Rhodium-catalyzed cyclopropanations of allylsilanes and allylstannanes: the role of the silyl/stannyl group in trans-cis stereoselection

The stereochemistry of cyclopropanation reactions between allylsilanes and allylstannanes 6 with four representative diazoalkanes in the presence of r hodium acetate has been studied. Cyclopropanations with ethyl diazoacetate show a preference for the formation of the trans cyclopropane esters 7, whe reas the corresponding reactions with (trimethylsilyl)diazomethane favor th e cis stereoisomers. In both cases, overall stereoselection is low, ranging from 1.3:1 to 2.4:1. The product ratios show a small dependence on the nat ure of the silyl or stannyl groups, with silyl substituents giving better s tereoselection than stannyl, and larger ligands on the metal center leading to lower selectivity. On the other hand, rhodium-catalyzed cyclopropanatio ns using methyl 2-diazo-4-phenyl-3-butenoate or 1-aryldiazoacetates occur w ith excellent stereocontrol. The stereochemical patterns found in these rea ctions are in accord with an open transition state model wherein the rhodiu m carbenoid approaches the allylmetal pi -bond from an antiperiplanar orien tation with respect to the allylic carbon-ML, bond. While the metal center may help stabilize developing P-cationic charge in the transition states, h yperconjugation effects appear to play a minor role, if any, in directing t he stereochemical course of cyclopropanation. (C) 2001 Published by Elsevie r Science B.V.