Stereochemistry and mechanism of the chemoselective allylation of alpha-substituted beta-keto esters with allyl bromide mediated by Zn in NH4Cl(aq)-THF

alpha-substituted beta-keto esters were chemoselectively allylated at the keto group by a Zn-mediated reaction with allyl bromide in NH4Cl(aq)-THF. The reactions gave diastereomeric homoallylic alcohols with d.e.s ranging from 28 to 74%. With ethyl 2-chloroacetoacetate, substitution of the chlorine atom by the allyl group took place. Similarly, on allylation of diethyl benzoylmalonate, diethyl malonate was eliminated. These reactions support a radical mechanism. The relative configurations of the formed diastereomers were determined by a combination of H-1 NMR spectroscopy and molecular modeling. The stereochemistry of the diastereomers supports a mechanism including strong association of the keto ester with the metal surface.