A Possible Relationship Between Oxidation Rates of Alcohols and Their Carbon-13 Chemical Shifts

Abstract

Variations in the oxidation rates of alicyclic alcohols with chromate and of aldopyranoses with bromine, parallel variations in 13C chemical shifts for the carbinols. Accepting that these shifts closely reflect electron densities, mechanisms for these oxidations are depicted in terms of the minimal change in electron configuration required on passage from the ground to the transition state. A net flow of electrons from H-1 to C-1 is visualized for the chromate reaction, and hence relief of strain is associated in the transition state, as well as in the ground state, with a relatively high electron density on carbon. In the bromine oxidations, a net flow of electrons from C-1 to H-1 is visualized, which thereby allows for a sterically-dependent contribution by the ring oxygen atom to resonance stabilization of the transition state. These patterns are consistent with chromate and bromine oxidation mechanisms that now find wide acceptance.