| Description |
Efforts towards understanding the nickel catalyzed cycloaddition of diynes and nitriles to make pyridines are described. During the course of this mechanistic study, a previously uncharacterized class of nickel-nitrile-NHC (N-heterocyclic carbene) dimers, which display cooperative η1 and η2 binding modes of the nitrile to nickel, were discovered. Crystal structures were obtained for select dimers. These dimers proved catalytically competent in the cycloaddition reaction. Pseudo first order kinetics revealed that the reaction was first order in dimer while being zero order in NHC ligand, nitrile, and diyne. While stoichiometric reactions with dimer and diyne in the absence of nitrile did give product, largely improved yields were observed in the presence of free nitrile. Stoichiometric competition studies utilizing various identities of free nitriles and dimers indicated a preference for incorporation of free nitrile into the product versus the dimer-bound nitrile. The results of this study suggest a mechanism involving partial dimer opening as the rate-determining step. This is then followed by nitrile binding, which precedes oxidative coupling with diyne. |
