Microwave-Assisted Synthesis of E -Aldimines, N -Heterocycles, and H 2 by Dehydrogenative Coupling of Benzyl Alcohol and Aniline Derivatives Using CoCl 2 as a Catalyst.

The acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines has been achieved mostly by employing precious-metal-based complexes or complexes of earth-abundant metal ions with sensitive and complicated ligand systems as catalysts mostly under harsh reaction conditions. Methodologies using readily available earth-abundant metal salts as catalysts without the requirement of ligand, oxidant, or any external additives are not explored. We report an unprecedented microwave-assisted CoCl 2 -catalyzed acceptorless dehydrogenative coupling of benzyl alcohol and amine for the synthesis of E -aldimines, N -heterocycles, and H 2 under mild condition, without any complicated exogenous ligand template, oxidant, or other additives. This environmentally benign methodology exhibits broad substrate scope (43 including 7 new products) with fair functional-group tolerance on the aniline ring. Detection of metal-associated intermediate by gas chromatography (GC) and HRMS, H 2 detection by GC, and kinetic isotope effect reveal the mechanism of this CoCl 2- catalyzed reaction to be via ADC. Furthermore, kinetic experiments and Hammett analysis with variation in the nature of substituents over the aniline ring reveal the insight into the reaction mechanism with different substituents.