N-hetero-carbene Pd complex, preparation method thereof and application of N-hetero-carbene Pd complex in olefin hydroesterification reaction

Abstract

The invention belongs to the technical field of olefin hydroesterification reaction and catalysts thereof, and particularly relates to an N-hetero-carbene Pd complex, a preparation method of the N-hetero-carbene Pd complex and application of the N-hetero-carbene Pd complex in olefin hydroesterification reaction. The preparation method of the N-hetero-carbene Pd complex comprises the following steps: mixing an imidazole cation precursor and an anion precursor, carrying out heating reaction, carrying out suction filtration, washing, and drying to obtain amphoteric onium salt; the preparation method comprises the following steps: under a protective atmosphere, dissolving amphoteric onium salt and a Pd compound in a solvent, carrying out a stirring reaction on the obtained solution under the action of a basic catalyst, washing, drying, and recrystallizing to obtain the N-hetero-carbene Pd complex. The N-hetero-carbene Pd complex provided by the invention is used for preparing organic carboxylic ester, the product is easy to separate, the N-hetero-carbene Pd complex is high in repeated utilization rate as a catalyst, the reaction conditions are mild, the raw material application range is wide, and the product selectivity is high.

Description

Technical Field

[0001] The invention belongs to the technical field of olefin hydroesterification reaction and catalysts thereof, and specifically relates to an N-heterocarbene Pd complex and a preparation method thereof, and application thereof in olefin hydroesterification reaction. Background Art

[0002] At present, the hydroesterification reaction of olefins and alkynes is an important reaction in the chemical industry. In this reaction, unsaturated hydrocarbons react with CO and oxygen nucleophiles (water or alcohol) to generate corresponding carboxylic acid derivatives. The reaction conditions are mild and the atomic utilization efficiency is high, so it is considered to be a green and sustainable chemical process. Complexes of transition metals such as Ru, Pd, Pt or Co can catalyze the hydroesterification reaction. Among these metal complexes, palladium / phosphine complexes are the most widely used because they have high activity and selectivity under relatively mild conditions. However, this catalytic system also has some problems. For example, homogeneous palladium catalysts have high separation energy consumption, the use of phosphine ligands will inevitably generate Pd black precipitates, and the use of bidentate phosphine ligands is expensive. Therefore, it is of great application prospect to develop a high-efficiency and low-cost heterogeneous catalytic system to realize the recovery of precious metal Pd and high-efficiency catalytic hydroesterification reaction.

[0003] As a new type of effective ligand, N-heterocyclic carbene ligands are increasingly valued in the chemical industry. Like tertiary phosphine ligands, N-heterocyclic carbene ligands are excellent electron donors. They can replace phosphine ligands to a certain extent and are often called "phosphine-like ligands", which expands the research field of ligands. N-heterocyclic carbene ligands have many advantages, such as low cost, simple preparation, low toxicity, and the complex formed with Pd is stable to water, heat and air. It is a very promising carbonylation catalyst material. However, there are no reports on the use of N-carbene Pd complexes in the hydroesterification reaction of olefins. This is mainly because carbene Pd is unstable during the reaction and easily agglomerates to produce Pd black particles, which quickly deactivates the catalyst.

[0004] In the paper published on June 1, 2009, "Synthesis of novel chiral N-heterocyclic carbene ligands and application of Pd complexes in Suzuki coupling reactions", 2.2.2.2 Ligand transfer method, NHC can also be transferred from one transition metal to another in the form of an intramolecular transition metal under the condition of adding silver oxide to generate NHC-Pd complexes. However, in the carbene palladium compound prepared by this method, Pd is coordinated with one imidazole group and has poor stability. If the Ag ion is coordinated with two imidazole groups first, it is difficult to carry out metal exchange to prepare the carbene Pd compound.

