Cu-ABNO Catalyst for the Synthesis of Quinolines and Pyrazines via Aerobic Double Dehydrogenation of Alcohols

In this report, a new imidazole- and amide-functionalized pincer-like Cu(II) complex (1) was synthesized and characterized. By employing 1 and 9-azabicyclo[3.3.1]nonane NH-Oxyl (ABNOH), a catalytic protocol for alcohol oxidation and the subsequent alcohol oxidation-triggered synthesis of quinolines and pyrazines were explored. Alcohols such as 2-aminoaryl alcohols were also oxidized efficiently. As carbonyls from 2-arylaminobenzyl alcohols and secondary alcohols are synthons for quinolines, we explored their synthesis directly from alcohols. The protocol was quite efficient and completed the reaction in only ∼5-10 h. Combinations such as (a) primary 2-arylaminobenzyl alcohols with secondary alcohols or their ketones and (b) secondary 2-arylaminobenzyl alcohols with secondary alcohols or their ketones were found to be very effective for the synthesis of quinolines. The protocol was also successful for the synthesis of various pyrazines from 1,2-diols and 1,2-diaminobenzenes in 10 h. Mechanistic investigations showed that the generated complex acted as an active catalyst: it activated O₂ and subsequently with the cooperation of 9-azabicyclo[3.3.1]nonane N-Oxyl (ABNO^•) activated the α-CH hydrogen of coordinated alkoxide. Then, Cu(II)/Cu(I) reduction led to the formation of carbonyl compounds, which via successive C-C/C-N coupling reactions resulted in heterocycles in the presence of KO^tBu and 1.