3.2. General procedure for preparation of phosphanes

3.2.1. Trifluoromethyl- and selenomethyl-substituted phenylphosphanes (I)

Trifluoromethyl- and selenomethyl-substituted phenylphosphanes were prepared in order to investigate the difference in effect of electron-withdrawing and electron-releasing groups in modifying arylphosphanes. Additionally, in the olefin hydroformylation, answers were sought to the question in which direction the functionality of substituents in phenylphosphanes should be modified for optimum performance.

First, hydroformylation tests were performed with known arylphosphanes containing an electron-releasing OMe-, SMe-, or NMe₂-substituted phenyl ring. The main interest here was the potential bidentate bonding from different donor atoms to the metal center [59], [67], [68]. Rhodium catalyst modified with the SMe-substituted phenylphosphane, (o-thio­methyl­­phenyl)­diphenylphosphane, has produced improved results in the hydroformylation of MMA [59], [68]. Trifluoromethyl-substituted phenyl­phosphanes 1–4 (Scheme 3) were prepared as a means of understanding in which way the electron-withdrawing functionality affects the hydroformylation results of olefins relative to the potentially bidentate ligands modified with electron-releasing functionality. Furthermore, a heterodonor phosphorus-seleno and electron-releasing ligand, (o-seleno­­methyl­­phenyl)­diphenyl­phosphane 5, was prepared and tested in the hydroformylation of olefins to supplement the series of above-mentioned heterodonor ligands.

The synthesis of trifluoromethyl- and selenomethyl-substituted phenyl­phosphanes has been reported earlier [69], [70], [71], [72]. The trifluoromethyl-substituted triphenylphosphane ligands (o-trifluoromethyl­phenyl)­diphenylphosphane 1, tris(o-trifluoro­methylphenyl)­phosphane 2, (p-trifluoro­methylphenyl)diphenylphosphane 3, and tris(p-trifluoro­methyl­phenyl)­­phosphane 4 (Scheme 3) were prepared from liquid 1-bromo-2-(trifluoromethyl)­benzene or 1-bromo-4-(trifluoromethyl)benzene and the appropriate chloro­phosphane, and likewise the (o-­seleno­­methylphenyl)­diphenylphosphane 5 (Scheme 3) was prepared from 1-bromo-2-(selenomethyl)benzene and diphenylchlorophosphane. Times for both reaction steps at 0°C were 1.5 h, and finally hydrolysis with hydrochloric acid (0.2 M) was carried out only with ligand 5 since the CF₃-substituted phenylphosphanes were known to oxidize easily and to convert slowly to the phosphane oxide during isolation from the reaction mixture [70]. The reaction of selenomethyl-substituted ligand 5 was also carried out at 0°C, because earlier the yield of tris(o-selenomethylphenyl)phosphane had not been improved when the reaction temperature was reduced to −78°C [72]. The ligands 1–4 were washed with hexane, and ligand 5 was recrystallized from ethanol. The final solid products were brown (1–2), orange (3–4), and white (5).

3.2.2. (9-Anthryl)phenylphosphanes (I)

Bulkier aromatic phosphanes containing anthryl ring(s) instead of phenyl ring(s) were synthetized in order to investigate the effect of steric stress on the catalytic activity and selectivity of the hydroformylation reaction.

The (9-anthryl)diphenylphosphane 6 and bis(9-anthryl)phenylphosphane 7 (Scheme 4) were synthetized according to the literature method of Wesemann et. al. [73]. A diethyl ether solution of 9-bromo­anthryl was added to a solution of n-butyllithium in ether at –30°C. The mixture was stirred for 30 min, after which the appropriate chlorophosphane in diethyl ether was added over a period of 30 min. The resulting mixture was refluxed for 3.5 h and finally the cooled mixture was pumped dry in vacuum. The raw product was dissolved in dichloromethane and filtered over degassed Al₂O₃ (basic). The ligands were yellow solid products.

