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[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.

Hydroboration of CO2 to Methyl Boronate Catalyzed by a Manganese Pincer Complex: Insights into the Reaction Mechanism and Ligand Effect

“…Then, Gonsalvi and Kirchner et al showed that Mn­(I) complexes supported by PNP pincer ligand bearing N–H moieties in the backbone were able to bring about the efficient and selective hydroboration of CO 2 to methoxyboranes, which is one of the few examples of CO 2 borohydride catalyzed by the manganese-based catalyst . Similar works have been reported since then, − and there are few studies on the mechanism of CO 2 hydroboration using manganese-based catalysts.…”

The transition metal-catalysed hydroboration reaction

“…In many cases for the hydroboration of carbonyls, the catalyst functions mainly as a Lewis acid, activating the carbonyl carbon for nucleophilic attack by the hydride of the borane reagent. A number of different transition metals have been used to catalyse this reaction, 430 including rhodium, 424,431 titanium, 401,402,[432][433][434][435][436][437][438] hafnium, 403 molybdenum, 325 silver, 406 cobalt, 169,[439][440][441][442] ruthenium, 404 palladium, 414,443 zinc, 412,413,444-450 iron, 415,[418][419][420]439,440,451,452 nickel, 409,410 manganese, 408,440,453 copper, 421,422 and yttrium. 454,455 Chiral diols combined with titanium(IV) complexes provide up to 99 : 1 enantiomeric ratios in the hydroboration of simple ketones with catecholborane.…”

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

“…458 Baik and Baker collaboratively reported a similar transformation with bi-ligated Mn-amide complex Mn111 (Table 30, entry 5). 459 Although the scope of the reaction was not extensively studied, the catalyst was active under mild conditions, and a lower reaction time was required for complete conversion.…”