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.