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.

Oriented Internal Electrostatic Fields Cooperatively Promote Ground- and Excited-State Reactivity: A Case Study in Photochemical CO2 Capture

“…Alternative proposed mechanisms for carboxylation of o-alkylphenyl ketones. 22,24 Our results support the notion that hydrogentransfer to form the o-quinodimethane must occur in the triplet state; this is followed by intersystem crossing at the conical intersection point of (E)-(Z) isomerisation to form the singlet (E)-or (Z)-dienol. On the singlet surface, the barrier for hydrogen atom transfer (HAT) from the (Z)-enol is substantially lower than on the triplet surface (24 kJ mol -1 versus 87 kJ mol -1 in DMSO, respectively), resulting in swift deactivation to the starting ketone.…”

“…In contrast, competition between CO 2 addition via Su and Su’s mechanism (83 kJ mol –1 at our level of theory) and HAT (24 kJ mol –1 ) is such that deactivation is dominant. At their chosen level of theory, Su and Su report a more favorable barrier difference of 27 kJ mol –1 (in favor of HAT); however, they fail to account for solvation effects, which are particularly significant given the initial phase difference between the ( Z )-enol and CO 2 .…”

“…In contrast, competition between CO2 addition via Su and Su's mechanism 24 (83 kJ mol -1 at our level of theory) and HAT (24 kJ mol -1 ) are such that deactivation is dominant.…”

“…Thus, to drive the reaction forward, isomerization from (Z)-A to (E)-A must occur on the triplet surface, such that the latter becomes kinetically trapped following ISC. While this PEDA mechanism appears to satisfactorily explain the process developed by Murakami and co-workers, a subsequent theoretical study by Su and Su 24 proposed an alternative. In particular, the E−Z isomerization was ignored, and it was instead proposed that CO 2 addition occurs directly to the (Z)-isomer via an eight-membered transition state, directly affording the keto-carboxylic acid 3 without generating the enol-lactone 2 (Scheme 1C).…”

C−H Bond Carboxylation with Carbon Dioxide

“…The reaction can be promoted by UV irradiation or sunlight. Recently, Su and Su studied the mechanism of this reaction through theoretical calculations …”