Chemists Confirm The Identity Of Pivotal Intermediate In Carbocation Rearrangements

Chemists have known for 70 years that carbocation rearrangements involve slow steps and fast steps. The slow steps are crucial in that they control the degree of branching in alkanes, and understanding how that works is vital for predicting product distributions in processes such as biomass and petroleum refining. But why the key steps are slow has been a confounding mystery. Daniel J. S. Sandbeck, Daniel J. Markewich, and Allan L. L. East of the University of Regina now appear to have found the answer via a set of computer simulations (J. Org. Chem. 2016, DOI: 10.1021/acs.joc.5b02553). The team first revisited decades-old studies in which chemists proposed that the rearrangements proceed through a protonated cyclopropane intermediate. Then using hexyl ion as a model, the researchers ran simulations and uncovered 70 transition states connecting primary, secondary, and tertiary ion versions. In the past, most chemists thought the cyclopropane intermediate must be protonated on an edge of the ring or at one of the corners. The Regina researchers found that neither assumption was correct but that the intermediate takes on a mesomeric structure that is a hybrid of the two. The finding suggests that branching is slow because the reaction pathway must pass through an unstable primary carbocation that involves the mesomeric structure.