.5 bar to about 2 bar. It will naturally be understood that the pressures and temperatures described above relate to conditions for performing the same reaction and therefore may be applied in combination. The reaction may thus be performed at a temperature in the range from about 50oC to about 150oC, more preferably at a temperature in the range from about 75oC to about 125oC, wherein [11C]CH4 or [13C]CH4 gas is added to the reaction vessel at a pressure in the range from about 0.5 bar to about 10 bar, even more preferably in the range from about 0.5 bar to about 2 bar. Thus [11C]CH4 or [13C]CH4 gas may be added to the reaction vessel at a pressure in the range from about 0.5 bar to about 10 bar and the reaction may then be performed at a temperature in the range from about 50oC to about 150oC. The iodine(III) compound may be incorporated into a catalyst support material, such as silica or a zeolite. Such catalyst support systems are well known to those skilled in the art and any suitable catalyst support system may be used in the process of the invention. NO2may be added to the reaction vessel after the iodine(III) compound has reacted with the [11C]CH4or [13C]CH4gas, in order to regenerate the iodine(III) compound. Preferably, the [11C]CH4or [13C]CH4gas is evacuated from the reaction vessel before NO2is added. This is preferable for safety reasons. The [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2product may be decanted as a liquid from the reaction vessel or may be washed out of the reaction vessel using a solvent. CH3OSO2CF3boils at 100 °C at atmostpheric pressure, and if the reaction is performed at a temperature of about 100 °C or higher (e.g. in the range from about 100 °C to about 150 °C), the [11C]CH3OSO2CF3or [13C]CH3OSO2CF3may be formed in the gaseous phase and the flow of methane through the reaction vessel may carry the [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3 out of the reaction vessel for use in subsequent processes. Performing the reaction in solution The reaction may be performed in solution instead of in the solid state. The invention thus provides a process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 comprising adding [11C]CH4 or [13C]CH4 gas to a solution of the iodine(III) compound in acid in a reaction vessel, wherein the acid is an acid that has a pKa of less than 3 on the aqueous scale or the acid is an acid that has a Hammett acidity function (H0) of less than -2, providing that the acid is not H2SO4 / SO3, wherein the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3 or -N(SO2CF3)2 group. Throughout the present disclosure, term “a solution of an iodine(III) compound in acid” as used herein means a solution formed by adding an iodine(III) compound to an acid. This applies regardless of the nature of the iodine species that are present in the resultant solution. However, as explained below, the solution of an iodine(III) compound in acid may also be formed in situ from the oxidation of I2 in CF3SO3H or HN(SO2CF3)2 with NO2. The process may further comprise adding NO2 to the reaction vessel. NO2 acts as an oxidant to regenerate the iodine(III) compound from the I2 that is formed when the iodine(III) compound reacts with [11C]CH4 or [13C]CH4. The solution may optionally be cooled to below 0 °C before NO2is added, but this is not critical and NO2 can be added at the temperature at which the reaction with methane is conducted or at ambient temperature (e.g. at about 25 °C). The pressure at which NO2is added to the reaction vessel is not particularly critical, but it may be added at a pressure of at least about 0.5 bar, preferably in the range from 1 bar to 10 bar. Preferably, NO2is added to the reaction vessel after the formation of I2. The formation of I2may be determined by UV-visible spectroscopy or conductivity measurements, preferably by UV-visible spectroscopy. In some acidic media, such as triflic acid, the formation of I2may be detected visually, by the formation of a precipitate of I2. The process may involve degassing the reaction vessel before NO2is added. This avoids mixing methane with the oxidant NO2, which is preferable from a safety perspective. The reaction vessel may be degassed by any standard degassing process. An example of a suitable degassing process is to use a freeze-thaw process, in which the reaction mixture is frozen and the headspace in the reaction vessel is evacuated under vacuum using a vacuum pump. The reaction vessel is then sealed off from the vacuum pump but left under static vacuum, and the solution allowed to thaw, thus drawing gas out of the solution. The solution is then frozen again and the headspace evacuated using the vacuum pump. This may preferably be repeated several times (e.g. two to four times) to degas the reaction mixture thoroughly. Other known methods of degassing may be used instead, such a sparging with inert gas. The use of NO2 makes the process catalytic by regenerating the iodine(III) compound. The process may therefore be performed in a continuous manner in which the process further comprises: (a) adding further [11C]CH4 or [13C]CH4 to the reaction vessel after adding NO2 to the reaction vessel; (b) allowing the [11C]CH4 or [13C]CH4 to react with the