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

A combined
experimental and mechanistic study of the chemoselective
hydroboration of carbonyls by the paramagnetic bis-amido Mn­[SMeNSMe]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.