Hydrogen atom transfer from a metal hydride (MHAT) has emerged as a powerful, if puzzling, technique in chemical synthesis. In catalytic MHAT reactions, earth-abundant metal complexes generate stabilized and unstabilized...

Primary benzylic boronate esters are useful intermediates in organic synthesis, but these reagents cannot be prepared by hydroboration. The benzylic C-H borylation of methylarenes would be a method to form these products, but such reactions without neat methylarene or a directing group are unknown. We report an approach to divert the borylation of methylarenes from aromatic positions to benzylic positions with a silylborane as reagent and a new iridium catalyst containing an electron-deficient phenanthroline as ligand. This system forms benzylic boronate esters selectively over the corresponding aryl boronate esters. An Ir diboryl monosilyl complex ligated by the phenanthroline was isolated and determined to be the resting state of the catalyst. Mechanistic studies show that this complex is kinetically competent to be an intermediate in the catalytic process. Kinetic studies of benzylic and aryl C-H borylation catalyzed by various Ir complexes show that the rate of aryl C-H borylation decreases with decreasing electron density at the metal center of the Ir catalyst, but that the rate of benzylic C-H borylation is less sensitive to the degree of electron density at the metal center of the Ir catalyst. Kinetic and computational studies suggest that the two borylation reactions respond differently to the degree of electron density at the metal center because they occur with different turnover-limiting steps. The turnover-limiting step in the borylation of aryl C-H bonds is known to be C-H oxidative addition, but the turnover-limiting step of the borylation of benzylic C-H bonds appears to be an isomerization prior to C-B reductive elimination.

The catalytic hydrofunctionalization of alkenes through radical-polar crossover metal hydrogen atom transfer (MHAT) offers a mild pathway for the introduction of functional groups in sterically congested environments. For M = Co, this reaction is often proposed to proceed through secondary alkylcobalt(IV) intermediates, which have not been characterized unambiguously. Here, we characterize a metastable (salen)Co-(isopropyl) cation, which is capable of forming C−O bonds with alcohols as proposed in the catalytic reaction. Electron nuclear double resonance (ENDOR) spectroscopy of this formally cobalt(IV) species establishes the presence of the cobalt−carbon bond, and accompanying DFT calculations indicate that the unpaired electron is localized on the cobalt center. Both experimental and computational studies show that the cobalt(IV)−carbon bond is stronger than the analogous bond in its cobalt(III) analogue, which is opposite of the usual oxidation state trend of bond energies. This phenomenon is attributable to an inverted ligand field that gives the bond Co δ− −C δ+ character and explains its electrophilic reactivity at the alkyl group. The inverted Co−C bond polarity also stabilizes the formally cobalt(IV) alkyl complex so that it is accessible at unusually low potentials. Even another cobalt(III) complex, [(salen)Co III ] + , is capable of oxidizing (salen)Co III (iPr) to the formally cobalt(IV) state. These results give insight into the electronic structure, energetics, and reactivity of a key reactive intermediate in oxidative MHAT catalysis.

The borylation of methylarenes with a silylborane reagent and a new iridium catalyst containing an electron-deficient phenanthroline ligand selectively leads to benzylic boronate esters over the corresponding aryl boronate esters. -(LARSEN, M. A.; WILSON, C. V.; HARTWIG*, J. F.; J. Am. Chem. Soc. 137 (2015) 26, 8633-8643, http://dx.