Despite steric shielding of the PC bond, the methylenephosphonium ions 1 and 2 react with 2,3-dimethylbutadiene in an ene reaction and a [2 + 4]cyclo-addition reaction, respectively, to give 3 and 4 (counterion in all cases AlCl). This reaction behavior is understandable from the pronounced electron-deficient character of the alkyl-substituted P atoms. Compounds 1 and 2 are formed in high yields from the corresponding P-chloro-substituted phosphorus ylides by chloride abstraction.
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Grützmacher et al. (1991) studied this question.
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