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(a q)-THF. The reactions gave diastereomeric homoallylic alcohols with d. e.s ranging from 28 to 74%. With ethyl 2-chloroacetoacetate, substitut ion 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 configurat ions 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 th e keto ester with the metal surface.