Synthesis method of aryl bidentate phosphine compound

Abstract

The invention discloses a synthesis method of an aryl bidentate phosphine compound, which comprises the following steps of: (1) reacting benzene or alkylbenzene with benzyl chloride or 4-alkyl benzyl chloride at 0-100 DEG C for 0.5-5 hours in the absence of oxygen and under the condition of a lewis acid catalyst to prepare a first intermediate; (2) dropwise adding an alkyl lithium reagent into tetramethylethylenediamine at the temperature of-30 to 40 DEG C under the conditions of no water, no oxygen and a solvent, stirring for 1 to 3 hours, dropwise adding the first intermediate into the mixed solution at normal temperature, and reacting for 4 to 20 hours at the temperature of-20 to 40 DEG C to prepare a second intermediate; and (3) under anhydrous and anaerobic conditions, adding diphenyl phosphine chloride into the second intermediate obtained in the step (2), and reacting at-20 to 80 DEG C for 5-60 hours to obtain the aryl bidentate phosphine compound. The synthesis method disclosed by the invention is simple in process, mild in condition and high in product yield, and has a relatively good industrial prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation and relates to a method for synthesizing an aromatic bidentate phosphine compound. Background Art

[0002] Olefin hydroesterification is a process in which olefins react with carbon monoxide and alcohols under transition metal catalysis to generate carboxylic acid esters with high added value. It has 100% atom economy and is one of the most important homogeneous catalytic reactions at present. For example, methyl propionate is often prepared by ethylene hydroesterification in industry. Methyl propionate and other propionate ester products are important organic chemical raw materials and are widely used in the fields of fragrances, lithium battery manufacturing, organic solvents, organic synthesis, etc. Many transition metals such as cobalt, ruthenium, rhodium and palladium can catalyze the ethylene hydroesterification reaction. The palladium catalytic system has been widely studied due to its simple reaction conditions and high catalytic activity. Common palladium catalysts are composed of palladium metal precursors, organic phosphine ligands and acid additives. Among them, Bronsted acid is an additive, and the palladium complex modified by the organic phosphine ligand is an efficient catalyst for the ethylene hydroesterification reaction. The electronic properties and spatial properties of the organic phosphine ligand synergistically affect the various steps of the catalytic reaction, and play a key role in the efficient catalysis of the ethylene hydroesterification reaction by transition metal palladium.

[0003] At present, the organic phosphine ligands used for the synthesis of methyl propionate by vinyl hydroesterification are mainly phosphine ligands such as alkyl phosphine, cycloalkyl phosphine, bidentate phosphine and multidentate phosphine. In particular, Lucite has disclosed a bidentate phosphine compound 1,2-bis(di-tert-butylphosphinomethyl)benzene based on a substituted aryl bridge, which can improve the conversion rate and reaction rate (Chem. Commun., 1999, 1877-1878; WO2004014552A1). Evonik Degussa has disclosed α,α'-bis(2-pyridyl(tert-butyl)phosphino)-o-xylene based on a benzene skeleton (US6335471B1; CN106366126A), which shows a better yield of esters and less by-products. In addition, the Lanzhou Institute of Chemical Physics discloses a method for preparing organic carboxylic acid esters based on an aromatic bidentate phosphine ligand combined catalyst, which has both the rigid skeleton structure of a rigid ligand and the flexibility of a flexible ligand, which is conducive to forming the most favorable coordination mode in space and a relatively stable active structure, and has good catalytic activity and selectivity in the reaction of synthesizing organic methyl propionate from vinyl hydroesterification (CN111087306A). Although the above-mentioned phosphine ligands show good performance in the vinyl hydroesterification reaction, the preparation method of the disclosed phosphine ligands has the disadvantages of complicated production process, high cost, harsh reaction conditions, difficult preparation, and long reaction time. Therefore, it is crucial to develop a synthesis method of a phosphine ligand for hydroesterification reaction that is cheap, simple in reaction process, and mild in reaction conditions. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problems of complex process, harsh reaction conditions and the need for easily produced raw materials in the existing preparation methods of phosphine ligands, and to provide a method for synthesizing an aromatic bidentate phosphine compound.