iodine(III) compound in the reaction mixture formed in step (a); (c) adding further NO2 to the reaction vessel during or after step (b); (d) repeating steps (a) to (c). As above, preferably the solution is cooled to below 0 °C before NO2 is added in step (c) and more preferably the solution is cooled to below -50 °C before NO2 is added. The pressure at which NO2 is added to the reaction vessel in step (c) is not particularly critical, but it may be added at a pressure of at least about 0.5 bar, preferably in the range from 1 bar to 10 bar. Preferably, the reaction vessel is degassed before NO2is added in step (c). The process may comprise adding O2 to regenerate NO2 from the NO that is formed when NO2 reacts with the I2formed from the reaction of the iodine(III) compound with [11C]CH4or [13C]CH4. O2may be added after each addition of NO2in order to regenerate NO2. For example, O2may be added after step (c) as well as after any previous addition of NO2to the reaction vessel. O2and NO2may both be added to the reaction vessel. If NO2and O2are both added to the reaction vessel, the reaction vessel is preferably degassed first. This avoids O2and NO2mixing with methane, which is preferable from a safety perspective. It is more preferable for O2not to be added to the reaction vessel in which [11C]CH4or [13C]CH4is reacted with the iodine(III) compound. Therefore, preferably, NO2is regenerated by transferring the NO formed in the process to a separate vessel (i.e. a vessel other than the reaction vessel) and mixing it with O2in that vessel to form NO2, before returning NO2to the reaction vessel. No catalyst is necessary to catalyse the reaction of NO with O2, so preferably, no catalyst is used to catalyse the reaction of NO with O2. However, if desired, a catalyst may be used for this reaction and suitable catalysts are well-known to those skilled in the art. The NO2 can then be returned to the reaction vessel. The solution phase reaction may be performed at the same temperature and pressure as described above in the context performing the process in the solid state. The reaction may proceed at room temperature, i.e. a temperature in the range from about 20oC to about 30oC, but proceeds faster at elevated temperatures. It is therefore preferable to perform the reaction at a temperature in the range from about 50oC to about 150oC, more preferably at a temperature in the range from about 75oC to about 125oC. There is no particular upper limit on the pressure at which the [11C]CH4 or [13C]CH4 gas is added to the reaction vessel, and this will largely be dictated by the nature of the reaction vessel used. The [11C]CH4 or [13C]CH4 gas may be added to the reaction vessel at any pressure of about 0.5 bar or above, such as a pressure in the range from about 0.5 bar to about 200 bar. However, a significant advantage of the process of the invention is that the reaction proceeds well at low pressures, which makes the process cheaper and simpler to operate and does not require specialist equipment that can withstand high pressures. Therefore, preferably, the [11C]CH4 or [13C]CH4 is added at a pressure of about 10 bar or less (e.g. at a pressure in the range from about 0.5 bar to about 10 bar), preferably 5 bar or less (e.g. at a pressure in the range from about 0.5 bar to about 5 bar), and more preferably at a pressure in the range from about 0.5 bar to about 2 bar. It will naturally be understood that the pressures and temperatures described above relate to conditions for performing the same reaction and therefore may be applied in combination. The reaction may thus be performed at a temperature in the range from about 50oC to about 150oC, more preferably at a temperature in the range from about 75oC to about 125oC, wherein [11C]CH4or [13C]CH4gas is added to the reaction vessel at a pressure in the range from about 0.5 bar to about 10 bar, even more preferably in the range from about 0.5 bar to about 2 bar. Thus [11C]CH4or [13C]CH4 gas may be added to the reaction vessel at a pressure in the range from about 0.5 bar to about 10 bar and the reaction may then be performed at a temperature in the range from about 50oC to about 150oC. The reaction generally proceeds rapidly, and at temperatures of 50oC or above, reaction is normally immediate, especially if the reaction is performed in solution with stirring. Those of ordinary skill in the art will readily be able to determine how long to perform the reaction for in order to achieve high conversion of [11C]CH4or [13C]CH4to the desired product. [11C]CH4or [13C]CH4gas is preferably reacted with the iodine(III) compound for a duration of at least one minute. The [11C]CH4 or [13C]CH4 may be reacted with the iodine(III) compound for a time in the range from 1 minute to 60 minutes, for example a time in the range from 1 minute to 30 minutes, preferably 1 minute to 10 minutes. The same reaction times may also be used for the reaction in the solid state described above. The ability to achieve a high yield of the product within this short timeframe is a significant benefit in the context of preparing radiotracer compounds for PET imaging, as explained above. Longer reaction times of up to 2 hours may be used, but this is not desirable in the context of preparing [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2, which may be used to prepare radiotracer compounds by methylation. At the end of the reaction, the desired product may be isolated from the reaction mixture, for example using distillation. Methyl triflate (CH3OTf) boils at 100 °C at atmospheric pressure and may be isolated from the reaction mixture by distillation. Therefore, the process of the invention may comprise performing distillation to isolate [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3 from the reaction mixture. The distillation may be performed under reduced pressure, i.e. at a pressure lower than 1 atm. Iodine(III) compound An iodine(III) compound is used as an oxidant in the process. An iodine(III) compound is a compound in which iodine is in the +3 oxidation state. The iodine(III) compound can be made by methods well known to those skilled in the art. The synthesis of iodine(III) compounds including I(OTf)3 is reported in Inorganic Chemistry, vol.28, no.4, 1989, and other iodine(III) compounds for use in the present invention can be synthesised analogously or by other methods known to those skilled in the art. In the process of the invention, the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3or -N(SO2CF3)2group. It is not vital for all groups attached to iodine in the iodine(III) compound to be -OSO2R or -N(SO2R)2groups. The iodine in the iodine(III) compound may, for example, be bonded to one or more aryl (Ar) groups. Thus, the iodine(III) compound may, for example, be of the formula ArIZ2or Ar2IZ, where Z is -OSO2CF3or -N(SO2CF3)2. Ar is preferably a phenyl group, more preferably a substituted phenyl group, wherein the substituents are preferably nitro or halo groups. Especially preferred Ar groups are C6F5or p-NO2C6H4, i.e. the iodine(III) compound may be of the formula (C6F5)IZ2, (C6F5)2IZ, (p- NO2C6H4)IZ2or (p NO2C6H4)2IZ, where Z is as defined above. However, the iodine(III) compound is very preferably homoleptic, meaning that all three substituents on the iodine atom are the same. In other words, the iodine(III) compound very preferably has the formula I(OSO2CF3)3 or I[N(SO2CF3)2]3. Therefore, in the process for the preparation of [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3, the iodine(III) compound is very preferably I(OSO2CF3)3. In the process for the preparation of [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2, the iodine(III) compound is very preferably I[N(SO2CF3)2]3. The homoleptic iodine(III) compounds I(OSO2CF3)3 and I[N(SO2CF3)2]3 may be prepared in situ by mixing I2 with triflic acid (CF3SO3H) or HN(SO2CF3)2, respectively, and adding NO2 as an oxidant. Therefore, the process of the invention may further comprise mixing I2 with CF3SO3H in a reaction vessel and adding NO2 to the reaction vessel, prior to adding [11C]CH4 or [13C]CH4 to the reaction vessel. Optionally, said method may comprise adding further NO2 to the reaction vessel after the [11C]CH4 or [13C]CH4 has reacted with the I(OSO2CF3)3 and I2 has been formed. Alternatively, the process of the invention may further comprise mixing I2 with HN(SO2CF3)2 in a reaction vessel and adding NO2 to the reaction vessel, prior to adding [11C]CH4 or [13C]CH4 to the reaction vessel. The I2 should preferably be mixed with the HN(SO2CF3)2 at a temperature of at least about 60 °C, more preferably in the range from about 60 °C to about 150 °C, so that HN(SO2CF3)2 is in liquid form and the temperature should be maintained within that range while NO2 is added. The reaction with [11C]CH4 or [13C]CH4 should also preferably be conducted at a temperature of at least about 60 °C, more preferably in the range from about 60 °C to about 150 °C. Optionally, said method may comprise adding further NO2 to the reaction vessel after the [11C]CH4 or [13C]CH4 has reacted with the I[N(SO2CF3)2]3and I2has been formed. Acids As explained above, the reaction may be performed by adding the [11C]CH4or [13C]CH4gas to a solution if the iodine(III) compound in acid. The acid is an acid that has a pKa of less than 3 on the aqueous pKa scale or is an acid that has a Hammett acidity function (H0) of less than -2. Preferably, the acid is an acid that has a pKa of less than 3 on the aqueous pKa scale The acid preferably has a pKa of less than 1 on the aqueous pKa scale, more preferably less than 0 and most preferably less than -1. The acid may be an acid that has a Hammett acidity function (H0) of less than -2, preferably less than -6 and more preferably less than -10. As explained above, those skilled in the art will be well aware of such acids from their common general knowledge. The acid is not H2SO4 / SO3 and preferably is not H2SO4. This is because the use of H2SO4 / SO3 as the acid leads to the production of CH3OSO3H rather than the desired reaction product. The iodine(III) compound preferably fully dissolves in the acid at the temperature that the reaction takes place, but that is not essential. Preferably, the acid is the conjugate acid of the -OSO2CF3 or -N(SO2CF3)2 group that is attached to the iodine atom in the iodine(III) compound. HN(SO2CF3)2, commonly known as bistriflimidic acid, is a solid at room temperature and melts at around 46-57 °C. As one skilled in the art will