Lewis acid catalyzed heavy atom tunneling - the case of 1-bicyclo[3.1.0]-hexa-3,5-dien-2-one.

For many thermal reactions, the effects of catalysis or the influence of solvents on reaction rates can be rationalized by simple transition state models. This is not the case for reactions controlled by quantum tunneling, which do not proceed transition states, and therefore lack the simple concept of transition state stabilization. 1-Bicyclo[3.1.0]-hexa-3,5-dien-2-one is a highly strained cyclopropene that rearranges to 4-oxocyclohexa-2,5-dienylidene heavy-atom tunneling. HO, CFI, or BF form Lewis acid-base complexes with both reactant and product, and the influence of these intermolecular complexes on the tunneling rates for this rearrangement was studied. The tunneling rate increases by a factor of 11 for the HO complex, by 23 for the CFI complex, and is too fast to be measured for the BF complex. These observations agree with quantum chemical calculations predicting a decrease in both barrier height and barrier width upon complexation with Lewis acids, resulting in the observed Lewis acid catalysis of the tunneling rearrangement.