[0005] To achieve the above object, the present invention adopts the following technical solution: A method for synthesizing an aromatic bidentate phosphine compound, characterized in that the reaction route is:

[0006] Wherein, R1 and R2 are hydrogen or C1-C4 alkyl, preferably, R1 and R2 are hydrogen, methyl, ethyl, isopropyl or tert-butyl; The steps include: (1) In the absence of oxygen and with a Lewis acid catalyst, benzene or alkylbenzene (1) and benzyl chloride or 4-alkylbenzyl chloride (2) are reacted at 0 to 100° C. for 0.5 to 5 hours to obtain a first intermediate (3); (2) In the absence of water, oxygen and solvent, an alkyl lithium reagent is added dropwise into tetramethylethylenediamine at -30 to 40°C, stirred for 1 to 3 hours, and the first intermediate (3) is added dropwise into the mixed solution at room temperature, and reacted at -20 to 40°C for 4 to 20 hours to obtain a second intermediate (4); (3) Under anhydrous and oxygen-free conditions, diphenylphosphine chloride is added to the second intermediate (4) obtained in step (2), and the reaction is carried out at -20 to 80° C. for 5 to 60 hours to obtain an aromatic bidentate phosphine compound (5).

[0007] Furthermore, in step (1), the molar ratio of benzene or alkylbenzene (1) to benzyl chloride or 4-alkylbenzyl chloride (2) is 20-2:1, preferably 10-5:1; and the Lewis acid catalyst is selected from ZnCl 2、 One of AlCl3, FeCl3, SnCl4, preferably AlCl3 or FeCl3; the reaction temperature is preferably 20-90°C, more preferably 40-80°C; the reaction time is preferably 0.5-3 hours, more preferably 0.5-1.5h.

[0008] Furthermore, the first intermediate (3) obtained in step (1) is recrystallized using isopropanol.

[0009] Furthermore, the solvent in step (2) is selected from one or more of chloroform, dichloromethane, 1,2-dichloroethane, ether, petroleum ether, tetrahydrofuran, 2-methyltetrahydrofuran, cyclopentane, n-pentane, cyclohexane, n-hexane, and n-heptane, preferably ether, n-hexane or tetrahydrofuran; the alkyl lithium reagent is selected from one of methyl lithium, tert-butyl lithium, n-butyl lithium, sec-butyl lithium, n-hexyl lithium, and lithium diisopropylamide, preferably n-butyl lithium; the molar ratio of tetramethylethylenediamine to alkyl lithium is 1-8:1, preferably 1-2:1; the molar ratio of alkyl lithium to the first intermediate (3) is 2-10:1, preferably 2-2.5:1; the reaction temperature is preferably 0-30°C; and the reaction time is preferably 6-18 hours.

[0010] Furthermore, the reaction solution of the second intermediate (3) obtained in step (2) is directly subjected to step (3) without separation.

[0011] Furthermore, in step (3), the molar ratio of diphenylphosphine chloride to the second intermediate (4) is 2-5:1, preferably 2-3:1; the reaction temperature is preferably 10-40° C.; and the reaction time is preferably 8-36 hours.

[0012] Furthermore, the reaction product of step (3) is extracted with dichloromethane, the organic phase is dried, and the solvent is removed by rotary evaporation. The resulting mixture is purified by a silica gel column to obtain the aromatic bidentate phosphine compound (5).

