Mechanistic Study of Metal-Ligand Cooperativity in Mn(II)-Catalyzed Hydroborations: Hemilabile SNS Ligand Enables Metal Hydride-Free Reaction Pathway

Abstract

A combined experimental and mechanistic study of the chemoselective hydroboration of carbonyls by the paramagnetic bis-amido Mn[(SNSMe)-N-Me](2) complex (1) is described. The catalyst allows for room-temperature hydroboration of carbonyls at low catalyst loadings (0.1 mol %) and reaction times (<30 min). A series of mechanistic studies highlight the significance of bifunctional amido bis(thioether) ligand L to the success of the reaction, insight otherwise difficult to attain in paramagnetic systems. Kinetic studies using variable time normalization analysis revealed no unusual reaction kinetics, indicating the absence of side reactions. A borylated analogue of L was observed and characterized via mass spectrometry. Density functional theory (DFT) calculations showed that thioether hemilability of L is crucial during catalysis for providing the active coordinating site. Also, the frequently proposed Mn-H intermediate was found not to be the active species responsible for catalysis. Instead, an inner-sphere reaction pathway with carbonyl coordination to the metal center and amido-promoted B-H reactivity is proposed to be operative.