[0005] CN201510960377.6 discloses a nitrogen heterocyclic carbene-palladium complex and a preparation method thereof. The prepared nitrogen heterocyclic carbene-palladium complex is Wherein L1 is a quinoline ligand, L2 is an azocyclic carbene ligand, and X1 and X2 are each independently anionic ligands (fluoride ion, chloride ion, bromide ion, iodide ion, acetate ion, trifluoroacetate ion, nitrate ion or tetrafluoroborate ion). The prepared azocyclic carbene-palladium complex is mainly used in catalyzing carbon-carbon or carbon-heteroatom coupling reactions with aryl halides as substrates. In this structure, Pd is coordinated with an allyl C and an N, and the stability of Pd is much less than the C coordination of the two allyl structures used in the present invention. The catalyst is used for olefin hydroesterification reaction, and Pd black is easily generated, which affects the stability of the catalyst. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an N-heterocarbene Pd complex, which is used to prepare organic carboxylic acid esters, and the product is easy to separate. The N-heterocarbene Pd complex has a high reusability as a catalyst, a wide range of reaction conditions and raw materials, and a high product selectivity.

[0007] The method for preparing the N-heterocarbene Pd complex of the present invention comprises the following steps:

[0008] (1) an imidazolium cation precursor and an anion precursor are mixed, heated to react, filtered, washed, and dried to obtain an amphoteric onium salt;

[0009] (2) Under a protective atmosphere, a zwitterionic onium salt and a Pd compound are dissolved in a solvent, the obtained solution is stirred and reacted under the action of an alkaline catalyst, and after washing, drying and recrystallization, an N-heterocarbene Pd complex is obtained.

[0010] The molar ratio of the imidazolium cation precursor to the anion precursor in step (1) is (1-10): (0.1-10), and the temperature is raised to 30-90° C. and the reaction is carried out for 2-12 hours.

[0011] The washing solvent used in step (1) is one of toluene, ether, n-hexane, nonane, methanol, deionized water, ethyl acetate and petroleum ether, and the drying temperature is 60-120°C.

[0012] The imidazole cation precursor of step (1) is one of N-methylimidazole, benzimidazole, 2-phenylimidazole, and 1-ethylimidazole, and the anion precursor is one of ethyl bromide, butyl bromide, propyl chloride, 1,4-butanesulfonate, and NaBF6.

[0013] The Pd compound in step (2) is one of palladium acetate, palladium chloride, palladium acetylacetonate, and allyl palladium chloride; the solvent is one of methanol, toluene, ethanol, deionized water, ethyl acetate, petroleum ether, pyridine, and N-methylimidazole; and the alkaline catalyst is one of potassium carbonate, sodium carbonate, sodium hydroxide, ammonia water, sodium bicarbonate, and calcium hydroxide.

[0014] The molar ratio of the amphoteric onium salt to the Pd compound is (0.1-1):(1-10), and the molar ratio of the alkaline catalyst to Pd is (1-10):(0.01-10). The reaction temperature of step (2) is -10-120°C, and the reaction time is 1-48h.

[0015] The protective atmosphere in step (2) is preferably N2.

[0016] The solvent used for washing in step (2) is one or more of methanol, ethanol, deionized water, saturated saline, ethyl acetate, and petroleum ether; the reagent used for drying is one of anhydrous calcium chloride, anhydrous sodium carbonate, anhydrous sodium sulfate, and molecular sieves; the reagent used for recrystallization is one of ethyl acetate, methyl propionate, petroleum ether, ether, and ethylene dichloride.

[0017] An N-heterocarbene Pd complex is prepared by the preparation method of the N-heterocarbene Pd complex.

[0018] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps: mixing alcohol, N-heterocarbene Pd complex and acid auxiliary agent, sealing, filling terminal olefin, pressurizing, heating and reacting. The reaction is carried out in a kettle reactor.

[0019] The molar ratio of H in the acid auxiliary agent to Pd in ​​the N-heterocarbene Pd complex is (500-1): (10-1), and the amount of terminal olefin added in a single kettle is 0.05mol-10mol. The pressure is increased to 1-10Mpa, preferably 1-5Mpa, the temperature is increased to 50-180°C, preferably 80-120°C, and the reaction is carried out for 2-10h.

[0020] The alcohol is one of methanol, ethanol, ethylene glycol and glycerol, and is also used as the solvent of the reaction system.

[0021] The acid auxiliary agent is one of acetic acid, hydrochloric acid, methanesulfonic acid, p-toluenesulfonic acid and sulfuric acid.

[0022] The terminal olefin is one of ethylene, propylene, butylene, butadiene and styrene, and the mass purity of the terminal olefin is 90%-100%. The pressurized gas is CO or a mixture of CO and N2, and the mass purity of the gas is 90%-100%.