[0013] Beneficial effects: Compared with the preparation method of bidentate phosphine ligands for hydroesterification reaction in the prior art, the preparation process and post-treatment method of the synthesis method of the aryl bidentate phosphine compound of the present invention are simple, the product yield is high, the production cost is low, the reaction conditions are mild, the raw materials are easy to obtain, and the use of highly toxic and corrosive precursor chemicals such as liquid bromine can be avoided. At the same time, it can also avoid the dehydrogenation of n-butyl lithium at a low temperature of -78°C. The reaction is safer, easy to scale up, and the post-treatment produces less waste, which has broad industrial application prospects. As an aryl bidentate phosphine ligand in a palladium catalytic system, it is used for the hydroesterification reaction of terminal olefins, carbon monoxide and alcohols, and can efficiently synthesize the corresponding organic carboxylic acid esters. DETAILED DESCRIPTION

[0014] The implementation process of the present invention and the beneficial effects produced are described in detail below through specific embodiments, which is intended to better understand the essence and characteristics of the present invention, but is not intended to limit the present invention.

[0015] Example 1 The synthesis method of the aromatic bidentate phosphine ligand 5a: bis(4-(methyl)-2-(diphenylphosphino)phenyl)methane is as follows:

[0016] (1) Preparation of the first intermediate 3 Toluene 1 (46.5 g, 504.6 mmol) and anhydrous aluminum chloride (0.75 g, 5.62 mmol) were added to a 500 mL round-bottom flask. After stirring for 10 minutes, the mixture was heated in a water bath and the temperature was controlled at 20-30 °C. p-Methylbenzyl chloride 2 (9.46 g, 67.3 mmol) was added dropwise from a constant pressure dropping funnel within 1 h. After the addition was completed, the temperature was raised to 80 °C and refluxed for 1 h. The mixture was cooled to room temperature, cold water was added, the organic phase was separated, and the mixture was washed with a saturated sodium carbonate solution and then with water. The organic phase was separated, dried, and recrystallized by adding isopropanol to obtain 10.96 g of the first intermediate 3 with a yield of 83%.

[0017] (2) Preparation of Second Intermediate 4 Under anhydrous and oxygen-free conditions, in a 250 mL Shrek bottle, n-butyllithium (45.6 mL, 1.6 M, n-hexane solution) was slowly dripped into an ether solution of tetramethylethylenediamine (8.48 g, 73 mmol) at 0°C and stirred for 2 hours. After returning to room temperature, a tetrahydrofuran solution of the first intermediate 3 (7.16 g, 36.5 mmol) was dripped into the above mixed solution, and the reaction was continued for 10 hours to obtain a reaction solution of the second intermediate 4.

[0018] (3) Preparation of aromatic bidentate phosphine ligand 5a: bis(4-(methyl)-2-(diphenylphosphino)phenyl)methane Under anhydrous and oxygen-free conditions, diphenylphosphine chloride (16.11 g, 73 mmol) was slowly added to the reaction solution of the second intermediate 4 obtained in the above step (2), and the mixture was stirred at 25°C for 16 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The resulting mixture was purified by silica gel column with the eluent being petroleum ether:ethyl acetate in a volume ratio of 30:1 to obtain 17.5 g of aromatic bidentate phosphine ligand 5a with a yield of 85%.

[0019] 1 H NMR (400 MHz, C6D6): δ = 2.36 (s, 6H), 4.72 (t, 2H), 7.03-7.09 (m, 12H), 7.08-7.14 (m, 4H), 7.25 (m, 2H), 7.40 (m, 8H).

[0020] Example 2 The synthesis method of the aromatic bidentate phosphine ligand 5b: bis(4-(ethyl)-2-(diphenylphosphino)phenyl)methane is as follows: (1) Preparation of the first intermediate 3 Ethylbenzene 1 (53.6 g, 504.6 mmol) and anhydrous aluminum chloride (0.75 g, 5.62 mmol) were added to a 500 mL round-bottom flask. After stirring for 10 minutes, the mixture was heated in a water bath and the temperature was controlled at 20-30 °C. 4-ethylbenzyl chloride 2 (10.4 g, 67.3 mmol) was added dropwise from a constant pressure dropping funnel within 1 h. After the addition was completed, the temperature was raised to 80 °C and refluxed for 1 h. The temperature was cooled to room temperature, cold water was added, the organic phase was separated, and the phase was washed with a saturated sodium carbonate solution and then with water. The organic phase was separated, dried, and isopropanol was added for recrystallization to obtain 11.77 g of the first intermediate 3 with a yield of 78%.

