Further Evidence for the Role of dπ-pπ Bonding in Rhodium-Mediated Hydroborations

Stereoelectronic effects governing coordination of the alkene functionality to rhodium have been postulated to account for diastereofacial selectivities in catalyzed hydroborations of allylic alcohol derivatives 1; this paper describes two sets of experiments to test this controversial hypothesis. The first one involves competition experiments between related allylic alcohol derivatives, and shows that allylic trifluoroacetates react at least 160–210 times faster than allylic acetates under the catalyzed conditions. Similar competition experiments with uncatalyzed hydroborations reveal rate differences of less than 5-fold in the opposite sense (i.e. allylic acetates react faster). The second set of experiments show that catalyzed and uncatalyzed hydroborations of 5-substituted 2-methyleneadamantanes 5 proceed with opposite diastereofacial selectivities. Results from the competition experiments and the stereoselectivity studies are consistent with the original postulate for stereoelectronic effects in catalyzed hydroborations, and provide pointers to some of the mechanistic features that characterize the process. Experiments to probe the reversibility of mechanistic steps in rhodium-catalyzed hydroborations are presented. Finally, selectivities observed in the catalyzed hydroborations of 5-substituted 2-methyleneadamantanes 5 are discussed with reference to the “Cieplak postulate”, a hypothesis that is shown to be inappropriate for predicting the stereochemical outcome of catalyzed hydroborations.