Phosphonium salts reductive 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... 

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. 

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... 

Unlike the corresponding phosphonium salts, addition of sulfonium salts to aldehydes results, not in the alkene products, but in the formation of epoxides (see Section 1.1.5.2). However, sulfones can be used to prepare alkenes, by way of the a-metallo derivatives, in what is termed the Julia olefination (alkenylation). Addition of the organometallic species to an aldehyde or ketone gives a p-hydroxy sulfone which, in the form of its 0-acyl or 0-sulfonyl derivative, undergoes reductive cleavage with, for example, sodium amalgam in methanol to form the alkene. The reaction is regioselective and can be used to prepare mono-, di- and trisubstituted alkenes (2.91). 

Initial Wittig reaction of the readily available arabinose derivative 138 and the phosphonium salt 139 led to the cis alkene 140 (Scheme 38). Stereoselective dihy-droxylation, exploiting the 1,3-allylic strain effect, then monoprotection of the diol, oxidation and cleavage led to the cyclic hemiacetal 142 after protection of the primary alcohol. Reduction of the hemiacetal was best performed with /j-PrsSiH, affording exclusively the p anomer 143. Deprotection and acidic methanolysis then led to the C-disaccharide analog 144 of p-D-Man(l-4)-D-Glc. On the other hand, simple oxidation of the C2-hydroxyl group in 143 followed by stereoselective reduction converted the manno series to the gluco series with the eventual formation of C-cellobioside 145. 

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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