Eosin Y-catalyzed reductive homocoupling of para-quinone methides under visible-light

In this manuscript, we demonstrate a visible-light driven dimerization of para-quinone methides using eosin Y catalyst via a reductive homocoupling process. This mild and operationally simple methodology was found to be compatible with a variety of differently substituted para-quinone methides and a broad range of tetra-arylethane derivatives were obtained in moderate to good yields (47%-87%).

Reactivity of metal dioxo complexes

Metal dioxo chemistry and its diverse reactivity are presented with an emphasis on the mechanisms of reactivity. Work from approximately the last decade is surveyed and organized by metal. In particular, the chemistry of cis-dioxo metal complexes is discussed at length. Reactions are grouped by generic type, including addition across a metal oxo bond, oxygen atom transfer, and radical atom transfer reactions. Attention is given to advances in deoxygenation chemistry, oxidation chemistry, and reductive transformations.

Transition metal-catalyzed deoxydehydration: missing pieces of the puzzle

Deoxydehydration (DODH) is a transformation that converts a vicinal diol into an olefin with the help of a sacrificial reductant.

Molybdenum-Catalyzed Deoxygenation Coupling of Lignin-Derived Alcohols for Functionalized Bibenzyl Chemicals

With the growing demand for sustainability and reducing CO₂ footprint, lignocellulosic biomass has attracted much attention as a renewable, carbon-neutral and low-cost feedstock for the production of chemicals and fuels. To realize efficient utilization of biomass resource, it is essential to selectively alter the high degree of oxygen functionality of biomass-derivates. Herein, we introduced a novel procedure to transform renewable lignin-derived alcohols to various functionalized bibenzyl chemicals. This strategy relied on a short deoxygenation coupling pathway with economical molybdenum catalyst. A well-designed H-donor experiment was performed to investigate the mechanism of this Mo-catalyzed process. It was proven that benzyl carbon-radical was the most possible intermediate to form the bibenzyl products. It was also discovered that the para methoxy and phenolic hydroxyl groups could stabilize the corresponding radical intermediates and then facilitate to selectively obtain bibenzyl products. Our research provides a promising application to produce functionalized aromatics from biomass-derived materials.

Deoxygenative Transition-Metal-Promoted Reductive Coupling and Cross-Coupling of Alcohols and Epoxides

The prospective utilization of abundant, CO2-neutral, renewable feedstocks is driving the discovery and development of new reactions that refunctionalize oxygen-rich substrates such as alcohols and polyols through C–O bond activation. In this review, we highlight the development of transition-metal-promoted reactions of renewable alcohols and epoxides that result in carbon–carbon bond-formation. These include reductive self-coupling reactions and cross-coupling reactions of alcohols with alkenes and arene derivatives. Early approaches to reductive couplings employed stoichiometric amounts of low-valent transition-metal reagents to form the corresponding hydrocarbon dimers. More recently, the use of redox-active transition-metal catalysts together with a reductant has enhanced the practical applications and scope of the reductive coupling of alcohols. Inclusion of other reaction partners with alcohols such as unsaturated hydrocarbons and main-group organometallics has further expanded the diversity of carbon skeletons accessible and the potential for applications in chemical synthesis. Catalytic reductive coupling and cross-coupling reactions of epoxides are also highlighted. Mechanistic insights into the means of C–O activation and C–C bond formation, where available, are also highlighted.

1 Introduction

2 Stoichiometric Reductive Coupling of Alcohols

3 Catalytic Reductive Coupling of Alcohols

3.1 Heterogeneous Catalysis

3.2 Homogeneous Catalysis

4 Reductive Cross-Coupling of Alcohols

4.1 Reductive Alkylation

4.2 Reductive Addition to Olefins

5 Epoxide Reductive Coupling Reactions

6 Conclusions and Future Directions

Vanadium Pyridonate Catalysts: Isolation of Intermediates in the Reductive Coupling of Alcohols

The reductive coupling of alcohols using vanadium pyridonate catalysts is reported. This attractive approach for C(sp³)-C(sp³) bond formation uses an oxophilic, earth-abundant metal for a catalytic deoxygenation reaction. Several pyridonate complexes of vanadium were synthesized, giving insight into the coordination chemistry of this understudied class of compounds. Isolated intermediates provide experimental mechanistic evidence that complements reported computational mechanistic proposals for the reductive coupling of alcohols. In contrast to previous mononuclear vanadium(V)/vanadium(III)/vanadium(IV) cycles, this pyridonate catalyst system is proposed to proceed by a vanadium(III)/vanadium(IV) cycle involving bimetallic intermediates.

