Solvolytic ring-opening reactions of cyclopropyl bromides.: An assessment of the Woodward-Hoffmann-DePuy rule

A theoretical study on the torquoselectivity of the electrocyclic ring opening of 2,3-disubstituted-mono and dibromo cyclopropanes was carried out at the B3LY-P/6-311++G* level. As a result of charge donation from the dissociating C-C bond to the antibonding orbital of the breaking C-Br bond, only one of the two allowed disrotatory paths for the disrotatory ring opening, which takes place in concert with the C-Br bond cleavage, is energetically accessible, both in the gas phase and in solution. The general trends in torquoselectivity for these systems are not significantly affected by stereoelectronic effects of the substituents or the introduction of a second bromine in C1. For the Woodward-Hoffmann-DePuy-allowed processes, the remarkable lowering in energy when the OH or NH2 groups rotate inward can be attributed to the formation of an intramolecular X-H-Br hydrogen bond in the transition state. Aromaticity in reactants and transition states for the model systems is evaluated using a ring current model to shed light on the reaction mechanism.