[0023] Specifically, the use of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0024] (1) The imidazolium cation precursor and the anion precursor are mixed in a mass ratio of (1-10): (0.1-10), heated to 30-90° C. for reaction for 2-12 hours, filtered, washed, and dried at 60-120° C. to obtain a zwitterionic onium salt.

[0025] (2) Under N2 atmosphere, amphoteric onium salt and Pd compound are dissolved in a solvent, wherein the molar ratio of amphoteric onium salt to Pd compound is (0.1-1):(1-10), and the obtained solution is reacted at -10-120°C with stirring for 1-48h under the action of alkaline catalyst, wherein the molar ratio of alkaline catalyst to Pd is (1-10):(0.01-10), and after washing, drying and recrystallization, an N-heterocarbene Pd complex is obtained.

[0026] The reaction paths of the above two steps are as follows:

[0027]

[0028] (3) In a tank reactor, add alcohol with a purity of 90% to 100%, N-heterocarbene Pd complex, and acid auxiliary agent into the reactor, seal it, fill it with terminal olefin, and then pressurize the reactor to 1-10 MPa with CO or a mixture of CO and N2 with a purity of 90% to 100%, heat it to 50-180° C., and react for 2-10 hours. The ratio of the molar number of H in the acid auxiliary agent to the molar number of Pd in ​​the N-heterocarbene Pd complex is (500-1):(10-1).

[0029] The N-heterocarbene Pd complex and the acid promoter catalyst system used in the hydroesterification reaction make the reaction conditions mild, will not cause corrosion to the equipment, have low reaction cost, and the N-heterocarbene Pd complex used in the reaction has high recycling rate as a catalyst. The present invention prepares the N-carbene Pd complex catalyst by synthesizing imidazolium ionic liquid, N-carbene Pd complex and the like, and uses it for the hydroesterification reaction of olefins. Two or more imidazolium ionic liquids are coordinated with metal Pd, which can not only inhibit the production of Pd black, solve the problem of poor catalyst stability and high price, but also solve the problem of difficulty in recycling homogeneous hydroesterification catalysts. In addition, the catalyst system can also obtain higher reaction efficiency for the hydroesterification reaction of olefins.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The preparation method of the N-heterocarbene Pd complex of the present invention fixes Pd on an imidazolium ionic liquid to form an N-heterocarbene structure, which is beneficial to preventing Pd(II) from being reduced to Pd(0), thereby preventing Pd(0) from aggregating to form palladium black, and is beneficial to improving the stability of the N-heterocarbene Pd complex as a catalyst.

[0032] (2) The method for preparing the N-heterocarbene Pd complex of the present invention fixes Pd on an imidazolium ionic liquid to form an N-heterocarbene structure, and no phosphine ligand is added to the reaction system, which is beneficial to reducing the cost of the catalyst.

[0033] (3) The N-heterocarbene Pd complex prepared by the method of the present invention fixes Pd on the imidazolium ionic liquid to form an N-heterocarbene Pd structure. The catalyst system of the N-heterocarbene Pd complex is solid at room temperature, which is beneficial to the separation between the catalyst and the product, reduces the separation energy consumption, and realizes the recovery and reuse of the catalyst.

[0034] (4) The N-heterocarbene Pd complex prepared by the present invention is used in the hydroesterification reaction of olefins. The imidazolium ionic liquid has functional groups such as allyl, N, and α-H, and the structure and properties are flexible and adjustable, which is beneficial to improving the reaction activity and stability of the catalyst. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with specific embodiments.

[0036] In the following examples and comparative examples, the mass purity of the alcohol is 90%-100%, the mass purity of CO or the mixed gas of CO and N2 is 90%-100%, and the other chemical reagents used are all commercially available products, and the equipment used is all existing equipment. H:Pd refers to the ratio of the molar number of H in the acid auxiliary agent to the number of Pd in ​​the N-heterocarbene Pd complex.

[0037] Example 1

[0038] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0039] (1) N-methylimidazole and 1,4-butanesultone were mixed in a molar ratio of 1:1, heated to 60° C. for reaction for 6 h, filtered, washed with toluene three times, and dried in vacuo at 100° C. to obtain an amphoteric onium salt.

