Theoretical Insight into the Multiple Roles of the Silyl-Phenanthroline Ligand in Ir-Catalyzed C(sp3)–H Borylation

Abstract

Silyl-phenanthroline (NN′Si) ligand ancillary iridium-catalyzed C(sp³)–H borylation is investigated theoretically. Density functional theory calculations clearly disclose that the (NN′Si)Ir^V(H)(Bpin)₃ (NN′Si = 6-[(di-tert-butylsilyl)methyl]-1,10-phenanthroline) complex is a resting state, and the (NN′Si)Ir^III(Bpin)₂ complex serves as an active species in the catalytic cycle. The remarkably high activity of this type of a catalyst arises from the rapid reductive elimination of HBpin from (NN′Si)Ir^V(H)(Bpin)₃ to generate the active species (NN′Si)Ir^III(Bpin)₂. The silyl group plays a crucial role in accelerating the crucial hydride-migration elementary step, which allows the isomerization of the (NN′Si)Ir^V(R)(H)(Bpin)₂ intermediate to achieve the C(sp³)–B reductive elimination and afford the borylated product. Although C(sp³)–H borylation with HBpin is thermodynamically unfavorable, the Ir-dihydride intermediate (NN′Si)Ir^V(H)₂(Bpin)₂ generated after product formation is slightly more stable than resting-state (NN′Si)Ir^V(H)(Bpin)₃ in this catalytic cycle, which is an important driving force for the HBpin reaction. Such success was not attained by many other traditional bidentate ligands. The unique regioselectivity of n-butyl ethyl ether and 2-methylheptane, induced by the NN′Si-pincer ligand, is well reproduced and the underlying reason for the selectivity is clearly elucidated.