Base-Tuned Dehydrogenative and Borrowing Hydrogen Strategies for the Synthesis of Benzimidazoles and N-Alkylated Derivatives from o-Phenylenediamines and Alcohols Using a Cationic MnI Catalyst

The synthesis of a neutral MnI complex, [Mn(CO)3Br(P∩N3∩N)-κ2-P,N] (Mn1), and a cationic MnI complex, [Mn(CO)2(P∩N3∩N)2-κ4-P,N,P,N]BF4 (Mn3), derived from phosphine- and pyridine-functionalized 1,2,3-triazole ligand o-Ph2P(C6H4)C(CH)-1,2,3-N3(CH2)(Py) (L), referred to as “P∩N3∩N”, is described. The cationic complex Mn3 was successfully applied as a catalyst in both base-tuned dehydrogenative and hydrogen borrowing strategies for the synthesis of benzimidazoles and N-alkylated derivatives from diamines and alcohols. Notably, this study represents the first example of a base-metal-catalyzed synthesis of mono-N-alkylated and unsymmetrical N,N′-dialkylated amines. A series of control experiments, complemented by density functional theory (DFT) calculations, provided mechanistic insights, indicating that the formation of a [Mn-H] intermediate constitutes the rate-limiting step in the catalytic cycle.