Quantum Chemical Characterization of Factors Affecting the Neutral and Radical-Cation Newman-Kwart Reactions

Details of the electronic and geometric structures of stationary points along the reaction coordinate of the Newman Kwart rearrangement, which transforms an O-arylthiocarbamate into an S-arylcarbamothioate, are examined using quantum-chemical methods for a large number of compounds considering both the thermal reactions of uncharged substrates as well as the corresponding reactions of radical-cation substrates generated under photoredox conditions. The uncharged mechanism, which has intrinsically high 298 K free energies of activation (in excess of 30 kcal mol^-1), has the character of nucleophilic aromatic substitution and is thus accelerated by electron-withdrawing substituents on the aromatic ring. The radical-cationic mechanism, by contrast, has 298 K free energies of activation that are typically below 20 kcal mol^-1 and is accelerated by electron donating substituents on the aromatic ring, which stabilize the hole character that is transferred to this fragment from the thiocarbamate fragment as the reaction proceeds. Opportunities to further accelerate the radical-cation reaction are revealed by computational assessment of alternative amino groups for the thiocarbamate functionality.