The Difference in Ir-Catalyzed C(sp²)–H and C(sp³)–H Bond Activation Assisted by a Directing Group: Cyclometalation via <i>Cis</i>- or <i>Trans</i>-Chelation?

Iridium-catalyzed C-H borylation of aromatic and aliphatic hydrocarbons assisted by a directing group was theoretically investigated. Density functional theory (DFT) calculations revealed both Ir-catalyzed C(sp²)-H and C(sp³)-H borylations via an Ir^III/Ir^V catalytic cycle, where the tetra-coordinated (C, N)Ir^III(Bpin)₂ complex with two vacant sites is an active species. Dramatically, the orientation of cyclometalation for C(sp²)-H bond activation assisted by a directing group is different from the C(sp³)-H one. The activation energy (Δ<i>G</i>^°‡ = 28.5 kcal mol^-1) of the C(sp²)-H bond via <i>trans</i>-chelation to form cyclometalation is lower than that (41.4 kcal mol^-1) via <i>cis</i>-chelation. In contrast, the Δ<i>G</i>^°‡ (26.6 kcal mol^-1) of the C(sp³)-H bond via <i>cis</i>-chelation to form cyclometalation is lower than that (34.3 kcal mol^-1) via <i>trans</i>-chelation. In addition, the rate-determining step of Ir-catalyzed C(sp²)-H borylation is oxidative addition of the C(sp²)-H bond, while that of C(sp³)-H analogues is hydride migration. Such differences arise from not only the differences in the steric hindrance of the C(sp²) and secondary C(sp³) atoms but also the differences in the trans effect and steric effect of the two vacant sites of active species. These findings were expected to facilitate further studies on the design and synthesis of innovative ligands for Ir-catalyzed C-H borylation.