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 rhodium acetate has been studied. Cyclopropanations with ethyl diazoacetate show a preference for the formation of the trans cyclopropane esters 7, whereas the corresponding reactions with (trimethylsilyl)diazomethane favor the 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 nature of the silyl or stannyl groups, with silyl substituents giving better stereoselection than stannyl, and larger ligands on the metal center leading to lower selectivity. On the other hand, rhodium-catalyzed cyclopropanations using methyl 2-diazo-4-phenyl-3-butenoate or 1-aryldiazoacetates occur with excellent stereocontrol. The stereochemical patterns found in these reactions are in accord with an open transition state model wherein the rhodium carbenoid approaches the allylmetal π-bond from an antiperiplanar orientation with respect to the allylic carbonML3 bond. While the metal center may help stabilize developing β-cationic charge in the transition states, hyperconjugation effects appear to play a minor role, if any, in directing the stereochemical course of cyclopropanation.