Air oxidized activated carbon catalyst for aerobic oxidative aromatizations of N-heterocycles

Abstract

A simple “reagent-free” thermal air treatment turns active carbon into a mildly oxidized material with increased quinoidic content that catalytically dehydrogenates saturated N-heterocycles to the corresponding aromatic compounds. Thermal decarboxylation improves the activity of the catalyst further, making it overall more efficient compared to other widely used carbocatalysts such as oxidized carbon nanotubes, graphene oxide and untreated active carbons. The substrate scope covers 1,2,3,4-tetrahydroquinolines (THQ), 1,2,3,4-tetrahydro-β-carbolines and related N-heterocyclic structures. The developed protocol also successfully dehydrogenates 3-(cyclohexenyl)indoles to 3-aryl indoles, opening a concise transition metal-free approach to (hetero)biaryls as exemplified with the synthesis of the core structure of progesterone receptor antagonist. Hammett plots, deuterium KIE measurements and computations at DFT level suggest that bimolecular hydride transfer mechanism is more likely to operate between THQs and the o-quinoidic sites of the catalyst, than the addition–elimination hemiaminal route. Comparison of structural parameters and catalytic performance of various oxidized carbon materials, prepared by different oxidative and optional post treatments, revealed that quinoidic content and surface area correlate with the obtained yields, while carboxylic acid content has a clear inhibiting effect for the studied oxidative dehydrogenations (ODHs). The carbocatalyst itself can be prepared from inexpensive and environmentally benign starting materials and its catalytic activity can be enhanced by a simple thermal oxidation in air that produces no reagent waste. Furthermore, oxygen is used as terminal oxidant, and the carbocatalyst is recyclable at least six times without a notable loss of activity.