[0021] (2) Preparation of Second Intermediate 4 Under anhydrous and oxygen-free conditions, in a 250 mL Shrek bottle, n-butyllithium (45.6 mL, 1.6 M, n-hexane solution) was slowly dripped into an ether solution of tetramethylethylenediamine (8.48 g, 73 mmol) at 0°C and stirred for 2 hours. After returning to room temperature, a tetrahydrofuran solution of the first intermediate 3 (8.19 g, 36.5 mmol) was dripped into the above mixed solution, and the reaction was continued for 12 hours to obtain a reaction solution of the second intermediate 4.

[0022] (3) Preparation of aromatic bidentate phosphine ligand 5b Under anhydrous and oxygen-free conditions, diphenylphosphine chloride (16.11 g, 73 mmol) was slowly added to the reaction solution of the second intermediate 4 obtained in the above step (2), and the mixture was stirred at 25°C for 18 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The resulting mixture was purified by silica gel column with the eluent being petroleum ether:ethyl acetate in a volume ratio of 30:1 to obtain 17.3 g of aromatic bidentate phosphine ligand 5b with a yield of 80%.

[0023] 1 H NMR (400 MHz, C6D6): δ = 1.18 (s, 6H), 2.72 (m, 4H), 4.80 (t, 2H), 6.97-7.06 (m, 12H), 7.07-7.14 (m, 4H), 7.22 (m, 2H), 7.38 (m, 8H).

[0024] Example 3 The synthesis method of the aromatic bidentate phosphine ligand 5c: bis(4-(isopropyl)-2-(diphenylphosphino)phenyl)methane is as follows: (1) Preparation of the first intermediate 3 Isopropylbenzene 1 (60.7 g, 504.6 mmol) and anhydrous aluminum chloride (0.75 g, 5.62 mmol) were added to a 500 mL round-bottom flask. After stirring for 10 minutes, the mixture was heated in a water bath and the temperature was controlled at 20-30 °C. 4-isopropylbenzyl chloride 2 (11.3 g, 67.3 mmol) was added dropwise from a constant pressure dropping funnel within 1 h. After the addition was completed, the mixture was heated to 80 °C and refluxed for 1 h. The mixture was cooled to room temperature, cold water was added, the organic phase was separated, and the mixture was washed with a saturated sodium carbonate solution and then with water. The organic phase was separated, dried, and recrystallized by adding isopropyl alcohol to obtain 12.9 g of the first intermediate 3 with a yield of 76%.

[0025] (2) Preparation of Second Intermediate 4 Under anhydrous and oxygen-free conditions, in a 250 mL Shrek bottle, n-butyllithium (45.6 mL, 1.6 M, n-hexane solution) was slowly dropped into an ether solution of tetramethylethylenediamine (8.48 g, 73 mmol) at 0°C and stirred for 2 hours. After returning to room temperature, a tetrahydrofuran solution of the first intermediate 3 (9.21 g, 36.5 mmol) was dropped into the above mixed solution, and the reaction was continued for 13 hours to obtain a reaction solution of the second intermediate 4.

[0026] (3) Preparation of aromatic bidentate phosphine ligand 5c Under anhydrous and oxygen-free conditions, diphenylphosphine chloride (16.11 g, 73 mmol) was slowly added to the reaction solution of the second intermediate 4 obtained in the above step (2), and the mixture was stirred at 25°C for 22 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The resulting mixture was purified by silica gel column with the eluent being petroleum ether:ethyl acetate in a volume ratio of 30:1 to obtain 17.0 g of aromatic bidentate phosphine ligand 5c with a yield of 75%.

[0027] 1 H NMR (400 MHz, C6D6): δ = 1.20 (s, 12H), 2.82 (m, 2H), 4.78 (t, 2H), 7.01-7.09 (m, 12H), 7.08-7.11 (m, 4H), 7.25 (m, 2H), 7.40 (m, 8H).

