Mechanistic Insights into Copper-Catalyzed Carboxylations

The
copper-NHC-catalyzed carboxylation of organoboranes
with CO2 was investigated using computational and experimental
methods.
The DFT and DLPNO-CCSD­(T) results indicate that nonbenzylic substrates
are converted via an inner-sphere carboxylation of an organocopper
intermediate, whereas benzylic substrates may simultaneously proceed
along both inner- and outer-sphere CO2 insertion pathways.
Interestingly, the computations predict that two conceptually different
carboxylation mechanisms are possible for benzylic organoboranes,
one being copper-catalyzed and one being mediated by the reaction
additive CsF. Our experimental evaluation of the computed reactions
confirms that carboxylation of nonbenzylic substrates requires copper
catalysis, whereas benzylic substrates can be carboxylated with and
without copper.