A general mechanism for H2 activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (PhDPBPh)­Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (ΔG ‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (ΔG ‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (ΔG ‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future.

Disclosed herein is a novel, redox-neutral protocol for the visible-light-induced radical alkynylation of unactivated olefins. The intramolecular migration of an alkynyl group, by cleaving an inert C-C σ bond, is realized for the first time. A wide range of synthetically useful trifluoroethylated linear alkynes are readily obtained under mild reaction conditions.

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H2 and the hydride transfer. The heterolytic cleavage of H2 is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H2 heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H2. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO2. This ligand assisted pathway is more favorable than the direct attack of hydride to CO2. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide.

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-NC(R)− C(R) 2 −OH]PdCl 2 } (C1, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; C2, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methylphenyl; C3, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-methyoxylphenyl; C4, R = Me, Ar = 2,6-bis(diphenylmethyl)-4-chlorophenyl; C5, R = Ph, Ar = 2,6-dimethylphenyl; C6, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes C1 and C2 were determined by X-ray diffraction. These bidentate N,O-palladium complexes were applied for direct arylation under aerobic conditions. The effects of the reaction conditions and ligand substitution on the catalytic activity were evaluated. Upon a low palladium loading of 0.5 mol %, the bulky palladium complex C6 was successfully used to catalyze the cross-coupling of a variety of five-membered heteroarenes and their benzo-condensed derivatives with (hetero)aryl bromides. The mechanistic investigation on the direct arylation supported the involvement of a Pd(0)/Pd(II) CMD process.

Lewis/Bronsted acid activation plays a key role in hypervalent iodine reagent-mediated reactions. In addition to generally accepted cis-activation or trans-activation, this study reveals another important Lewis/Bronsted acid activation mode, the double-activation. Different from the generally proposed iodine­(III)­iranium SN2 mechanism, the hypervalent difluoro-iodoarene-promoted fluorocyclization of unsaturated alcohol prefers to undergo the metathesis mechanism via an iodine­(III)-π intermediate.