appreciate, when using this acid or any other acid that is a solid at room temperature, it is necessary to conduct the reaction at a temperature above the melting point of the acid, i.e. the temperature should remain above the melting point of the acid while the reaction takes place. Preferably, when the acid is HN(SO2CF3)2, the process is performed at a temperature of at least 60 °C, more preferably in the range from about 60 °C to about 150 °C. The acid may be a carboxylic acid or a sulfonic acid or HNO3. The carboxylic acid or sulfonic acid may contain a halogenated alkyl or aryl group, preferably wherein the halogen atoms are fluorine or chlorine atoms. More preferably, the acid is a perfluorinated C1-8 alkyl sulfonic acid or perfluorinated C1-8alkyl carboxylic acid. The perfluorinated C1-8alkyl sulfonic acid or perfluorinated C1-8alkyl carboxylic acid may be a perfluorinated C1-4alkyl sulfonic acid, perfluorinated C1-4alkyl carboxylic acid, perfluorinated C2-8 alkyl sulfonic acid or perfluorinated C3-8 alkyl carboxylic acid. Methyl triflate (CH3OTf) boils at 100 °C under atmospheric pressure. In a process for the preparation of [11C]CH3OTf or [13C]CH3OTf, the use of acids which boil at a higher temperature than CH3OTf is preferred because it facilitates simpler distillation of [11C]CH3OTf or [13C]CH3OTf from the reaction mixture. The use of triflic acid (HOTf) is preferable because it boils at a higher temperature than CH3OTf and therefore [11C]CH3OTf or [13C]CH3OTf can easily be separated from the reaction mixture by distillation. Similarly, perfluorinated C2-8alkyl sulfonic acids and perfluorinated C3-8alkyl carboxylic acids are useful when preparing [11C]CH3OTf or [13C]CH3OTf because they have sufficient acidity whilst having high boiling points and therefore the reaction proceeds well in these solvents and the [11C]CH3OTf or [13C]CH3OTf can easily be separated from the reaction mixture. Examples of suitable acids to use in the process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 include CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3(CF2)3SO3H, CF3(CF2)4SO3H, CF3(CF2)5SO3H, CF3(CF2)6SO3H, CF3(CF2)7SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CF3(CF2)3CO2H, CF3(CF2)4CO2H, CF3(CF2)5CO2H, CF3(CF2)6CO2H, CF3(CF2)7CO2H, CCl3CO2H, FSO3H, HN(SO2CF3)2, HN(SO2CF2CF3)2 or HNO3. Preferably, the acid may be selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CCl3CO2H, HN(SO2CF3)2, HN(SO2CF2CF3)2, FSO3H or HNO3. More preferably, the acid may be selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, HN(SO2CF3)2 or FSO3H. All of these acids boil significantly above the boiling point of methyl triflate whilst also being strong acids that result in highly reactive reaction media. As explained above, it is preferred for the acid to be the conjugate acid of the -OSO2CF3 or -N(SO2CF3)2 group that is attached to the iodine atom in the iodine(III) compound. In the process for the preparation of [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3, the most preferred acid is triflic acid (CF3SO3H). In the process for the preparation of [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2, the most preferred acid is HN(SO2CF3)2. The separation of [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 formed in HN(SO2CF3)2 may be perfomed by sublimation of the HN(SO2CF3)2 and the I2 formed in the reaction, allowing the unsublimed [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 product to be isolated. It will be appreciated that each of the acids disclosed in this section can be used in conjunction with the iodine(III) compounds disclosed above, especially the preferred iodine(III) compounds I(OSO2CF3)3or I[N(SO2CF3)2]3. Therefore, the present invention provides a process for the preparation of [11C]CH3OSO2CF3or [13C]CH3OSO2CF3comprising adding [11C]CH4or [13C]CH4to a solution of I(OSO2CF3)3in acid, wherein the acid is a carboxylic acid or a sulfonic acid that has a pKa or less than 3 on the aqueous scale or is a carboxylic acid or a sulfonic acid that has a Hammett acidity function (H0) of less than -2, preferably wherein the acid is a perfluorinated C1-8alkyl sulfonic acid or perfluorinated C1-8alkyl carboxylic acid (e.g. a perfluorinated C1-4alkyl sulfonic acid, perfluorinated C1-4alkyl carboxylic acid). The acid may be selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3(CF2)3SO3H, CF3(CF2)4SO3H, CF3(CF2)5SO3H, CF3(CF2)6SO3H, CF3(CF2)7SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CF3(CF2)3CO2H, CF3(CF2)4CO2H, CF3(CF2)5CO2H, CF3(CF2)6CO2H, CF3(CF2)7CO2H, CCl3CO2H, FSO3H, HN(SO2CF3)2, HN(SO2CF2CF3)2 or HNO3, preferably selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CCl3CO2H, HN(SO2CF3)2, HN(SO2CF2CF3)2, FSO3H or HNO3, more preferably selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, HN(SO2CF3)2 or FSO3H and most preferably the acid is CF3SO3H. The present invention also provides a process for the preparation of [11C]CH3N(SO2CF3)2 or [13C]CH3N(SO2CF3)2 comprising adding [11C]CH4 or [13C]CH4 to a solution of I[N(SO2CF3)2]3 in acid, wherein the acid is a carboxylic acid or a sulfonic acid that has a pKa or less than 3 on the aqueous scale or is a carboxylic acid or a sulfonic acid that has a Hammett acidity function (H0) of