3.2.3. Alkyl-substituted arylphosphanes (II, III, IV)

The modification of steric properties was continued by increasing the bulk of the arylphosphanes with o-alkyl substituents: the bulk of alkyl group was increased step by step from methyl to ethyl, isopropyl, and cyclohexyl. Functional groups were varied in the hope of finding relationships between the catalytic behavior and steric properties of the ligands since the ortho-alkyl-substituted ligands are electronically fairly similar.

All the phosphane ligands, (o-methylphenyl)diphenylphosphane 8, bis(o-methyl­phenyl)­­phenylphosphane 9, (o-ethylphenyl)diphenylphosphane 10, bis(o-ethylphenyl)­phenyl­­­phosphane 11, (o-isopropylphenyl)diphenylphosphane 12, (o-cyclohexylphenyl)­diphenyl­phosphane 13, and (o-phenylphenyl)­diphenyl­phosphane 14 (see Scheme 5), were prepared by a modified literature method [74]. Undiluted solution of n-butyl­lithium reagent was added dropwise to a solution of brominated organic reagent in diethyl ether, after which a solution of the appropriate chlorophosphane in diethyl ether was added. Times of both reaction steps at 0°C were two hours. The ligands were recrystallized from ethanol and as pure products they were white or translucent solids. Attempts to recrystallize ligand 10 were at first unsuccessful but after many attempts and an exceptionally long time (several weeks) was eventually achieved. Single crystals of 9–11 and 13 for X-ray crystallographic analysis were grown from a mixture of dichloromethane/hexane at room temperature. The o-phenyl-substituted phenylphosphane 14 was prepared to assess the difference in catalytic behavior between o-alkyl- and o-aryl-substituted ligands.

All the phosphane ligands introduced in this section, except 13, have been mentioned in the literature [75], [76], [77]. However, it was considered to prepare all these ligands, with systematically increasing bulk, and as well to obtain a full range of spectroscopic and structural data for them.

The ligands (2,4,5-trimethyl­­phenyl)diphenylphosphane 15 and (2,5-dimethylphenyl)­di­phenyl­­phosphane 16 (Scheme 6) were prepared to find out how the presence of an ortho-alkyl substituent together with meta-alkyl and para-alkyl substituents and with a meta-alkyl substituent affect the hydroformylation results. Furthermore, (2-methyl­naphthyl)­­diphenyl­phosphane 17 was prepared to combine the steric crowding of the polyaromatic ring with an ortho-methyl substituent. Ligands 15–17 were prepared like ligands 8–14 and the recrystallization from ethanol likewise gave white solid products. A different method has been reported for the synthesis of ligand 16, but with lower yield and without NMR data [78].

Preparation of meta-alkyl-substituted phenyl­phosphanes was undertaken with the goal of achieving improved activity of the hydroformylation reaction. The newly prepared ligands (m-isopropyl­phenyl)­diphenyl­phosphane 18, bis(m-isopropyl­­phenyl)­phenyl­phosphane 19, and tris(m-isopropylphenyl)­phosphane 20 (Scheme 7) have the important bulky alkyl-substituent, but its steering role is less important in meta position.

Ligands 18–20 were prepared by the method described for o-alkyl-substituted phenylphosphanes. Lengthening of the time of the first reaction step to 3.5 h did not increase the yields. The ligands were purified by column chromatography on silica gel using dichloromethane/hexane (1:2) as eluent. The pure ligands 18–19 were translucent and oily, whereas the ligand 20 was a white solid. Single crystals of 18 for X-ray crystallographic determination were grown from a mixture of dichloromethane/hexane at room temperature.

3.2.4. Alkyl-substituted pyridylphosphanes (III, V)

In compound 21, (3-methyl-2-pyridyl)diphenylphosphane, the o-alkyl substituent was combined with pyridyl ring. o-Alkyl and pyridyl functions are in different side chains of the phosphane in ligands (2,5-dimethyl­­phenyl)bis(3-pyridyl)­phosphane 22 and (2,5-di­methyl­phenyl)bis(4-pyridyl)­phosphane 23 (Scheme 8). The pyridyl ring was chosen to be a second modifying piece since pyridyl­phosphane catalysts have shown higher reaction rates than the corresponding phenylphosphanes, probably because of their stronger π -back-bonding character [48], [55].