[0040] (2) Under N2 atmosphere, amphoteric onium salt and palladium acetate were dissolved in pyridine at a molar ratio of 1:1. The resulting solution was catalyzed by K2CO3, with a molar ratio of K2CO3 to Pd of 2:1. The solution was stirred at 90°C for 24 h, washed with deionized water, saturated sodium chloride solution, and ethyl acetate, and dried over anhydrous sodium sulfate. The obtained viscous substance was recrystallized from petroleum ether to obtain an N-heterocarbene Pd complex.

[0041] (3) In a 250 ml autoclave, 100 ml of methanol, 5 g of the N-heterocarbene Pd complex prepared above, and methanesulfonic acid (H:Pd=100:1) were added to the autoclave, and after sealing, ethylene (0.08 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 4 MPa. The temperature was slowly raised to 110° C. by a temperature controller, and the reaction was carried out for 4 hours. The autoclave was cooled to room temperature, and the autoclave was unloaded to obtain methyl propionate product. The liquid obtained by the reaction was quantitatively analyzed by Agilent 6890, and the ethylene conversion rate was 98%, and the selectivity of the product methyl propionate was 99%.

[0042] Example 2

[0043] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0044] (1) Benzimidazole and ethyl bromide were mixed in a molar ratio of 2:1, heated to 80°C for reaction for 6 hours, filtered, washed with ether three times, and dried in vacuo at 80°C to obtain a zwitterionic onium salt.

[0045] (2) Under N2 atmosphere, amphoteric onium salt and palladium chloride are dissolved in N-methylimidazole at a molar ratio of 1:2. The obtained solution is stirred at 120°C for 1 hour under the catalytic action of NaOH, with a molar ratio of NaOH to Pd of 1:1. The solution is washed with deionized water and dried with anhydrous sodium sulfate. The obtained viscous substance is recrystallized from petroleum ether to obtain an N-heterocarbene Pd complex.

[0046] (3) In a 250 ml autoclave, 100 ml of methanol, 1 g of the N-heterocarbene Pd complex prepared above, and acetic acid (H:Pd=500:1) were added to the autoclave, and after sealing, styrene (0.08 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 2 MPa. The temperature was slowly raised to 100° C. by a temperature controller, and the reaction was carried out for 3 hours. The autoclave was cooled to room temperature, and the autoclave was unloaded to obtain methyl phenylpropionate. The obtained liquid was quantitatively analyzed by Agilent 6890, and the styrene conversion rate was 93%, and the selectivity of the product methyl phenylpropionate was 92%.

[0047] Example 3

[0048] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0049] (1) 2-Phenylimidazole and NaBF6 were mixed in a molar ratio of 1:10, heated to 50°C for reaction for 12 h, filtered, washed with ether three times, and dried in vacuo at 60°C to obtain a zwitterion salt.

[0050] (2) Under N2 atmosphere, amphoteric onium salt and acetylacetonate palladium are dissolved in toluene at a molar ratio of 0.1:10. The obtained solution is catalyzed by calcium hydroxide, and the molar ratio of calcium hydroxide to Pd is 5:0.01. The solution is stirred at -10°C for 48 hours. After washing with deionized water, saturated sodium chloride solution, and ethyl acetate, it is dried over anhydrous sodium sulfate. The obtained viscous substance is recrystallized in ethyl acetate to obtain an N-heterocarbene Pd complex.

[0051] (3) In a 250 ml autoclave, 100 ml of methanol, 10 g of the N-heterocarbene Pd complex prepared above, and acetic acid (H:Pd=50:1) were added to the autoclave, and after sealing, ethylene (0.2 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 2 MPa. The temperature was slowly raised to 160° C. by a temperature controller, and the reaction was carried out for 6 hours. The autoclave was cooled to room temperature, and the autoclave was unloaded to obtain methyl propionate. The obtained liquid was quantitatively analyzed by Agilent 6890, and the ethylene conversion rate was 99%, and the selectivity of the product methyl propionate was 96%.

[0052] Example 4

[0053] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0054] (1) 1-Ethylimidazole and chloropropane were mixed in a molar ratio of 10:0.1, heated to 90°C for reaction for 2 hours, filtered, washed with ether three times, and dried in vacuo at 60°C to obtain a zwitterion salt.

