Chromium-catalyzed sustainable C-C and C-N bond formation: C-alkylation and Friedländer quinoline synthesis using alcohols

The synthesis of a novel phosphine-based pincer chromium(ii) complex CrCl2(PONNH) (Cr-1) is reported in this study. The complex exhibited promising catalytic performance in C-C and C-N bond formation using the borrowing hydrogen methodology. Cr-1 catalyzed the alpha-alkylation of ketones using primary alcohols as alkyl surrogates in the presence of catalytic amount of a base. Cr-1 was also found to catalyze the beta-alkylation of secondary alcohols using primary alcohols. In addition, the dehydrogenative annulation of 2-aminobenzyl alcohols with ketones to form quinolines was achieved using Cr-1 as the catalyst. Based on the mechanistic investigation, a plausible mechanism based on metal-ligand cooperation is proposed. The reactions are redox-neutral, atom-efficient, and produce water as the only by-product, thus contributing to green chemistry.