The effects of substituents at phosphorus on the mode of decomposition of phosphonium betaines in protic solvents

Abstract

A delicate balance of steric and electronic effects controls the course of decomposition of phosphonium betaines generated in protic solvents. Elecron-withdrawing heteroaryl substituents (e.g. 2-furyl and 2-thienyl) promote intramolecular betaine collapse to form the normal Wittig products, as also does enclosure of the phosphorus in the dibenzophosphole ring system. Except in such cyclic systems, both electron-donating and bulky groups (e.g. o-tolyl and t-butyl) reduce the rate of intramolecular collapse and allow a dehydration step to occur, with the formation of a vinylphosphonium intermediate (IV). The decomposition of (IV) leading to abnormal products depends on the carbanionic stability of the group cleaved from phosphorus. Carbanions of moderate stability such as phenyl, p-methoxyphenyl, or m-chlorophenyl require the additional stabilisation of negative charge afforded in the transition state of the reaction leading to the rearrangement product (V). For carbanions of greater stability, such as 2-heteroaryl and m-trifluorophenyl carbanions, simple cleavage of the leaving group can occur, with formation of the vinylphosphine oxide (VI). The presence of an aryl substituent on the carbon α to phosphorus promotes betaine collapse to the normal Wittig products even when the phosphorus bears a t-butyl group [as in the reaction of the salt (XIII) with benzaldehyde in the presence of ethoxide ion].