[0028] Example 4 The synthesis method of the aromatic bidentate phosphine ligand 5d: bis(4-(tert-butyl)-2-(diphenylphosphino)phenyl)methane is as follows: (1) Preparation of the first intermediate 3 In a 500mL round-bottom flask, tert-butylbenzene 1 (67.7 g, 504.6 mmol) and anhydrous aluminum chloride (0.75 g, 5.62 mmol) were added. After stirring for 10 minutes, the mixture was heated in a water bath and the temperature was controlled at 20-30°C. 4-tert-butylbenzyl chloride 2 (12.3 g, 67.3 mmol) was added dropwise from a constant pressure dropping funnel within 1 hour. After the addition was completed, the mixture was heated to 80°C and refluxed for 1 hour. The mixture was cooled to room temperature, cold water was added, the organic phase was separated, and the mixture was washed with a saturated sodium carbonate solution and then with water. The organic phase was separated, dried, and recrystallized by adding isopropanol to obtain 13.78 g of the first intermediate 3 with a yield of 73%.

[0029] (2) Preparation of Second Intermediate 4 Under anhydrous and oxygen-free conditions, in a 250 mL Shrek bottle, n-butyllithium (45.6 mL, 1.6 M, n-hexane solution) was slowly dripped into an ether solution of tetramethylethylenediamine (8.48 g, 73 mmol) at 0°C and stirred for 2 hours. After returning to room temperature, a tetrahydrofuran solution of the first intermediate 3 (10.24 g, 36.5 mmol) was dripped into the above mixed solution, and the reaction was continued for 15 hours to obtain a reaction solution of the second intermediate 4.

[0030] (3) Preparation of aromatic bidentate phosphine ligand 5d Under anhydrous and oxygen-free conditions, diphenylphosphine chloride (16.11 g, 73 mmol) was slowly added to the reaction solution of the second intermediate 4 obtained in the above step (2), and the reaction was stirred at 25°C for 24 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The resulting mixture was purified by silica gel column using petroleum ether:ethyl acetate in a volume ratio of 30:1 as the eluent to obtain 16.8 g of aromatic bidentate phosphine ligand 5d with a yield of 71%.

[0031] 1 H NMR (400 MHz, C6D6): δ = 1.11 (s, 18H), 4.82 (t, 2H), 7.02-7.10 (m, 12H), 7.09-7.12 (m, 4H), 7.26 (m, 2H), 7.42 (m, 8H).

[0032] Example 5 The synthesis method of the aromatic bidentate phosphine ligand 5e: (5-(tert-butyl)-2-(diphenylphosphino)-4-isopropylphenylbenzyl)diphenylphosphine is as follows: (1) Preparation of the first intermediate 3 Isopropylbenzene 1 (60.7 g, 504.6 mmol) and anhydrous aluminum chloride (0.75 g, 5.62 mmol) were added to a 250 mL round-bottom flask. After stirring for 10 minutes, the mixture was heated in a water bath and the temperature was controlled at 20-30 °C. 4-tert-butylbenzyl chloride 2 (12.3 g, 67.3 mmol) was added dropwise from a constant pressure dropping funnel within 1 h. After the addition was completed, the temperature was raised to 80 °C and refluxed for 1 h. The mixture was cooled to room temperature, cold water was added, the organic phase was separated, and the mixture was washed with a saturated sodium carbonate solution and then with water. The organic phase was separated, dried, and recrystallized by adding isopropanol to obtain 13.27 g of the first intermediate 3 with a yield of 74%.

[0033] (2) Preparation of Second Intermediate 4 Under anhydrous and oxygen-free conditions, in a 250 mL Shrek bottle, n-butyllithium (45.6 mL, 1.6 M, n-hexane solution) was slowly dripped into an ether solution of tetramethylethylenediamine (8.48 g, 73 mmol) at 0°C and stirred for 2 hours. After returning to room temperature, a tetrahydrofuran solution of the first intermediate 3 (9.72 g, 36.5 mmol) was dripped into the above mixed solution, and the reaction was continued for 15 hours to obtain a reaction solution of the second intermediate 4.