[0055] (2) Under N2 atmosphere, amphoteric onium salt and allylpalladium chloride were dissolved in ethanol at a molar ratio of 0.1:1. The obtained solution was catalyzed by ammonia water, and the molar ratio of NH3 to Pd was 10:0.01. The solution was stirred at 50°C for 20 hours. After washing with deionized water and ethyl acetate, it was dried over anhydrous sodium sulfate. The obtained viscous substance was recrystallized in ethyl acetate to obtain N-heterocarbene Pd complex.

[0056] (3) In a 250 ml autoclave, 100 ml of ethanol, 1 g of the N-heterocarbene Pd complex prepared above, and acetic acid (H:Pd=100:1) were added to the autoclave, and after sealing, ethylene (0.1 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 2 MPa. The temperature was slowly raised to 100° C. by a temperature controller, and the reaction was carried out for 4 hours. After cooling to room temperature, the autoclave was unloaded to obtain ethyl propionate. The obtained liquid was quantitatively analyzed by Agilent 6890, and the ethylene conversion rate was 93%, and the selectivity of the product ethyl propionate was 93%.

[0057] Example 5

[0058] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0059] (1) Benzimidazole and butyl bromide were mixed in a molar ratio of 1:0.1, heated to 40° C. for reaction for 10 h, filtered, washed with toluene three times, and dried in vacuo at 100° C. to obtain an amphoteric onium salt.

[0060] (2) Under N2 atmosphere, amphoteric onium salt and palladium acetate were dissolved in ethyl acetate at a molar ratio of 1:10. The obtained solution was catalyzed by sodium bicarbonate, and the molar ratio of sodium bicarbonate to Pd was 10:0.01. The solution was stirred at 0°C for 24 hours, washed with deionized water, saturated sodium chloride solution, and ethyl acetate, and dried with anhydrous sodium sulfate. The obtained viscous substance was recrystallized from petroleum ether to obtain N-heterocarbene Pd complex.

[0061] (3) In a 250 ml autoclave, 100 ml of methanol, 8 g of the N-heterocarbene Pd complex prepared above, and methanesulfonic acid (H:Pd=100:1) were added to the autoclave, and after sealing, ethylene (0.08 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 4 MPa. The temperature was slowly raised to 80° C. by a temperature controller, and the reaction was carried out for 2 h. The autoclave was cooled to room temperature, and the autoclave was unloaded to obtain methyl propionate product. The liquid obtained by the reaction was quantitatively analyzed by Agilent 6890, and the ethylene conversion rate was 97%, and the selectivity of the product methyl propionate was 96%.

[0062] Example 6

[0063] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0064] (1) N-methylimidazole and 1,4-butanesultone were mixed in a molar ratio of 1:5, heated to 60° C. for reaction for 8 h, filtered, washed with petroleum ether for 3 times, and dried in vacuo at 100° C. to obtain a zwitterion salt.

[0065] (2) Under N2 atmosphere, amphoteric onium salt and palladium acetate were dissolved in petroleum ether at a molar ratio of 1:5. The obtained solution was catalyzed by sodium carbonate, and the molar ratio of sodium carbonate to Pd was 1:10. The solution was stirred at 60°C for 36 hours, washed with deionized water, saturated sodium chloride solution, and ethyl acetate, and dried with anhydrous sodium sulfate. The obtained viscous substance was recrystallized in petroleum ether to obtain N-heterocarbene Pd complex.

[0066] (3) In a 250 ml autoclave, 100 ml of methanol, 5 g of the N-heterocarbene Pd complex prepared above, and methanesulfonic acid (H:Pd=100:1) were added to the autoclave, and after sealing, styrene (0.1 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 4 MPa. The temperature was slowly raised to 110° C. by a temperature controller, and the reaction was carried out for 4 hours. The reaction was cooled to room temperature, and the autoclave was unloaded to obtain methyl phenylpropionate product. The liquid obtained by the reaction was quantitatively analyzed by Agilent 6890, and the styrene conversion rate was 95%, and the selectivity of the product methyl phenylpropionate was 98%.

[0067] Example 7

[0068] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0069] (1) Benzimidazole and butyl bromide were mixed in a molar ratio of 1:1, heated to 30°C for reaction for 11 hours, filtered, washed with deionized water for 3 times, and dried in vacuo at 100°C to obtain a zwitterionic onium salt.

