Mechanistic analysis of the photochemical carboxylation of o-alkylphenyl ketones with carbon dioxide

Abstract

The mechanisms for photochemical carboxylation reactions are studied theoretically using two model systems: o-methylbenzophenone and o-methylacetophenone, with the M06-L and the 6-311G(d,p) basis set. Three reaction routes are used to ascertain the actual photochemical reaction mechanism for the o-alkylphenyl ketone molecules. The structures of the intersystem crossing, which play a central role in these photocarboxylations, are determined. The intermediates and transition structures for the ground singlet states and the excited triplet states are also computed to allow a qualitative interpretation of the entire reaction path. The theoretical findings show that when o-alkylphenyl ketone molecules are produced in the T₁ state by photoexcitation at 365 nm, intersystem crossing to the S₀ surface is the most probable pathway for deactivation. When they relax to the S₀ state, the o-alkylphenyl ketone molecules can either react with CO₂ to undergo the insertion reaction, which produces the carboxylic acid, or revert to the reactants. In particular, the theoretical investigations strongly suggest that the insertion of CO₂ in these photocarboxylation reactions for o-alkylphenyl ketones occurs on the S₀ surface, rather than on the excited T₁ surface.