less than -2, preferably wherein the acid is a perfluorinated C1-8 alkyl sulfonic acid or perfluorinated C1-8 alkyl carboxylic acid (e.g. a perfluorinated C1-4 alkyl sulfonic acid, perfluorinated C1-4 alkyl carboxylic acid). The acid may be selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3(CF2)3SO3H, CF3(CF2)4SO3H, CF3(CF2)5SO3H, CF3(CF2)6SO3H, CF3(CF2)7SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CF3(CF2)3CO2H, CF3(CF2)4CO2H, CF3(CF2)5CO2H, CF3(CF2)6CO2H, CF3(CF2)7CO2H, CCl3CO2H, FSO3H, HN(SO2CF3)2, HN(SO2CF2CF3)2 or HNO3, preferably selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CCl3CO2H, HN(SO2CF3)2, HN(SO2CF2CF3)2, FSO3H or HNO3, more preferably selected from the group consisting of CF3SO3H, CH3SO3H, CF3CF2SO3H, HN(SO2CF3)2 or FSO3H and most preferably the acid is CF3SO3H or HN(SO2CF3)2. Preferably, the solution of the iodine(III) compound in acid does not contain chloride ions. One advantage of the process of the present invention over the work of Periana et al described in J. Am. Chem. Soc.2014, 136, 8393-8401, which is discussed in the background section, is that the process of the present invention does not require the presence of chloride ions to obtain a high yield. As will be appreciated, the process of the invention has numerous other advantages over that prior work, particularly the the catalytic nature of the process with respect to iodine and the fact that the reaction proceeds under much milder conditions. The volume of acid used is not particularly critical but may be in the range of 10-5000 mL / g relative to the mass of iodine(III) compound, for example in the range of 50-1000 mL / g relative to the mass of iodine(III) compound. Process for the preparation of a radiotracer compound The [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 produced by the processes described above may be used to introduce a [11C]CH3 group into a target compound to form a radiotracer compound. [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 can act as methylating agents, transferring an [11C]CH3 group to a target compound in order to form a radiotracer compound. [11C]CH3OSO2CF3 is a particularly well-established methylating agent that is used for this purpose and is the preferred methylating agent to use in the process of preparing a radiotracer compound. The present invention thus provides a process for the preparation of a radiotracer compound containing an [11C]CH3 group comprising performing the process described above for forming [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 and reacting [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 produced by said process with a target compound, thereby forming the radiotracer compound. Preferably, the process involves using [11C]CH3OSO2CF3 as the methylating agent. The target compound is preferably a compound that contains a nucleophilic nitrogen, oxygen or sulfur atom which can be methylated. For example, the target compound may be a compound that contains a -OH, -SH or -NH moiety (the nitrogen in an -NH moiety would naturally be bonded to one or two other groups to satisfy the valency of the nitrogen atom). Numerous common radiotracer compounds are typically produced by methylation using a11C labelled methylating agent such as [11C]CH3OTf. Common examples include [11C]raclopride, [11C]choline, [11C]methionine, [11C]HED (hydroxyephedrine), [11C]DTBZ (dihydrotetrabenazine), [11C]DASB (3- amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile), [11C]CFN (carfentanil), [11C]FMZ (flumazenil) and [11C]PiB (Pittsburgh compound B), the structures of which are provided below. Therefore, preferably, the process for the preparation of a radiotracer compound containing an [11C]CH3group comprises reacting a target compound with [11C]CH3OSO2CF3or [11C]CH3N(SO2CF3)2, preferably [11C]CH3OSO2CF3, to form [11C]raclopride, [11C]choline, [11C]methionine, [11C]HED (hydroxyephedrine), [11C]DTBZ (dihydrotetrabenazine), [11C]DASB (3- amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile), [11C]CFN (carfentanil), [11C]FMZ (flumazenil) or [11C]PiB (Pittsburgh compound B). The structure of the target compounds to be methylated using [11C]CH3OSO2CF3or [11C]CH3N(SO2CF3)2in order to form these radiotracer compounds will be self-evident to those skilled in the art; in most cases, those target compounds will contain a hydrogen atom (e.g. as part of an -NH, -OH or -SH group) at the position in which the [11C]CH3group is to be introduced. The target compound should be reacted with around 1 molar equivalent of the [11C]CH3OSO2CF3or [11C]CH3N(SO2CF3)2, e.g. about 1 to about 1.2 equivalents. Those skilled in the art will be familiar with methods of preparing the above radiotracer compounds by methylation of the relevant target compound that can act as a precursor to the relevant radiotracer compound. EXAMPLES The invention is illustrated by the following examples. These examples are not limiting on the scope of the invention, which is defined by the appended claims. Example 1: Stoichiometric methane activation using I(OTf)3 in HOTf I(OTf)3(50 mg) was added to a J. Youngs NMR tube. HOTf (0.6 mL) was then added to this to make a yellow solution. A 0.01 M 1,3,5-trimethoxybenzene in DMSO-d6 capillary insert was