Mechanism of Rhodium-Catalyzed Formyl Activation: A Computational Study

Metal-catalyzed transfer hydroformylation is an important way of cleaving C–C bonds and constructing new double bonds. The newly reported density functional theory (DFT) method, M11-L, has been used to clarify the mechanism of the rhodium-catalyzed transfer hydroformylation reported by Dong et al. DFT calculations depict a deformylation and formylation reaction pathway. The deformylation step involves an oxidative addition to the formyl C–H bond, deprotonation with a counterion, decarbonylation, and β-hydride elimination. After olefin exchange, the formylation step takes place via olefin insertion into the Rh–H bond, carbonyl insertion, and a final protonation with the conjugate acid of the counterion. Theoretical calculations indicate that the alkalinity of the counterion is important for this reaction because both deprotonation and protonation occur during the catalytic cycle. A theoretical study into the formyl acceptor shows that the driving force of the reaction is correlated with the stability of the unsaturated bond in the acceptor. Our computational results suggest that alkynes or ring-strained olefins could be used as formyl acceptors in this reaction

Mechanism of Synergistic Cu(II)/Cu(I)-Mediated Alkyne Coupling: Dinuclear 1,2-Reductive Elimination after Minimum Energy Crossing Point

An in-depth theoretical study of synergistic Cu­(II)/Cu­(I)-mediated alkyne coupling was performed to reveal the detailed mechanism for C–C bond formation, which proceeded via an unusual dinuclear 1,2-reductive elimination. Because the reactant for dinuclear 1,2-reductive elimination was calculated to be triplet while the products were singlet, the minimum energy crossing point (MECP) was introduced to the Cu/TMEDA/alkyne system to clarify the spin crossing between triplet state and singlet state potential energy surfaces. Computational results suggest that C–H bond cleavage solely catalyzed by the Cu­(I) cation is the rate-determining step of this reaction and Cu­(II)-mediated dinuclear 1,2-reductive elimination after the MECP is a facile process. These conclusions are in good agreement with our previous experimental results

Ir(III)/Ir(V) or Ir(I)/Ir(III) Catalytic Cycle? Steric-Effect-Controlled Mechanism for the <i>para</i>-C–H Borylation of Arenes

Density functional theory method N12 was used to study the mechanism of the [Ir­(cod)­OH]₂/Xyl–MeO–BIPHEP-catalyzed <i>para</i>-selective C–H borylation reaction. The results revealed that the use of a bulky diphosphine ligand such as Xyl–MeO–BIPHEP was unfavorable for the previously proposed iridium­(III)/iridium­(V) catalytic cycle because it resulted in considerable steric repulsion in the hepta-coordinated iridium­(V) intermediate. Inspired by this steric effect, we have proposed a novel iridium­(I)-/iridium­(III)-based catalytic cycle for this transformation and shown that it can be used to account for the experimental results. The iridium­(I)/iridium­(III) catalytic cycle induced by this steric effect consists of several steps, including (i) the oxidative addition of the C–H bond of the substrate to an active iridium­(I) boryl complex; (ii) the reductive elimination of a C–B bond; (iii) the oxidative addition of B₂pin₂ to an iridium­(I) hydride complex; and (iv) the reductive elimination of a B–H bond. Notably, the computed regioselectivity of this reaction was consistent with the experimental observations. The high <i>para</i>-selectivity of this reaction was also explained using structural analysis and a 2D contour model, which revealed that the strong steric repulsion between the diphosphine ligand and the <i>meta</i>-substituents resulted in a higher energy barrier for <i>meta</i>-C–H activation

Ir(III)/Ir(V) or Ir(I)/Ir(III) Catalytic Cycle? Steric-Effect-Controlled Mechanism for the <i>para</i>-C–H Borylation of Arenes

Density functional theory method N12 was used to study the mechanism of the [Ir­(cod)­OH]₂/Xyl–MeO–BIPHEP-catalyzed <i>para</i>-selective C–H borylation reaction. The results revealed that the use of a bulky diphosphine ligand such as Xyl–MeO–BIPHEP was unfavorable for the previously proposed iridium­(III)/iridium­(V) catalytic cycle because it resulted in considerable steric repulsion in the hepta-coordinated iridium­(V) intermediate. Inspired by this steric effect, we have proposed a novel iridium­(I)-/iridium­(III)-based catalytic cycle for this transformation and shown that it can be used to account for the experimental results. The iridium­(I)/iridium­(III) catalytic cycle induced by this steric effect consists of several steps, including (i) the oxidative addition of the C–H bond of the substrate to an active iridium­(I) boryl complex; (ii) the reductive elimination of a C–B bond; (iii) the oxidative addition of B₂pin₂ to an iridium­(I) hydride complex; and (iv) the reductive elimination of a B–H bond. Notably, the computed regioselectivity of this reaction was consistent with the experimental observations. The high <i>para</i>-selectivity of this reaction was also explained using structural analysis and a 2D contour model, which revealed that the strong steric repulsion between the diphosphine ligand and the <i>meta</i>-substituents resulted in a higher energy barrier for <i>meta</i>-C–H activation

