Nucleophilicity parameters for phosphoryl-stabilized carbanions and phosphorus ylides: implications for Wittig and related olefination reactions.

The kinetics of the reactions of four phosphoryl-stabilized carbanions 1a-d and four phosphorus ylides 1e-h with benzhydrylium ions 2a-h and structurally related quinone methides 2i-m have been determined by UV-vis spectroscopy. The second-order rate constants (k) correlated linearly with the electrophilicity parameters E of 2a-m, as required by the correlation log k = s(N + E) (J. Am. Chem. Soc. 2001, 123, 9500-9521), allowing us to calculate the nucleophile-specific parameters N and s for phosphoryl-substituted carbanions and phosphorus ylides. In this way, a direct comparison of the nucleophilic reactivities of Horner-Wadsworth-Emmons carbanions and Wittig ylides became possible. Ph(2)PO- and (EtO)(2)PO-substituted carbanions are found to show similar reactivities toward Michael acceptors, which are 10(4)-10(5) times higher than those of analogously substituted phosphorus ylides. The relative reactivities of these nucleophiles toward benzaldehydes differ significantly from those toward carbocations and Michael acceptors, in accordance with a concerted [2 + 2] cycloaddition being the initial step of these olefinations reactions. Effects of the counterion (K(+), Na(+), or Li(+)) on the nucleophilicities of the phosphoryl-stabilized carbanions in DMSO have been studied. Whereas the effects of K(+) and Na(+) are almost negligible for all types of carbanions investigated, Li(+) coordination reduces the reactivities of phosphonate-substituted acetic ester anions (1a) by a factor of 10(2) while the reactivities of phosphonate-substituted acetonitrile anions (1b) remain almost unaffected.