Phosphonium cleavage, reductive

The reported 2 e reductive cleavage of phenylsulphonylacetonitrile (PhSO CH CN) and the observation that in protic media the products were PhSO and CH CN, suggested that this reaction could be a useful source of CHjCN. However, careful re-examination showed that in acetonitrile solution the reaction is pseudo one-electron, analogous to the phosphonium and sulphonium salts (Scheme 8), and that phenylsulphonylacetonitrile is sufficiently acidic rapidly to protonate "CHjCN assuming additivity of substituent effects an estimate of pK 14-16 was made for PhSOjCHjCN, cf. pK 31 for CH,CN... 

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). 

The phosphonium salts undergo reductive cleavage by reaction 2142 ... 

Convenient alternatives to direct deprotonation of ethers are tin-lithium exchange [199, 258-261], halogen-magnesium exchange [262], or reductive cleavage of 0,Se-acetals [263, 264], Another synthetic equivalent of a-metalated ethers are (alkoxymethyl)phosphonium salts [265]. 

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. 

In contrast to phosphine oxides, whereby the phosphoryl group is reduced, the reduction of phosphonium salts involves cleavage of a P—C bond. There are several ways this can be achieved, including base-induced cleavage, thermal decomposition, hydride or electrolytic reduction.2... 

Single P C bond cleavage has been described. For example, phosphonium salts 2.56 are reduced to phosphines by LAH in THF under reflux the cleaved bond corresponds to reduction of the most stable carbanion [H2] (Figure 2.28). The P—C bonds of allylic phosphonates 2.57 or phosphonium salts can also be cleaved by... 

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