Theoretical Insight\ninto the Multiple Roles of the\nSilyl-Phenanthroline Ligand in Ir-Catalyzed C(sp<sup>3</sup>)–H\nBorylation

Silyl-phenanthroline\n(NN′Si) ligand ancillary iridium-catalyzed\nC(sp<sup>3</sup>)–H borylation is investigated theoretically.\nDensity functional theory calculations clearly disclose that the (NN′Si)Ir<sup>V</sup>(H)(Bpin)<sub>3</sub> (NN′Si = 6-[(di-<i>tert</i>-butylsilyl)methyl]-1,10-phenanthroline) complex is a resting state,\nand the (NN′Si)Ir<sup>III</sup>(Bpin)<sub>2</sub> complex serves\nas an active species in the catalytic cycle. The remarkably high activity\nof this type of a catalyst arises from the rapid reductive elimination\nof HBpin from (NN′Si)Ir<sup>V</sup>(H)(Bpin)<sub>3</sub> to\ngenerate the active species (NN′Si)Ir<sup>III</sup>(Bpin)<sub>2</sub>. The silyl group plays a crucial role in accelerating the\ncrucial hydride-migration elementary step, which allows the isomerization\nof the (NN′Si)Ir<sup>V</sup>(R)(H)(Bpin)<sub>2</sub> intermediate\nto achieve the C(sp<sup>3</sup>)–B reductive elimination and\nafford the borylated product. Although C(sp<sup>3</sup>)–H\nborylation with HBpin is thermodynamically unfavorable, the Ir-dihydride\nintermediate (NN′Si)Ir<sup>V</sup>(H)<sub>2</sub>(Bpin)<sub>2</sub> generated after product formation is slightly more stable\nthan resting-state (NN′Si)Ir<sup>V</sup>(H)(Bpin)<sub>3</sub> in this catalytic cycle, which is an important driving force for\nthe HBpin reaction. Such success was not attained by many other traditional\nbidentate ligands. The unique regioselectivity of <i>n</i>-butyl ethyl ether and 2-methylheptane, induced by the NN′Si-pincer\nligand, is well reproduced and the underlying reason for the selectivity\nis clearly elucidated.