[0070] (2) Under N2 atmosphere, amphoteric onium salt and palladium chloride were dissolved in deionized water at a molar ratio of 1:2. The resulting solution was catalyzed by K2CO3, with a molar ratio of K2CO3 to Pd of 1:2. The solution was stirred at -10°C for 6 h, washed with deionized water, saturated sodium chloride solution, and ethyl acetate, and dried over anhydrous sodium sulfate. The obtained viscous substance was recrystallized from petroleum ether to obtain an N-heterocarbene Pd complex.

[0071] (3) In a 250 ml autoclave, 100 ml of methanol, 5 g of the N-heterocarbene Pd complex prepared above, and methanesulfonic acid (H:Pd=100:1) were added to the autoclave, and after sealing, ethylene (0.05 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 1 MPa. The temperature was slowly raised to 50° C. by a temperature controller, and the reaction was carried out for 4 hours. The reaction was cooled to room temperature, and the autoclave was unloaded to obtain methyl propionate product. The liquid obtained by the reaction was quantitatively analyzed by Agilent 6890, and the ethylene conversion rate was 99%, and the selectivity of the product methyl propionate was 98%.

[0072] Example 8

[0073] The application of the N-heterocarbene Pd complex in the olefin hydroesterification reaction comprises the following steps:

[0074] (1) 1-Ethylimidazole and 1,4-butanesultone were mixed in a molar ratio of 5:1, heated to 60°C for reaction for 5 hours, filtered, washed with methanol three times, and dried in vacuo at 100°C to obtain an amphoteric onium salt.

[0075] (2) Under N2 atmosphere, amphoteric onium salt and acetylacetonate palladium were dissolved in methanol at a molar ratio of 0.5:1. The resulting solution was catalyzed by K2CO3, with a molar ratio of K2CO3 to Pd of 5:1. The solution was stirred at 90°C for 12 h, washed with deionized water, saturated sodium chloride solution, and ethyl acetate, and dried over anhydrous sodium sulfate. The obtained viscous substance was recrystallized from petroleum ether to obtain an N-heterocarbene Pd complex.

[0076] (3) In a 250 ml autoclave, 100 ml of methanol, 5 g of the N-heterocarbene Pd complex prepared above, and methanesulfonic acid (H:Pd=100:1) were added to the autoclave, and after sealing, styrene (10 mol) was charged, and then the autoclave was replaced with CO for 3 times, and then CO was charged until the pressure of the autoclave was 4 MPa. The temperature was slowly raised to 180° C. by a temperature controller, and the reaction was carried out for 10 hours. The reaction was cooled to room temperature, and the autoclave was unloaded to obtain methyl phenylpropionate product. The liquid obtained by the reaction was quantitatively analyzed by Agilent 6890, and the styrene conversion rate was 99%, and the selectivity of the product methyl phenylpropionate was 96%.

[0077] Example 9

[0078] The reaction conditions were consistent with those in the example, except that the experiment was repeated 5 times. The reaction activity was maintained at 97%-99%, and the selectivity was stable at 99%, indicating that the catalyst had excellent stability.

[0079] Comparative Example 1

[0080] In a 250ml autoclave, PdCl2 (0.05mmol), methanesulfonic acid (0.5mmol), and methanol (100ml) were added in sequence, the autoclave was sealed, the autoclave was replaced with CO for 3 times, ethylene (0.6mol) was flushed in, and then CO gas was filled in until the pressure of the autoclave was 4MPa. The temperature was slowly raised to 80°C by a temperature controller, the reaction was continued for 10h, cooled to room temperature, the autoclave was unloaded, and the liquid obtained by the reaction was quantitatively analyzed by Agilent 6890. The product methyl propionate was not detected.

[0081] Comparative Example 2

[0082] In a 250ml high-pressure reactor, Pd(OAc)2 (0.05mmol), methanesulfonic acid (0.5mmol), 1,2-bis[(di-tert-butylphosphino)methyl]benzene (0.4mmol), and methanol (100ml) were added in sequence. The reactor was sealed, replaced with CO for 3 times, ethylene (0.2mol) was flushed in, and then CO gas was filled in until the pressure of the reactor reached 3MPa. The temperature was slowly raised to 80℃ by a temperature controller, and the reaction was continued for 6h. The reaction was cooled to room temperature, the reactor was unloaded, and the liquid obtained by the reaction was quantitatively analyzed by Agilent 6890. The ethylene conversion rate was 92%, and the selectivity of the product methyl propionate was 90%.

