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)IrV(H)(Bpin)3 (NN'Si = 6-[(di-tert-butylsilyl)methyl]-1,10-phenanthroline) complex is a resting state, and the (NN'Si)IrIII(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)IrV(H)(Bpin)3 to generate the active species (NN'Si)IrIII(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)IrV(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)IrV(H)2(Bpin)2 generated after product formation is slightly more stable than resting-state (NN'Si)IrV(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.