Aliphatic C(sp3)-H Bond Activation Using Nickel Catalysis: Mechanistic Insights on Regioselective Arylation.

Abstract

Transition-metal-catalyzed C(sp3)-H bond activation in aliphatic compounds are of current interest. Lack of mechanistic insights on Ni-catalyzed C(sp3)-H activation using 8-aminoquinoline as a directing group motivated us to examine an interesting direct arylation of an aliphatic tertiary amide by using density functional theory. The catalysis employed Ni(II) precatalyst, 4-iodoanisole as an arylating agent, sodium carbonate, and mesitylenic acid as additives in DMF solvent. Examination of a comprehensive set of mechanistic pathways helped us learn that the most preferred route begins with a bidentate chelate binding of deprotonated substrate to the Ni. The C-H activation in the catalyst-substrate complex via a cyclometalation deprotonation provides a five-membered nickelacycle intermediate, which upon the rate-limiting oxidative insertion to aryl iodide forms a Ni(IV)-aryl intermediate. The ensuing reductive elimination furnishes the desired arylated product. We note that the explicit inclusion of sodium carbonate, mesitylenic acid, and solvent molecules on sodium ion all are critical in identifying the most favorable pathway. Of the two types of C(sp3)-H bonds in the substrate [2-methyl-2-phenyl-N-(quinolin-8-yl)heptanamide], the energies for the regiocontrolling reductive elimination is predicted to be more in favor of the methyl group than the methylene of the pentyl chain, in excellent agreement with the previous experimental observation.