Theoretical Investigation into the Mechanism of Cyanomethylation of Aldehydes Catalyzed by a Nickel Pincer Complex in the Absence of Base Additives

Abstract

Density functional theory (DFT) was used to study the reaction mechanism of cyanomethylation of aldehydes catalyzed by nickel pincer complexes under base-free conditions. The C-bound cyanomethyl complex, which was initially thought to be the active catalyst, is actually a precatalyst, and in order for the catalytic reaction to commence, it has to convert to the less-stable N-bound isomer. The carbon-carbon bond formation then proceeds via direct coupling of the N-bound isomer and the aldehyde to give a zwitterionic intermediate with a pendant alkoxide function, which is further stabilized by hydrogen-bonding interaction with water molecules (or alcohol product). The N-bound alkoxide group of the zwitterionic intermediate is subsequently substituted by MeCN via an associative mechanism, followed by deprotonation of the coordinated MeCN to afford the final product. It was found that the transition structure for the exchange reaction (substitution of MeCN for the alkoxide group) is the highest energy point on the catalytic cycle, and its energy crucially influences the catalyst efficiency. The Ni complexes ligated by bulky and weak trans-influencing pincer ligands are not appropriate catalysts for the cyanomethylation reaction due to the involvement of very-high-energy transition structures for the exchange reaction. In contrast, benzaldehydes with electron-withdrawing substituents are capable of stabilizing the exchange reaction transition structure due to the increased stability of the zwitterionic intermediate, leading to acceleration of the catalytic reaction.