Hydrosilylation of Ketones With Manganese(I) Thiopyridine Catalysts

Abstract

Hydrosilylation of ketones using manganese complexes has emerged as an efficient and safer alternative to traditional reduction methods. Reported manganese(I) systems are typically monomeric Mn(I) carbonyl bromide catalysts, while dimeric systems have remained largely unexplored. Here, three manganese complexes of the dimeric type [{Mn(6-R-PyS)(CO)₃}₂] (R = H 1, CH₃ 2, CF₃ 3) featuring thiopyridine ligands and Mn₂S₂ cores are found to be catalysts in the visible light-induced hydrosilylation of ketones at room temperature. Notably, optimal reactivity was achieved when the irradiation wavelength was set at 427 nm. Furthermore, a catalyst loading of 0.1 mol% was sufficient to achieve full conversion within 90 min over a wide scope of acetophenones and aliphatic ketones. However, the reactivity was drastically reduced when the sterically demanding, electron withdrawing CF₃ group was introduced to the ligand. An induction time of 10 min was observed that can be attributed to the formation of the active species, after which the reaction was found to proceed without irradiation. Further investigations into the mechanism revealed that upon irradiation of solutions of the complexes, CO is released forming an undefined paramagnetic species. In conclusion, for fast catalysis with the dinuclear Mn(I) complexes sterically unhindered ligands and activation with light are required. Furthermore, the catalytic reaction is amenable to continuous flow chemistry as exemplified by the hydrosilylation of acetophenone with 0.1 mol% of catalyst 1. When carried out in a photoflow reactor, the reaction is completed after 14.2-min resistant time.