Ruthenium pincer complex catalyzed efficient synthesis of quinoline, 2-styrylquinoline and quinazoline derivatives via acceptorless dehydrogenative coupling reactions

The synthesis of N-heterocycles has been considered an emerging area of chemical research due to their extensive utilization in pharmaceuticals, materials science, and natural product synthesis. The development of green, atom-economical, and sustainable strategies for the construction of N-heteroaromatic compounds by employing readily available starting materials is highly desired in the scientific community. In this context, alcohol is found as a cheap, abundant in nature, and biorenewable substrate produced from a diverse range of sustainable resources. The transition metal-catalyzed acceptorless dehydrogenative coupling (ADC) of alcohols provides a prominent synthetic strategy to synthesize N-heterocycles in a sustainable and environmentally benign way since only hydrogen and water are formed as the eco-friendly by-products. Herein, we report an efficient method for synthesis of quinoline and quinazoline derivatives via ADC of alcohols catalysed by a bifunctional ruthenium NNN-pincer complex. With a lower catalyst loading (0.1 mol%), base loading (1 mol%) and shorter reaction time (6 h) under aerial conditions, a wide variety of substituted quinolines and quinazolines were synthesized from 2-aminobenzyl alcohols, secondary alcohols, and nitriles. Further, the catalyst was found to be effective for the synthesis of 2-styrylquinoline derivatives via a one-pot three-component ADC strategy of 2-aminobenzyl alcohol, isopropanol, and primary alcohols. Notably, this catalytic system showed a remarkably high TON of 440 000 for 2-phenylquinoline and a TON of 290 000 for 2-phenylquinazoline, which are the highest reported values hitherto for transition metal-based catalysts. Mechanistic studies were performed to understand the reaction pathway and the role of the catalyst in different steps.

Automated summary

The synthesis of N-heterocycles is important in pharmaceuticals, materials science, and natural product synthesis. This study presents an efficient method for the synthesis of quinoline and quinazoline derivatives through an alcohol-catalyzed acceptorless dehydrogenative coupling (ADC). The use of a bifunctional ruthenium NNN-pincer complex as the catalyst allows the synthesis of a wide range of substituted quinolines and quinazolines with high yields and selectivity.