Science of Synthesis

Schobert, R.; Gordon, G. J., Science of Synthesis, (2004) 27, 1002.

Phosphonium salts when subjected to electrochemical reduction give either a phosphine[‌358‌,‌359‌] or a phosphorus ylide. Phosphorus ylide formation has been observed and investigated in polar aprotic solvents.[‌360‌] The mechanism is thought to include a two-electron transfer step;[‌361‌] the initial transfer of an electron at the cathode results in cleavage of the P—C bond of the phosphonium salt ‌58‌ forming a phosphine and a carbon radical. The carbon radical is then rapidly reduced by a second electron to a carbanion, which deprotonates the starting phosphonium ion and gives a hydrocarbon and an ylide ‌59‌ (‌Scheme 20‌). Ylides formed from benzyl-, allyl-, cinnamyl-, and polyenylphosphonium salts have been detected by cyclic voltammetry.[‌362‌] So-called electrogenerated bases have been developed; they are formed by cathodic reduction of organic precursors. For example, (fluoren-9-ylidene)methane derivatives can be electrochemically reduced to the corresponding anions at lower potentials than those required for the cleavage of phosphonium ions; such an anion abstracts a proton from a phosphonium salt affording a phosphorus ylide, which can be trapped with simple aldehydes to give alkenes ‌60‌.[‌361‌,‌363‌] The rate of base generation can be controlled by the current density; moreover the structure of the probase can be modified to give analogues that have a range of pKa values. The reduction potentials of several ylides have been measured by both direct reduction of the corresponding phosphonium salts[‌362‌] and through the use of an electrogenerated base.[‌363‌] Using triphenylphosphine and cyclic silylated enol ethers a number of (2-oxacycloalkyl)triphenylphosphonium salts have been synthesized by electrochemical methods; such products are normally difficult to prepare.[‌364‌]

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References

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