added to the NMR tube for use as a concentration reference and deuterium lock. Typically, an NMR measurement was carried out at this point to determine if there were any contamination. The N2in the NMR tube was then removed by freeze-pump-thawing the solution three times. CH4(1 Bar) was then added to the NMR tube. An NMR measurement was typically carried out at this point to determine successful CH4addition. The solution was then heated at the desired temperature. The colour of the solution typically displayed the following colours in order: yellow, turquoise, green, brown. The end point is determined by the visible formation of I2. An NMR measurement was then taken to determine CH4activation.1H NMR (400 MHz, DMSO) δ 9.79 (s, HOTf), 6.11(s, (1,3,5-TMB)), 4.03 (s, CH3OTf, 3.73 (s, (1,3,5- TMB)), -0.09 (s, CH4). Figure 1 depicts the % conversion of CH4 using I(OTf)3 and HOTf as a function of temperature in an NMR tube with no stirring, wherein the CH4 was added at 1 bar. It can be seen that the reaction progresses rapidly at temperatures below 100 °C when methane is added at 1 bar. This is a remarkable result given that previous systems for the oxidation of methane have required the use of much higher temperatures and pressures to achieve appreciable methane conversion. Example 2: Solid state reactivity of I(OTf)3 with CH4 I(OTf)3 (50 mg) was placed into a J. Youngs NMR tube. The N2 was removed in vacuo and then CH4 (1 Bar) was added. The NMR tube was then heated at 100 °C. After 5 minutes the white solid had turned into a green oily solid. The NMR tube was heated for 24 h at 100 °C which afforded a brown oily liquid with sublimed I2 on the walls of the NMR tube. HOTf (0.6 mL) was added to the NMR tube to afford a brown solution. A 0.01 M 1,3,5-trimethoxybenzene in DMSO-d6 capillary insert was added to the NMR tube for use as a concentration reference and deuterium lock. An NMR measurement was then taken to determine CH4 activation. The yield of MeOTf was 10.5% with respect to CH4.1H NMR (400 MHz, DMSO_weak) δ 10.06 (s, HOTf), 6.09(s, 1,3,5-TMB), 3.93 (s, MeOTf), 3.70 (s, 1,3,5-TMB), -0.21 (s, CH4). Example 3: Catalytic CH4 oxidation using I(OTf)3 + NO2 + CH4 in HOTf I(OTf)3(50 mg) was added to a J. Youngs ampoule. HOTf (5 mL) was added to this to give a yellow solution. The solution was then degassed and CH4 (1 Bar) was added. (A) The solution was allowed to stir at 100 °C until I2was observed (brown solution colour). NO2(0.0313 g) was then added at -78 °C, and allowed to thaw to give a turquoise solution, then degassed again and finally CH4(1 bar) was added. Again, this was heated to 100 °C and allowed to react. The steps from A were then repeated 3 more times. NO2was added again to homogenise the solution. A 0.6 mL aliquot of this reaction was then taken and added to an NMR tube.1H-NMR showed that 4.25 eq. MeOTf were produced. Example 4: Catalytic CH4oxidation using I2+ NO2+ CH4in HOTf I2(10 mg) was added to a J. Youngs ampoule. HOTf (5 mL) was added to this to give a brown suspension. The solution was then degassed and NO2(0.0313g) was added to give a turquoise solution. The solution was then degassed again and CH4(1 Bar) was added. (A) The solution was allowed to stir at 100 °C until I2 was observed (brown solution colour). NO2 (0.0313 g) was then added at -78 °C, and allowed to thaw to give a turquoise solution, then degassed again and finally CH4 (1 bar) was added. Again, this was heated to 100 °C and allowed to react. The steps from A were then repeated 3 more times. NO2 was added again to homogenise the solution. A 0.6 mL aliquot of this reaction was then taken and added to an NMR tube.1H-NMR showed that 2.76 eq. MeOTf were produced. Example 5: Gas phase infrared (IR) data of the headspace of a reaction between I2 in trifuloromethanesulphonic acid, and NO2 I2 (17.5 mg) was added to trifluoromethanesulphonic acid (2 mL) in an ampoule. The suspension was then frozen and NO2 (5 mL) was added. Once the NO2 was frozen, the ampoule was evacuated and sealed. It was then allowed to warm to room temperature where the solution turned from light pink to a deep turquoise solution. The headspace of the reaction was then vacuum transferred to an evacuated 10 cm gas phase IR cell (KBr windows). An IR spectrum was measured, which displayed characteristic absorbances associated with NO gas (1959 cm-1, 1928 cm-1– see R. E. Nightingale, A. R. Downie, D. L. Rotenberg, B. J. Crawford and R. A. J. Ogg, J. Phys. Chem., 1954, 58, 1047–1050). This IR spectrum is shown in Figure 2. Absorbances associated with NO2 (1750 cm-1(N2O4), 1600 cm-1(NO2), 1250 cm-1(N2O4), 750 cm-1(N2O4)) (see Figure 3) are not present in the spectrum (see R. E. Nightingale, A. R. Downie, D. L. Rotenberg, B. J. Crawford and R. A. J. Ogg, J. Phys. Chem., 1954, 58, 1047–1050). The absorbances below 1500 cm-1are associated with trifluoromethanesulphonic acid vapour. The NO2 used for these reactions does not contain any NO gas, so this must be a product of the reaction between the NO2and I2in trifluoromethanesulphonic acid. This result demonstrates that NO is formed in the reaction, which can be subsequently re-oxidised by O2and reused in the catalytic cycle. EMBODIMENTS OF THE INVENTION A