Mechanistic Insights into the Nickel-Catalyzed Regioselective Carboxylation of Allylic Alcohols

The
recently reported nickel-catalyzed direct carboxylation of
allylic alcohol eliminates the necessity for pre-activation of substrates
in traditional strategies and generates linear β, γ-unsaturated
carboxylic acids with high E/Z selectivity.
Motivated by the significant experimental advantages and the mechanistic
ambiguity (in the C–O activation mode of the allylic alcohol,
the rate- and regio-/stereoselectivity-determining steps, etc.), we conducted a detailed study on the mechanism of
Ni-catalyzed carboxylation of allylic alcohols with density functional
theory calculations. It is found that the reaction occurs via the activation of allylic alcohol, oxidative ligation,
reduction, and carboxylation steps. The rate-determining step is the
first step, in which the moisture of the reaction system plays a critical
role. Through the hydrogen bonding network formed between water and
the substrate, “proton-relay” occurs easily to generate
the allylic hydrogen carbonate and to undergo the subsequent oxidative
ligation steps. Meanwhile, the steric hindrance between CO2 and the allylic group during the coordination of CO2 is
mainly responsible for the regioselectivity on the terminal carbon
atom, and the E/Z selectivity is
mainly determined by the thermodynamic stability of the E-substrates.