[0083] Comparative Example 3

[0084] This comparative example 3 is the same as example 1, except that the ratio of anions to cations in step 1 is changed to 200:1, and an N-heterocarbene Pd compound is prepared for hydroesterification reaction, with an ethylene conversion rate of 10% and a selectivity of 50%.

[0085] Comparative Example 4

[0086] This comparative example 3 is the same as example 2, except that the ratio of the amphoteric onium salt to the Pd compound in step 2 is changed to 2:1, and an N-heterocarbene Pd compound is obtained for the hydroesterification reaction, with an ethylene conversion rate of 20% and a selectivity of 80%.

[0087] Comparative Example 5

[0088] This comparative example 3 is the same as example 1, except that the reaction temperature in step 2 is 120° C., and the obtained N-heterocarbene Pd compound is used for the hydroesterification reaction, with an ethylene conversion rate of 15% and a selectivity of 70%.

[0089] Of course, the above contents are only preferred embodiments of the present invention and cannot be considered to limit the scope of the embodiments of the present invention. The present invention is not limited to the above examples, and equal changes and improvements made by ordinary technicians in the technical field within the essential scope of the present invention should all fall within the scope of the patent coverage of the present invention.

Claims

1. A method for preparing an N-heterocarbene Pd complex, characterized in that: The following steps are involved: (1) an imidazolium cation precursor and an anion precursor are mixed, heated to react, filtered, washed, and dried to obtain an amphoteric onium salt; (2) Under a protective atmosphere, a zwitterionic onium salt and a Pd compound are dissolved in a solvent, the obtained solution is stirred and reacted under the action of an alkaline catalyst, and after washing, drying and recrystallization, an N-heterocarbene Pd complex is obtained.

2. The method for preparing the N-heterocarbene Pd complex according to claim 1, wherein: The molar ratio of the imidazolium cation precursor to the anion precursor in step (1) is (1-10): (0.1-10), and the temperature is raised to 30-90° C. and the reaction is carried out for 2-12 hours.

3. The method for preparing the N-heterocarbene Pd complex according to claim 2, characterized in that: The imidazole cation precursor of step (1) is one of N-methylimidazole, benzimidazole, 2-phenylimidazole, and 1-ethylimidazole, and the anion precursor is one of ethyl bromide, butyl bromide, propyl chloride, 1,4-butanesulfonate, and NaBF6.

4. The method for preparing the N-heterocarbene Pd complex according to claim 1, characterized in that: The Pd compound in step (2) is one of palladium acetate, palladium chloride, palladium acetylacetonate, and allyl palladium chloride; the solvent is one of methanol, toluene, ethanol, deionized water, ethyl acetate, petroleum ether, pyridine, and N-methylimidazole; and the alkaline catalyst is one of potassium carbonate, sodium carbonate, sodium hydroxide, ammonia water, sodium bicarbonate, and calcium hydroxide.

5. The method for preparing the N-heterocarbene Pd complex according to claim 1, characterized in that: The molar ratio of the amphoteric onium salt to the Pd compound is (0.1-1):(1-10), and the molar ratio of the basic catalyst to Pd is (1-10):(0.01-10).

6. The method for preparing the N-heterocarbene Pd complex according to claim 1, characterized in that: The reaction temperature of step (2) is -10-120°C, and the reaction time is 1-48h.

7. An N-heterocarbene Pd complex, characterized in that: The invention is prepared by the preparation method of the N-heterocarbene Pd complex according to any one of claims 1 to 6.

8. Use of the N-heterocarbene Pd complex according to claim 7 in olefin hydroesterification reaction, characterized in that: The following steps are involved: The alcohol, N-heterocarbene Pd complex and acid auxiliary agent are mixed, the terminal olefin is charged, the pressure is increased, the temperature is increased, and the reaction is carried out.

9. Use of the N-heterocarbene Pd complex according to claim 8 in olefin hydroesterification reaction, characterized in that: The molar ratio of H in the acid auxiliary agent to Pd in ​​the N-heterocarbene Pd complex is (500-1):(10-1), and the single-pot addition amount of the terminal olefin is 0.05 mol-10 mol.

10. Use of the N-heterocarbene Pd complex according to claim 8 in olefin hydroesterification reaction, characterized in that: Increase the pressure to 1-10Mpa, raise the temperature to 50-180℃, and react for 2-10h.

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