process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2comprising reacting [11C]CH4or [13C]CH4with an iodine(III) compound, wherein the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3or -N(SO2CF3)2group. The process of embodiment 1, wherein the process is a process for the preparation of [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3 and the iodine(III) compound is I(OSO2CF3)3. The process of embodiment 1 or embodiment 2, wherein [11C]CH4 or [13C]CH4 gas is passed over the iodine(III) compound in the solid state. The process of embodiment 3, wherein the reaction is conducted at a temperature in the range from about 50 °C and about 150 °C, preferably in the range from about 75 °C to about 125 °C. The process of embodiment 3 or embodiment 4, wherein the [11C]CH4 or [13C]CH4 is added to the reaction vessel at a pressure of about 10 bar or less, preferably about 5 bar or less, more preferably in the range from about 0.5 bar to about 2 bar. The process of any one of embodiments 3-5, wherein the iodine(III) compound is incorporated into a catalyst support material. The process of any one of embodiments 3-6, wherein NO2 is passed over the iodine compound that remains after [11C]CH4 or [13C]CH4 gas has been passed over the iodine(III) compound, in order to regenerate the iodine(III) compound. The process of embodiment 1 or embodiment 2, wherein the [11C]CH4 or [13C]CH4 is added to a solution of the iodine(III) compound in acid in a reaction vessel, wherein the acid is an acid that has a pKa of less than 3 on the aqueous scale or wherein the acid is an acid that has a Hammett acidity function (H0) of less than -2, providing that the acid is not H2SO4 / SO3. The process of embodiment 8, further comprising adding NO2to the reaction vessel. The process of embodiment 9, wherein NO2is added to the reaction vessel after the formation of I2. The process of any one of embodiments 9-10, further comprising adding O2to regenerate the NO2. The process of any one of embodiments 9-11, comprising: (a) adding further [11C]CH4or [13C]CH4to the reaction vessel after adding NO2to the reaction vessel; (b) allowing the [11C]CH4 or [13C]CH4 to react with the iodine(III) compound in the reaction mixture formed in step (a); (c) adding further NO2 to the reaction vessel during or after step (b); (d) repeating steps (a) to (c). The process of embodiment 12, wherein O2 and NO2 are added to the reaction vessel in step (c). The process of any one of embodiments 8-13, wherein the acid is a carboxylic acid or a sulfonic acid, wherein the carboxylic acid or sulfonic acid has a pKa of less than 3 on the aqueous scale, or wherein the acid is HNO3. The process of embodiment 14, wherein the carboxylic acid or sulfonic acid contains a halogenated alkyl or aryl group, preferably wherein the halogen atoms are fluorine or chlorine atoms. The process of any one of embodiments 8-15, wherein the acid has a pKa on the aqueous scale of less than 1, preferably less than 0, more preferably less than -1. The process of embodiment 14, wherein the acid is a perfluorinated C1-8 sulfonic acid or a perfluorinated C1-8 carboxylic acid. The process of embodiment 14, wherein the acid is is CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CCl3CO2H, HN(SO2CF3)2, HN(SO2CF2CF3)2, FSO3H or HNO3. The process of embodiment 14, wherein the acid is CF3SO3H, CH3SO3H, CF3CF2SO3H, HN(SO2CF3)2or FSO3H. 20. The process of embodiment 14, wherein the iodine(III) compound is I(OTf)3and the acid is CF3SO3H, thereby producing [11C]CH3OSO2CF3or [13C]CH3OSO2CF3; or wherein the iodine(III) compound is I[N(SO2CF3)2]3and the acid is HN(SO2CF3)2,thereby producing [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2. 21. The process of any one of embodiments 8-13, wherein the solution of the iodine(III) compound in acid is formed by mixing I2with CF3SO3H and adding NO2to the reaction vessel. 22. The process of any one of embodiments 8-21, further comprising performing distillation to isolate [11C]CH3OSO2CF3 or [13C]CH3OSO2CF3 from the reaction mixture. 23. The process of any one of embodiments 1-2 and 8-22, wherein the [11C]CH4 or [13C]CH4 is allowed to react with the iodine(III) compound at a temperature in the range from about 10 °C to about 200 °C, preferably in the range from about 10 °C to about 150 °C, more preferably in the range from about 50 °C to about 125 °C. 24. The process of any one of embodiments 8-23, wherein the [11C]CH4 or [13C]CH4 is added to the reaction vessel at a pressure of about 10 bar or less, preferably about 5 bar or less, more preferably in the range from about 0.5 bar to about 2 bar. 25. A process for the preparation of a radiotracer compound containing an [11C]CH3 group comprising performing the process of any preceding embodiment and reacting [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 produced by said process with a target compound, thereby forming the radiotracer compound. The invention has been described in detail above, including preferred embodiments thereof. However, the invention is not limited except by the appended claims. Those skilled in the art will appreciate that modifications can be made to the preferred ways of implementing the invention that are described above, whilst remaining within the scope of the claims.

