When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study on the Mechanistic Preference in Ru-PNNH Pincer Complex Catalyzed Dehydrogenative Coupling Reaction

Metal ligand cooperation (MLC) plays an important role in the development of homogeneous catalysts. Two major MLC modes have generally been proposed, known as the M-L bond mode and the (de)­aromatization mode. To reveal the role of the dual potential functional sites on the MLC process, we present a detailed mechanistic study on a novel-designed Ru-PNNH complex possessing dual potential MLC functional sites for the M-L bond mode and the (de)­aromatization mode, respectively. Our results indicate that the Ru-PNNH complex prefers the M-L mode exclusively during different stages of the catalytic cycle. The unusual double deprotonation process and the mechanistic preference are rationalized. The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the amido group. The origin of the unexpected exclusive mechanistic preference on the M-L bond mechanism is due to the conjugation effect of the amido group, which stabilizes the dearomatized complex and diminishes the driving force of the (de)­aromatization mode. This study highlights the pivotal role of the ligand’s electronic effect on the MLC mechanism and should provide valuable information for the development of highly efficient bifunctional catalysts

The Effect of HSAB on Stereoselectivity: Copper- and Gold-Catalyzed 1,3-Phosphatyloxy and 1,3-Halogen Migration Relay to 1,3-Dienes

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu­(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (<i>Z</i>)-1,3-dienes. In contrast, the relative softer Au­(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (<i>E</i>)-1,3-dienes, where the <i>E</i> type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard–soft/acid–base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates

Copper-Catalyzed Regioselective C–H Sulfonylation of 8‑Aminoquinolines

Copper­(I)-catalyzed 5-sulfonation of quinolines via bidentate-chelation assistance has been developed. The reaction is compatible with a wide range of quinoline substrates and arylsulfonyl chlorides. Experimental and theoretical (DFT) investigation implicated that a single-electron-transfer process is involved in this sulfonylation transformation

Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

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

General H₂ Activation Modes for Lewis Acid–Transition Metal Bifunctional Catalysts

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, (^PhDPB^Ph)­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, (^PhDPB^Ph)Ni catalyst most prefers the synergetic heterolytic mode (Δ<i>G</i>^‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (Δ<i>G</i>^‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (Δ<i>G</i>^‡ = ∼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

Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α‑Hydroxyimine Palladium Complexes

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-NC­(R)–C­(R)₂–OH]­PdCl₂} (<b>C1</b>, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; <b>C2</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methylphenyl; <b>C3</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methyoxylphenyl; <b>C4</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-chlorophenyl; <b>C5</b>, R = Ph, Ar = 2,6-dimethylphenyl; <b>C6</b>, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes <b>C1</b> and <b>C2</b> 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 <b>C6</b> 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

Aerobic and Efficient Direct Arylation of Five-Membered Heteroarenes and Their Benzocondensed Derivatives with Aryl Bromides by Bulky α‑Hydroxyimine Palladium Complexes

In the present work, a series of α-hydroxyimine palladium complexes with bulky substituents (i.e., {[Ar-NC­(R)–C­(R)₂–OH]­PdCl₂} (<b>C1</b>, R = Me, Ar = 2-diphenylmethyl-4,6-dimethylphenyl; <b>C2</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methylphenyl; <b>C3</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-methyoxylphenyl; <b>C4</b>, R = Me, Ar = 2,6-bis­(diphenylmethyl)-4-chlorophenyl; <b>C5</b>, R = Ph, Ar = 2,6-dimethylphenyl; <b>C6</b>, R = Ph, Ar = 2,6-diisopropylphenyl)) were synthesized and characterized. The structures of palladium complexes <b>C1</b> and <b>C2</b> 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 <b>C6</b> 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