An Efficient Synthesis of Benzimidazoles via Palladium-Catalyzed Amine Exchange Reaction from Trialkylamines to o-Phenylenediamine in an Aqueous Medium

摘要

Transition metal-catalyzed alkyl group transfer between alkylamines has been known as amine exchange reaction (amine scrambling reaction) and used for the synthesis of unsymmetrical amines and /V-heterocycles and the study of the metabolism of amines. During the course of our studies directed towards transition metal-catalyzed C-N bond activation of alkylamines, we developed an alkyl (or alkanol) group transfer from alkylamines (or alkanolamines) to N-atom of anilines as well as a-carbon of ketones, which leads to a regioselective α-alkylation of ketones. The former transfer eventually leads to indoles and quinolines under the employed reaction conditions. However, except for our findings, there have been known only a few examples for the synthesis of N-heterocycles using such an amine exchange reaction. It is known that hydropyrimidines, imidazolidines and imidazoles could be formed by palladium-catalyzed intermolecular amine exchange reaction between diamines and alkylamines. Diamines were found to be cyclized to pyrrolidine, piperidine, and azepane via ruthenium-catalyzed intramolecular amine exchange reaction. On the other hand, in connection with this report, Murahashi el al. reported that N-methylbenzylamine reacts with ophenylenediamine in the presence of Pd/C to give 2-phenylbenzimidazole and 1-benzyl-2-phenylbenzimidazole in 37% and 25% yields, respectively. Under these circumstances, the present reaction was disclosed during the course of seeking for a more efficient catalytic system on an intrinsic amine exchange reaction. Herein this report describes an efficient synthesis of benzimidazoles via palladium-catalyzed amine exchange reaction from trialkylamines to o-phenylenediamine in an aqueous medium.