Claims

1. CLAIMS 1. A process for the preparation of [11C]CH3OSO2CF3, [13C]CH3OSO2CF3, [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2comprising reacting [11C]CH4or [13C]CH4with an iodine(III) compound, wherein the iodine in the iodine(III) compound is bonded to at least one -OSO2CF3or -N(SO2CF3)2group.

2. The process of claim 1, wherein the iodine(III) compound has the formula I(OSO2CF3)3or I[N(SO2CF3)2]3.

3. The process of claim 1 or claim 2, wherein the process is a process for the preparation of [11C]CH3OSO2CF3or [13C]CH3OSO2CF3and the iodine(III) compound is I(OSO2CF3)3.

4. The process of any one of claims 1-3, wherein [11C]CH4or [13C]CH4gas is passed over the iodine(III) compound in the solid state.

5. The process of claim 4, wherein the reaction is conducted at a temperature in the range from about 50 °C and about 150 °C, preferably about 75 °C to about 125 °C.

6. The process of claim 4 or claim 5, wherein the [11C]CH4 or [13C]CH4 is added to the reaction vessel at a pressure of about 10 bar or less, preferably about 5 bar or less, more preferably about 0.5 bar to 2 bar.

7. The process of any one of claims 4-6, wherein the iodine(III) compound is incorporated into a catalyst support material.

8. The process of any one of claims 4-7, wherein NO2 is passed over the iodine compound that remains after [11C]CH4 or [13C]CH4 gas has been passed over the iodine(III) compound, in order to regenerate the iodine(III) compound.

9. The process of any one of claims 1-3, wherein the [11C]CH4 or [13C]CH4 is added to a solution of the iodine(III) compound in acid in a reaction vessel, wherein the acid is an acid that has a pKa of less than 3 on the aqueous scale or wherein the acid is an acid that has a Hammett acidity function (H0) of less than -2, providing that the acid is not H2SO4 / SO3.

10. The process of claim 9, further comprising adding NO2 to the reaction vessel.

11. The process of claim 10, wherein NO2is added to the reaction vessel after the formation of I2.

12. The process of claim 10 or claim 11, wherein the reaction vessel is cooled to below 0°C before NO2is added.

13. The process of any one of claims 10-12, further comprising adding O2to regenerate the NO2.

14. The process of any one of claims 10-13, comprising: (a) adding further [11C]CH4 or [13C]CH4 to the reaction vessel after adding NO2 to the reaction vessel; (b) allowing the [11C]CH4 or [13C]CH4 to react with the iodine(III) compound in the reaction mixture formed in step (a); (c) adding further NO2 to the reaction vessel during or after step (b); (d) repeating steps (a) to (c).

15. The process of claim 14, wherein O2 and NO2 are added to the reaction vessel in step (c).

16. The process of claim 14 or claim 15, wherein step (a) comprises degassing the reaction vessel before the further [11C]CH4 or [13C]CH4 is added.

17. The process of any one of claims 9-16, wherein the acid is a carboxylic acid or a sulfonic acid, wherein the carboxylic acid or sulfonic acid has a pKa of less than 3 on the aqueous scale, or wherein the acid is HNO3.

18. The process of claim 17, wherein the carboxylic acid or sulfonic acid contains a halogenated alkyl or aryl group, preferably wherein the halogen atoms are fluorine or chlorine atoms.

19. The process of any one of claims 9-18, wherein the acid has a pKa on the aqueous scale of less than 1, preferably less than 0, more preferably less than -1.

20. The process of claim 17, wherein the acid is a perfluorinated C1-8sulfonic acid or a perfluorinated C1-8carboxylic acid.

21. The process of claim 17, wherein the acid is CF3SO3H, CH3SO3H, CF3CF2SO3H, CF3CF2CF2SO3H, CF3CO2H, CF3CF2CO2H, CF3CF2CF2CO2H, CCl3CO2H, HN(SO2CF3)2, HN(SO2CF2CF3)2, FSO3H or HNO3.

22. The process of claim 17, wherein the acid is CF3SO3H, CH3SO3H, CF3CF2SO3H, HN(SO2CF3)2or FSO3H.

23. The process of claim 17, wherein the iodine(III) compound is I(OTf)3and the acid is CF3SO3H, thereby producing [11C]CH3OSO2CF3or [13C]CH3OSO2CF3.

24. The process of claim 17, wherein the iodine(III) compound is I[N(SO2CF3)2]3and the acid is HN(SO2CF3)2,thereby producing [11C]CH3N(SO2CF3)2or [13C]CH3N(SO2CF3)2.

25. The process of any one of claims 9-16, wherein the solution of the iodine(III) compound in acid is formed by mixing I2with CF3SO3H and adding NO2to the reaction vessel.

26. The process of any one of claims 9-23 or 25, further comprising performing distillation to isolate [11C]CH3OSO2CF3or [13C]CH3OSO2CF3from the reaction mixture.

27. The process of any one of claims 1-3 and 9-26, wherein the [11C]CH4 or [13C]CH4 is allowed to react with the iodine(III) compound at a temperature in the range from about 10 °C and about 200 °C, preferably in the range from about 10 °C and about 150 °C, more preferably in the range from about 50 °C and about 125 °C.

28. The process of any one of claims 9-27, wherein the [11C]CH4 or [13C]CH4 is added to the reaction vessel at a pressure of about 10 bar or less, preferably about 5 bar or less, more preferably in the range from about 0.5 bar to about 2 bar.

29. A process for the preparation of a radiotracer compound containing an [11C]CH3 group comprising performing the process of any preceding claim and reacting [11C]CH3OSO2CF3 or [11C]CH3N(SO2CF3)2 produced by said process with a target compound, thereby forming the radiotracer compound.

30. The process of claim 29, comprising reacting [11C]CH3OSO2CF3 produced by the process of any one of claims 1-28 with a target compound, thereby forming the radiotracer compound.

31. The process of claim 29, wherein the radiotracer compound is [11C]raclopride, [11C]choline, [11C]methionine, [11C]HED (hydroxyephedrine), [11C]DTBZ (dihydrotetrabenazine), [11C]DASB (3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile), [11C]CFN (carfentanil), [11C]FMZ (flumazenil) or [11C]PiB (Pittsburgh compound B).

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