Triazole-phosphine Pd(II)-Enabled Dehydrogenation of Alcohols or Amines: A Combination of Experimental and Theoretical Study

We describe a novel triazole-phosphine Pd(II) (TPP) complex-catalyzed dehydrogenation reaction of alcohols or amines by using iodobenzene as the oxidant, in which a unique butterfly TPP dimer is first prepared via a three-component reaction of 1,2,3-triazole, P(Cy)3, and PdCl2 and the competitive cross-coupling reaction of iodobenzene with alcohols or amines could be avoided under TPP catalysis. In particular, the primary alcohols and imines can be further oxidized into acids or nitriles in a tunable manner, respectively. Preliminary mechanistic results by density functional theory calculation suggest that this reaction follows the Pd(II)–Pd(IV) catalytic pathway and the process of TPP-catalyzed oxidation dehydrogenation of alcohol or amine to form unsaturated bonds and Pd(II)–H species generated before the oxidative addition of TPP with iodobenzene, thereby avoiding competitive cross-coupling

C(sp²)–C(sp²) Reductive Elimination from Well-Defined Diarylgold(III) Complexes

A series of well-defined phosphine-ligated diarylgold­(III) complexes <i>cis</i>-[Au­(L)­(Ar_F)­(Ar′)­(Cl)] were prepared, and detailed kinetics of the C­(sp²)–C­(sp²) reductive elimination from these complexes were studied. The mechanism of the reductive elimination from the complexes <i>cis</i>-[Au­(L)­(Ar_F)­(Ar′)­(Cl)] was further studied by theoretical calculations. The combination of experimental and theoretical results suggests that the biaryl reductive elimination from organogold­(III) complexes <i>cis</i>-[Au­(L)­(Ar_F)­(Ar′)­(Cl)] proceeds through a concerted biaryl-forming pathway from the four-coordinated Au­(III) metal center. These studies also disclose that the steric hindrance of the phosphine ligands plays a major role in promoting the biaryl-forming reductive elimination from diarylgold­(III) complexes <i>cis</i>-[Au­(L)­(Ar_F)­(Ar′)­(Cl)], while electronic properties of these ligands have a much smaller effect. Futhermore, it was found that the complexes with more weakly electron withdrawing aryl ligands undergo reductive elimination more quickly and the elimination rate is not sensitive to the polarity of the solvents

Triazole-phosphine Pd(II)-Enabled Dehydrogenation of Alcohols or Amines: A Combination of Experimental and Theoretical Study

We describe a novel triazole-phosphine Pd(II) (TPP) complex-catalyzed dehydrogenation reaction of alcohols or amines by using iodobenzene as the oxidant, in which a unique butterfly TPP dimer is first prepared via a three-component reaction of 1,2,3-triazole, P(Cy)3, and PdCl2 and the competitive cross-coupling reaction of iodobenzene with alcohols or amines could be avoided under TPP catalysis. In particular, the primary alcohols and imines can be further oxidized into acids or nitriles in a tunable manner, respectively. Preliminary mechanistic results by density functional theory calculation suggest that this reaction follows the Pd(II)–Pd(IV) catalytic pathway and the process of TPP-catalyzed oxidation dehydrogenation of alcohol or amine to form unsaturated bonds and Pd(II)–H species generated before the oxidative addition of TPP with iodobenzene, thereby avoiding competitive cross-coupling

Direct Observation of Reduction of Cu(II) to Cu(I) by Terminal Alkynes

X-ray absorption spectroscopy and <i>in situ</i> electron paramagnetic resonance evidence were provided for the reduction of Cu­(II) to Cu­(I) species by alkynes in the presence of tetramethylethylenediamine (TMEDA), in which TMEDA plays dual roles as both ligand and base. The structures of the starting Cu­(II) species and the obtained Cu­(I) species were determined as (TMEDA)­CuCl₂ and [(TMEDA)­CuCl]₂ dimer, respectively

Cu(II)–Cu(I) Synergistic Cooperation to Lead the Alkyne C–H Activation

An efficient alkyne C–H activation and homocoupling procedure has been studied which indicates that a Cu­(II)/Cu­(I) synergistic cooperation might be involved. <i>In situ</i> Raman spectroscopy was employed to study kinetic behavior, drawing the conclusion that Cu­(I) rather than Cu­(II) participates in the rate-determining step. IR, EPR, and X-ray absorption spectroscopy evidence were provided for structural information, indicating that Cu­(I) has a stronger interaction with alkyne than Cu­(II) in the C–H activation step. Kinetics study showed Cu­(II) plays a role as oxidant in C–C bond construction step, which was a fast step in the reaction. X-band EPR spectroscopy showed that the coordination environment of CuCl₂(TMEDA) was affected by Cu­(I). A putative mechanism with Cu­(I)–Cu­(II) synergistic cooperation procedure is proposed for the reaction