[0034] (3) Preparation of aromatic bidentate phosphine ligand 5e Under anhydrous and oxygen-free conditions, diphenylphosphine chloride (16.11 g, 73 mmol) was slowly added to the reaction solution of the second intermediate 4 obtained in the above step (2), and the reaction was stirred at 25°C for 24 hours. After the reaction was completed, water was added to quench the reaction, and the mixture was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and the solvent was removed by rotary evaporation. The resulting mixture was purified by silica gel column with the eluent being petroleum ether:ethyl acetate in a volume ratio of 30:1 to obtain 16.9 g of aromatic bidentate phosphine ligand 5e with a yield of 73%.

[0035] 1 H NMR (400 MHz, C6D6): δ = 1.19 (d, 6H), 1.28 (d, 9H), 2.81 (m, 1H), 4.80 (t, 2H), 7.04-7.12 (m, 12H), 7.08-7.16 (m, 4H), 7.28 (m, 2H), 7.45 (m,8H).

[0036] The above are only preferred embodiments of the present invention, and the implementation scope of the present invention is not limited thereto. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for synthesizing an aromatic bidentate phosphine compound, characterized in that: The reaction route is Wherein, R1 and R2 are hydrogen or C1-C4 alkyl; The steps include: (1) In the absence of oxygen and with a Lewis acid catalyst, benzene or alkylbenzene (1) and benzyl chloride or 4-alkylbenzyl chloride (2) are reacted at 0 to 100° C. for 0.5 to 5 hours to obtain a first intermediate (3); (2) In the absence of water, oxygen and solvent, an alkyl lithium reagent is added dropwise into tetramethylethylenediamine at -30 to 40°C, stirred for 1 to 3 hours, and the first intermediate (3) is added dropwise into the mixed solution at room temperature, and reacted at -20 to 40°C for 4 to 20 hours to obtain a second intermediate (4); (3) Under anhydrous and oxygen-free conditions, diphenylphosphine chloride is added to the second intermediate (4) and reacted at -20 to 80°C for 5 to 60 hours to obtain an aromatic bidentate phosphine compound (5).

2. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The molar ratio of the alkylbenzene (1) and 4-alkylbenzyl chloride (2) in step (1) is 20-2:1; the Lewis acid catalyst is selected from ZnCl 2、 One of AlCl3, FeCl3, SnCl4; reaction temperature 20-90°C; reaction time 0.5-3 hours.

3. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The first intermediate (3) obtained in step (1) is recrystallized using isopropanol.

4. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The solvent in step (2) is selected from one or more of chloroform, dichloromethane, 1,2-dichloroethane, ether, petroleum ether, tetrahydrofuran, 2-methyltetrahydrofuran, cyclopentane, n-pentane, cyclohexane, n-hexane and n-heptane.

5. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The alkyl lithium reagent in step (2) is selected from one of methyl lithium, tert-butyl lithium, n-butyl lithium, sec-butyl lithium, n-hexyl lithium and lithium diisopropylamide.

6. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: In step (2), the molar ratio of the alkyl lithium to tetramethylethylenediamine is 1-8:1; and the molar ratio of the alkyl lithium to the first intermediate (3) is 2-10:

1.

7. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The reaction temperature in step (2) is 0-30° C. and the reaction time is 6-18 hours.

8. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The reaction solution of the second intermediate (4) obtained in step (2) is directly subjected to step (3) without separation.

9. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The molar ratio of diphenylphosphine chloride to the second intermediate (4) in step (3) is 2-5:1; the reaction temperature is 10-40° C.; and the reaction time is 8-36 hours.

10. The method for synthesizing an aromatic bidentate phosphine compound according to claim 1, characterized in that: The reaction product of step (3) is extracted with dichloromethane, the organic phase is dried, and the solvent is removed by rotary evaporation. The resulting mixture is purified by silica gel column to obtain the aromatic bidentate phosphine compound (5).

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