Manganese-catalyzed [2+2+2] Cycloaddition Reactions

We report the cyclotrimerization reactions of triynes using Mn(I) complexes derived from MnBr(CO)5 and phosphine ligands such as 1,1-bis(diphenylphosphino)methane (dppm). These reactions are driven by irradiation under mild conditions (30-80 °C) without the need of additional photoinitiators. Our catalytic screening revealed that counter anions and ligands significantly influence the process. This method accommodates a broad range of functionalities in the substrates, including alkyl, aryl, Bpin, TMS, PPh2, pyridyl, and thienyl moieties, without notable interference in the transformation. Additionally, this method enables reactions with oligoalkynes like (un)substituted hexaynes, producing two-fold cyclization products in excellent yields. Under stoichiometric conditions, the cyclization of diynes with phosphaalkynes results in the first photochemical synthesis of phosphinines. Experimental and theoretical mechanistic studies indicate that the dissociation of the diphosphine ligand precedes the involvement of the Mn-carbonyl species in the catalytic cycle. The ligand plays a crucial role in stabilizing the catalyst during the transformation and preventing the formation of unreactive cluster species.