A Protic Mn(I) Complex Based on a Naphthyridine-N-oxide Scaffold: Protonation/Deprotonation Studies and Catalytic Applications for Alkylation of Ketones

Abstract

A Mn(I) complex (1) bearing a proton responsive hydroxy unit on 1,8-naphthyridine-N-oxide scaffold (L1H) was synthesized. The molecular structure of 1 revealed the lactim form of the ligand. The corresponding deprotonated lactam complexes [18-C-6-K·2] and 3 were prepared and structurally characterized. The acid–base equilibrium between the lactim and lactam forms was studied by 1H NMR and UV–vis spectra. The catalytic efficiency of 1 was evaluated by performing α-alkylation reaction of ketones with primary alcohols. The scope of the α-alkylation reaction is broad in terms of both ketones and alcohols. The efficacy of the protic catalyst is demonstrated in the alkylation of the bioactive steroids progesterone and pregnenolone. A controlled catalyst [Mn(L2)(CO)3Br] (4), which is structurally similar to 1 but devoid of the proton responsive hydroxy unit, shows significantly reduced catalytic efficiency validating the crucial role of the hydroxy functionality in 1. Kinetic study, control reactions, and deuterium labeling experiments were conducted to gain mechanistic insights.