Polar bromination of cyclopropane: a DFT study

Hitherto the polar addition of bromine to cyclopropane has been considered as a two-step process. Current calculations have established the energetics for the proposed cation-anion pairs required by these mechanisms. An energetically lower pathway is proposed here in the form of a syn-cycloaddition process. Compared to the two-step process, a significantly lower activation enthalpy for this process has been found. The stereochemical consequences of the cyclic mechanism are retention-retention for the two adding moieties. This result is consistent with published experimental data on the bromination of a deuterated cyclpropane.