N-Heteroaryl Carbamates from Carbon Dioxide via Chemoselective Superbase Catalysis : Substrate Scope and Mechanistic Investigation

Abstract

We report a mild superbase-catalyzed and nitrogen-selective carboxylation of N-heteroaryls, with subsequent alkylation enabling the synthesis of drug-like O-alkyl carbamates in good yields (av. 86%). Our findings suggest a partial revision of the current mechanistic understanding as superbases upon mixing with indoles and azoles generally form uncharged hydrogen-bonded complexes and not ionic salts as previously proposed. However, when these complexes are exposed to CO2, carbamate salts are formed. These can be categorized into two subgroups, stable and fluxional carbamate salts, where the latter undergo fast and reversible CO2 exchange, thus being poor substrates for alkylation. Experiments and DFT calculations indicate that the fluxional behavior is primarily caused by substrate-specific electronic destabilization effects. The degree of destabilization depends on the number of nitrogen atoms within and the functional group substitution on the heterocyclic ring structures. Fluxionality can be compensated for by the use of lower temperatures and/or higher CO2 pressures as both measures stabilize the carbamate salts sufficiently, enabling subsequent alkylation.