Study of the Wittig reaction of benzyltriphenylphosphonium salt and benzaldehyde via ylide-mediated phase-transfer catalysis - Substituent and solvent effects

The Wittig reaction of benzyltriphenylphosphonium salt and benzaldehyde in two-phase organic solvent/water (NaOH) medium was investigated, focusing on the effects of substituent and organic solvent. The substituents chosen for study include CH3, F, Cl, Br, CH3O, NO2, and CF3. The organic solvents include polar solvents (CHCl3 and CH2Cl2) and nonpolar solvents (n-C6H14, C6H6, and CCl4). It was found that the rate of agitation was insignificant and the reaction of benzylidenetriphenylphosphorane and benzaldehyde in the organic phase is the decisive step for determining the Z/E ratio of the product stilbene. In general, the polar solvents exhibit more favorable Z-selectivity. The Z/E ratio of the product stilbene may change substantially by interchanging the substituents on the benzyl group of phosphorus atom and on the phenyl group of aryl aldehyde. In contrast to the meta- and para-substituted benzaldehydes and the ortho-substituted benzylidene ylides, the ortho-substituted benzaldehydes bearing heteroatom substituent exhibit a pronounced enhancement for the Z-selectivity with the order of effectiveness of substituents being CF3 > (Cl, Br)> CH3O > F > NO2. The concerted asynchronous cycloaddition mechanism involving a four-centered transition state is suggested to be operating in these systems. (C) 1999 Elsevier Science B.V. All rights reserved.