Oxo-Rhenium-Catalyzed Radical Addition of Benzylic Alcohols to Olefins

Although carbon radicals generated from a variety of alcohol derivatives have proven valuable in coupling and addition reactions, the direct use of alcohols as synthetically useful radical sources is less known. In this report, benzylic alcohols are shown to be effective radical precursors for addition reactions to alkenes when treated with triphenylphosphine or piperidine with the catalyst ReIO₂(PPh₃)₂ (I).

Mechanistic Features of the Oxidation-Reductive Coupling of Alcohols Catalyzed by Oxo-Vanadium Complexes

The oxo-vanadium-catalyzed redox disproportionation of activated alcohols (oxidation-reductive coupling, Ox-RC) produces carbonyl compounds and hydrocarbon dimers. A mechanistic study of this novel reaction is reported herein. Following our initial disclosure, new findings include the following: (1) The [(salimin)VO₂]^--catalyzed Ox-RC of Ph₂CHOH in the presence of fluorene affords the products of H-atom abstraction and all possible hydrocarbon dimers. (2) Electronic substituent effects on the relative rates of Ox-RC with respect to 4-X-BnOH reactants and Bu₄N[(Y-salimin)VO₂] catalysts (1a-c) reveal (a) a correlation of the oxidation rate of X-BnOH reactants with the radical σ parameter and (b) correlation of the oxidation rate for (Y-salimin)VO₂^- with the standard Hammett σ parameter. (3) The ease of electrochemical reduction of 1a-c is Y = NO₂ > OMe > H. (4) Ambient ¹H NMR studies of the interaction of 1 with alcohols suggest only a weak equilibrium association. (5) Density functional theory computational modeling of the Ox-RC reaction supports a ping-pong-type catalytic pathway, beginning with alcohol oxidation by (salimin)VO₂^-, preferably by stepwise-H-atom transfer from the alcohol to 1, affording the carbonyl product and the reduced (salimin)V(III)(OH)₂^-. The reduction half-reaction likely begins with condensation of the latter species with R₂CHOH to give the alkoxide complex (salimin)V(OR)OH^-; homolysis of the R···OV(III)(salimin) bond affords (salimin)V(IV)OH(O)^- and the R-radical; the latter dimerizes and the former can disproportionate via H-transfer to reform catalyst (salimin)VO₂^- (1) and (salimin)V(OH)₂^-.

Oxidation–reductive coupling of alcohols catalyzed by oxo-vanadium complexes

Oxo-vanadium complexes catalyze the novel oxidation–reductive coupling of benzylic and allylic alcohols.

Perrhenate-Catalyzed Deoxydehydration of a Vicinal Diol: A Comparative Density Functional Theory Study

Oxo-rhenium compounds, such as perrhenate salts, have demonstrated preferable activity in catalyzing the deoxydehydration (DODH) reaction in the presence of reductants. Here, the first computational details of the reported DODH mechanisms are presented using the density functional theory (DFT) (M06/6-311+G(d,p)/LANL2DZ) to investigate the conversion of a vicinal diol into the corresponding alkene by ReO₄^- as a catalyst. The DFT studies were carried out to evaluate the DODH mechanisms, from the energy point of view, for the conversion of phenyl-1,2-ethanediol to styrene by perrhenate anion in the presence of PPh₃ as a reductant through a detailed comparison of two potential pathways including pathway A and pathway B. Pathway A includes the sequence of condensation of oxo-Re(VII) with diol before the reduction of Re(VII) to Re(V), whereas pathway B involves the reduction of oxo-Re(VII) to oxo-Re(V) before the condensation process. In pathway B, two basic routes (B1 and B2) are possible, which can take place through different reaction steps, including the extrusion of alkene from Re(V)-diolate in route B1, and the second reduction of the Re(V)-diolate by reductant and then the extrusion of alkene from the Re(III)-diolate intermediate in route B2. The intermediates and the Gibbs free energy changes, including ΔG°_g and ΔG°_sol, have been calculated for alternative pathways (A and B) in the gas and solvent (chlorobenzene and methanol) phases and compared to each other. In addition, the transition states and the activation energy barriers for two pathways (A and B) in the gas phase and in chlorobenzene have been calculated. The key transition states include the nucleophilic attack of PPh₃ on an Re═O bond, the dissociation of OPPh₃ from the rhenium moiety, the transfer of an H atom of diol to the oxo ligand in an oxo-Re bond through the condensation step, and the extrusion of styrene from the Re-diolate complexes. The DFT results indicate that the DODH reaction is thermodynamically feasible through both pathways (A and B). However, the calculations reveal that the perrhenate-catalyzed DODH reaction through pathway A has the lowest overall activation barrier energy among the DODH mechanism routes.