Oxygen Atom Transfer in the Oxidation of Dimethyl Sulfoxide by Oxoammonium Cations

Cyclic oxoammonium salts and DMSO are known as important reagents for their diverse and unique reactivity. In the present work, we have studied the reaction of six- and five-membered oxoammonium salts with DMSO. The reaction includes similar to 100% selective transfer of the O atom from the >N+=O group to the S atom of DMSO and structural rearrangement of the remaining cationic framework, leading to the formation of hydrolytically unstable iminium salts. The logarithms of the bimolecular rate constants kappa of the reaction correlated linearly with the reduction potentials E->N+(=O)/>N-O center dot, a relationship known for other electrophile-nucleophile combinations. The kinetic data and results of the DFT calculations allow for the suggestion that the studied process pr oceeds via the prereactive charge-transfer complex >N+=O center dot center dot center dot S (O)Me-2 and its direct concerted rearrangement to the iminium salts. An alternative mechanism that includes intermediate steps with discrete nitrenium cations can be ruled out on the basis of product analysis and DFT computations. The obtained results allow a deeper understanding of the redox chemistry of a pair of nitroxide radicals-oxoammonium cations.

Automated summary

The reaction between six- and five-membered oxoammonium salts and DMSO shows selective transfer of the O atom and formation of hydrolytically unstable iminium salts. Kinetic data and DFT calculations suggest a mechanism involving a charge-transfer complex leading to the formation of iminium salts. Analysis of products and computations rule out an alternative mechanism with discrete nitrenium cations.