(3-Methyl-2-pyridyl)diphenylphosphane 21 was prepared according to the literature method by adding a solution of 2-bromo-3-methylpyridine in diethyl ether to a cooled solution of n-butyllithium in diethyl ether at −100°C [79]. The low reaction temperature, which was essential to decrease the formation of side products, was maintained by means of an ethanol/liquid nitrogen bath. After 1.5 h stirring, a solution of chlorodiphenyl­phosphane in diethyl ether was added and stirring was continued at −100°C for 1 h. After slow warming to room temperature, the raw product was extracted with sulfuric acid and the aqueous layer was separated and made alkaline with sodium hydroxide. The solid product was extracted back to the organic phase with diethyl ether and dried in vacuum. The ligand 21 has been mentioned in the patent literature [80].

Addition of TMEDA was essential to obtain ligands 22 and 23. The syntheses were performed according to the method of Bowen et al. [81] by adding the appropriate 3- or 4-bromopyridine in diethyl ether to the cooled diethyl ether mixture of n-butyllithium and TMEDA at −115°C. After 5 min stirring, two thirds of the dichloro(2,5-dimethyl­phenyl)­phosphane was added and stirring was continued for 0.5 h. The rest of the dichloro(2,5-dimethyl­phenyl)­phosphane was then added and the mixture was stirred for a further 2.5 h at −100°C, before warming to room temperature overnight. The raw products were extracted and dried like ligand 21. The products were purified by column chromatography on silica gel using dichloromethane/hexane/methanol (10:3:1) as eluent. Ligands 21 and 22 were white solids and 23 was transparent solid. Single crystals of 23 for X-ray crystallographic analysis were grown from dichloro­methane/hexane mixture at room temperature.

3.2.5. o-Alkyl-substituted arylalkylphosphanes (V)

Earlier studies on 1-hexene hydroformylation have shown that Rh-catalyst modified with linear trialkyl­phosphanes produce only alcohols, whereas the catalyst with triphenyl­phosphane or no phosphanes at all mainly form acetals [82]. The Rh-catalysts modified with triisopropylphosphanes promote the formation of aldehydes and the catalysts with mixed ethylphenyl­phosphanes the formation of mixtures of aldehydes and alcohols [82]. In other words, catalytic properties of alkylphosphanes are very much dependent on the site and degree of branching. In general, more stable metal complexes are formed with alkylphosphanes than with arylphosphanes owing to their greater σ-electron donor ability [37]. However, little attention has been paid to trialkylphosphane and mixed arylalkyl­phosphane rhodium complexes as catalysts for hydroformylation [39], it was to fill this gap that ortho-alkyl-substituted arylalkylphophanes were prepared. Isopropyl or cyclohexyl groups were combined with three different ortho-alkyl-containing aromatic groups — o-methylphenyl, o-cyclohexylphenyl, and 2-methyl­naphthyl — giving the following ligands: (o-methyl­phenyl)­­diisopropyl­­phosphane 24, (o-cyclo­hexyl­phenyl)­­diisopropyl­phosphane 25, (o-methyl­­­phenyl)­dicyclo­hexylphosphane 26, (o-cyclo­hexyl­phenyl)­dicyclo­hexyl­phosphane 27, bis(o-methyl­­­phenyl)­­isopropyl­phosphane 28, (2-methyl­naphthyl)­diisopropyl­phosphane 29, and (2-methyl­­naphthyl)­di­cyclo­­hexyl­phosphane 30 (Scheme 9). The ligands were synthesized by the same method as the o-alkyl-substituted arylphosphanes but with use of the appropriate chloroalkyl­phosphanes. The ligands 24 and 29-30 were purified by column chromatography on silica gel using dichloromethane/hexane (1:2) as eluent, the ligands 26-27 were recrystalized from ethanol, the ligand 25 was washed with hexane, and 28 was dissolved in ethanol and filtered over silica gel. The pure ligands 24-25 and 28 were oily and translucent compounds, whereas 26-27 and 29-30 were white solids. Single crystals of 27 for X-ray crystallographic analysis were grown from dichloromethane/hexane mixture at room temperature.