Reduction of Phosphonium Salts

The one-electron electrochemical reduction of 1,2-vinylene and buta-l,4-dienylene bisphosphonium salts at a mercury cathode produces an ylide character by the reaction pathway depicted in reactions 11—13. The mechanism is altered when OH is generated in the unbuffered aqueous-organic medium this reaction is depicted as reaction 14. The electrochemical reduction of phosphonium salt in the presence of tri-p-anisylphosphine produces a mixture of the saturated or semi-saturated bisphosphonium salts through either reaction scheme 15 or alternatively 16. 

Reduction of phosphonium salts as a route to tertiary phosphines is common, but its use for the synthesis of P—H compounds is unusual. 

The procedures to break the C—bond are rather limited. The main methods are alkaline hydrolysis and LAH reduction of phosphonium salts. Electrochemical reductions can also be used for this purpose. 

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... 

Those compounds were synthesised by alkaline hydrolysis or cathodic reduction of phosphonium salts or by reduction of phosphine oxides with silanes. In both cases, the optically pure precursor had to be prepared by chemical resolution (see Chapter 2, Section 2.2), a step that is time-consuming, inefficient and very dependent on the structure of the compound being resolved. 

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). 

Redox systems involving R2P and R2PX substituted benzenes and biphenyls have been studied by cyclic voltammetry and ESR spectrometry. The successive addition of one electron has been postulated25. The formation of a free radical on one-electron reduction has been confirmed by the observation of an ESR signal showing splittings from 31P. The synthesis of phosphonium salts in nearly quantitative yields from phosphane has also been established. 

In the absence of proton donors the reduction of 1 proceeds in two one-electron steps in contrast to the two-electron reduction found for Pli4As under the same conditions. Upon addition of phenol, the first reduction wave grows to a two-electron wave at the expense of the second reduction wave" and thereby parallels the behaviour observed for the reduction in aprotic solvents of phosphonium salts bearing a-hydrogens . In both... 

Further examples that illustrate the application of ESR spectroscopy in electrochemistry can be found in papers concerning the reduction of phosphonium and ammonium salts [382], 2,5-diphenyl-1,3,4-oxadiazole [336], and bianthrone [388], the electron transfer... 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

Sodium amalgam-reduction of the salt (69) (obtained from the reaction of acyl-stabilized ylides with trifluoromethane sulphonic anhydride) provides a new synthesis of acetylenes (Scheme 11). A new synthesis of ethynyltriphenyl-phosphonium salts (70) is provided by the reaction of acylmethylenetriphenyl-phosphoranes with dibromotriphenylphosphorane and base. ... 

In a related synthesis of phosphonium salts, Pd2(dba)3 catalyzed addition of alkenes and the acid Tf2NH to PPhs to yield anti-Markovnikov adducts (Scheme 61) [108]. By analogy to related chemistry with alkynes [109], a plausible mechanism involves protonation of Pd, alkene insertion, and P-C reductive elimination. 

Spheroidene (97) was synthesized [16,17] by a Wittig reaction of 67 with apo-8 -neuro-sporenal (74) and sodium methoxide (Scheme 21). The aldehyde 74 was prepared by the Wittig reaction of phosphonium salt 26 with the Cio-dialdehyde 20 and subsequent Horner-Emmons reaction with the phosphonoester 28. Reduction of the ester group with LiAlH4 and oxidation with Mn02 gave 74. 

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