Optically active phosphonium salts, reduction

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

The reduction of optically active phosphonium salts by lithium aluminum hydride, which probably does involve 70 as an intermediate, affords racemic phosphines, presumably by pseudorotation in 70 before it decomposes 63). 

Sodium-naphthalene reduction of organotrineopentoxyphosphonium salts led to the instantaneous loss of phosphonium ion phosphonates and phosphites were obtained748 (reaction 224). Alkali metal amalgams are efficient reagents for the reductive cleavage of both achiral and optically active phosphonium salts configuration is retained750 (Table 23). 

Optically active quaternary arsonium [396,397] and phosphonium [398] salts are electro-reductively decomposed to tertiary arsines and phosphines, respectively, with retention of configuration. Since the tertiary products can be converted again into different quaternary salts with alkylating regents, this decomposition reaction may be useful for transformation between a variety of optically active quaternary salts [397]. 

Metals such as Na or alkali metal amalgams can also be used in the cleavage of the C—P" bond. In the latter case, reductive cleavage of achiral and optically active quaternary phosphonium salts succeeds in high yields with retention of configuration. ... 

The reductive cleavage of achiral and optically active quaternary phosphonium and arsonium salts with alkali metal amalgams to form tertiary phosphines and arsines succeeds in high yield with retention of configuration [124]. The reduction with the amalgams was found to give better yields than the conventional cathodic cleavage. 

Using a method of electrolytic reduction, Horner [3] in 1961 obtained optically active phosphines from the corresponding phosphonium salts. These reactions proceed with retention of configuration (13.57) and the products racemise only slowly in boiling toluene. 

Mixed tert. amines and phosphines as well as mixed quaternary ammonium and phosphonium salts can be conveniently prepared by successive alkylations and reductions because of the varying ease with which the groups are removed. The benzyl group is most easily removed. F. e., also with a polished Pb-cathode, s. L. Horner and A. Mentrup, A. 646, 49, 65 (1961) optically active phosphines from phosphonium salts s. Tetrah. Let. 1961, 161 optically active arsines cf. Tetrah. Let. 1962, 203. 

Other Compounds. Horner and co-workers [129-131] have described the production of optically active phosphines and arsines in the cathodic cleavage of quaternary phosphonium and arsonium salts. This reaction, like its reverse quaternization reaction, takes place with retention of configuration. Shapoval, Skobets, and Markova [132, 133] have described the production of a. stereoregular isocyanate polymer during reduction at a nickel cathode in dimethyl-formamide. The formation of a stereoregular product evidently demonstrates the orienting effect of the electrode on the molecules of the monomer in the polymerization process. 

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