The Difference\nin Ir-Catalyzed C(sp<sup>2</sup>)–H\nand C(sp<sup>3</sup>)–H Bond Activation Assisted by a Directing\nGroup: Cyclometalation via <i>Cis</i>- or <i>Trans</i>-Chelation?

Iridium-catalyzed C–H borylation of aromatic and\naliphatic\nhydrocarbons assisted by a directing group was theoretically investigated.\nDensity functional theory (DFT) calculations revealed both Ir-catalyzed\nC(sp<sup>2</sup>)–H and C(sp<sup>3</sup>)–H borylations\nvia an Ir<sup>III</sup>/Ir<sup>V</sup> catalytic cycle, where the\ntetra-coordinated (C, N)Ir<sup>III</sup>(Bpin)<sub>2</sub> complex\nwith two vacant sites is an active species. Dramatically, the orientation\nof cyclometalation for C(sp<sup>2</sup>)–H bond activation\nassisted by a directing group is different from the C(sp<sup>3</sup>)–H one. The activation energy (Δ<i>G</i><sup>°‡</sup> = 28.5 kcal mol<sup>–1</sup>) of the\nC(sp<sup>2</sup>)–H bond via <i>trans</i>-chelation\nto form cyclometalation is lower than that (41.4 kcal mol<sup>–1</sup>) via <i>cis</i>-chelation. In contrast, the Δ<i>G</i><sup>°‡</sup> (26.6 kcal mol<sup>–1</sup>) of the C(sp<sup>3</sup>)–H bond via <i>cis</i>-chelation to form cyclometalation is lower than that (34.3 kcal\nmol<sup>–1</sup>) via <i>trans</i>-chelation. In\naddition, the rate-determining step of Ir-catalyzed C(sp<sup>2</sup>)–H borylation is oxidative addition of the C(sp<sup>2</sup>)–H bond, while that of C(sp<sup>3</sup>)–H analogues\nis hydride migration. Such differences arise from not only the differences\nin the steric hindrance of the C(sp<sup>2</sup>) and secondary C(sp<sup>3</sup>) atoms but also the differences in the trans effect and steric\neffect of the two vacant sites of active species. These findings were\nexpected to facilitate further studies on the design and synthesis\nof innovative ligands for Ir-catalyzed C–H borylation.