Theoretical Insight into Catalysis of the Aluminabenzene–Iridium Complex for C(sp3)–H Borylation of NEt3: How to Control α- and β-Regioselectivities?

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

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.

The Difference in Ir-Catalyzed C(sp2)-H and C(sp3)-H Bond Activation Assisted by a Directing Group: Cyclometalation via Cis- or Trans-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 IrIII/IrV catalytic cycle, where the tetra-coordinated (C, N)IrIII(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 (ΔG°‡ = 28.5 kcal mol-1) of the C(sp2)-H bond via trans-chelation to form cyclometalation is lower than that (41.4 kcal mol-1) via cis-chelation. In contrast, the ΔG°‡ (26.6 kcal mol-1) of the C(sp3)-H bond via cis-chelation to form cyclometalation is lower than that (34.3 kcal mol-1) via trans-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.

DFT calculations in solution systems: solvation energy, dispersion energy and entropy

DFT calculations of reaction mechanisms in solution have always been a hot topic, especially for transition-metal-catalyzed reactions. The calculation of solvation energy is performed using either the polarizable continuum model (PCM) or the universal solvation model SMD. The PCM calculation is very sensitive to the choice of atomic radii to form a cavity, where the self-consistent isodensity PCM (SCI-PCM) has been recognized as the best choice and our IDSCRF radii can provide a similar cavity. Moving from a gas-phase case to a solution case, dispersion energy and entropy should be carefully treated. The solvent-solute dispersion is also important in solution systems, and it should be calculated together with the solute dispersion. Only half of the solvent-solute dispersion energy from the PCM calculation belongs to the solute molecules to maintain a thermal equilibrium between a solute molecule and its cavity, similar to the treatment of electrostatic energy. Relative solute dispersion energy should also be shared equally with the newly formed cavity. The entropy change from a gas phase to a liquid phase is quite large, but the modern quantum chemistry programs can only calculate the gas-phase translational entropy based on the idea-gas equation. In this review, we will provide an operable method to calculate the solution translational entropy, which has been coded in our THERMO program.

Theoretical Insight into the Multiple Roles of LiHMDS in Pd-Catalyzed Borylation of Fluorobenzene

Pd-catalyzed borylation of fluorobenzene was theoretically studied. DFT calculations revealed that the reaction occurs through an unprecedented 3 + 6-membered ring transition state, in which one LiHMDS (HMDS = hexamethyldisilazane) acts as a ligand and another LiHMDS is essential to provide Li···N and Li···F interactions, overcoming the large destabilization of the strong phenyl-F bond distortion. The characteristic feature of LiHMDS was elucidated by comparing it with HMDS and NaHMDS analogues.