A General Regioselective Synthesis of Alcohols by Cobalt-Catalyzed Hydrogenation of Epoxides

Regioselective Ring-Opening of Terminal Epoxides Catalyzed by a Porous Metal Silicate Material

Reductive ring-opening of epoxides is a green pathway for synthesizing highly value-added alcohols. In this study, we present a practically applicable approach for the synthesis of anti-Markovnikov-type alcohols with high yields from aliphatic and aromatic epoxides under mild conditions by developing porous metal silicate (PMS) catalysts. A PMS material PMS-20 consists of cobalt and nickel bimetal redox-active sites, exhibiting exceptional catalytic activity and selectivity in the reductive ring-opening of terminal epoxides with >99% yield of primary alcohols. Comparing with the existing methods using noble metals, PMS-20 exhibits broad substrate scope and excellent functional group tolerance by synergistic work between cobalt and nickel species, which is clarified by dual chamber cell system characterization and theoretical calculation results.

Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4 ]- as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry

We describe a unique catalytic system with an efficient coupling of Ti- and Cr-catalysis in a reaction network that allows the use of [BH4 ]- as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH4 ]- to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical SN 2-type reactivity of [BH4 ]- salts is observed.

Vanadium-catalysed regioselective hydroboration of epoxides for synthesis of secondary alcohols

Regioselective epoxide ring-opening through hydroboration catalysed by a vanadium(III) dialkyl complex supported by a redox-active terpyridine ligand is reported. Secondary alcohols were obtained in high yields via effective Markovnikov hydroboration of terminal epoxides, showcasing a new catalytic application of an earth-abundant vanadium(III) complex.

The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes

The homogeneous catalysis of epoxide hydrogenolysis to give alcohols has recently received significant attention. Catalyst systems have been developed for the selective formation of either the Markovnikov (branched) or anti-Markovnikov (linear) alcohol product. Thus far, the reported catalysts exhibiting Markovnikov selectivity all feature the potential for Noyori/Shvo-type bifunctional catalysis, with either a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide ring-opening have involved cooperative C-O bond hydrogenolysis involving the metal hydride and the acidic pendant group on the ligand, in analogy to the well-documented mechanism of polar double-bond hydrogenation exhibited by catalysts of this type. In this work, we present a combined computational/experimental study of the mechanism of epoxide hydrogenolysis catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find that, at least for these ruthenium systems, the previously proposed bifunctional pathway for epoxide ring-opening is energetically inaccessible; instead, the ring-opening proceeds through opposite-side nucleophilic attack of the ruthenium hydride on the epoxide carbon, without the involvement of the ligand N-H group. For both catalyst systems, the rate law and overall barrier predicted by density functional theory (DFT) are consistent with the results from kinetic studies.

Vinyl cation-mediated intramolecular hydroarylation of alkynes using pyridinium reagents

Once considered to be exotic species of limited synthetic utility, vinyl cations have recently been shown to be highly versatile intermediates in a variety of processes. Here, we report a method for the synthesis of aryl-substituted benzocycloheptenes and -hexenes using the hydrotriflate salt of an electron-poor pyridine as a uniquely efficient proton source for a vinyl cation mediated Friedel-Crafts cyclization. The mild conditions made possible by this reagent allowed a range of simple and functionalized alkynes bearing pendant aryl groups to serve as suitable substrates for this scalable and convenient protocol.

Sustainable Production of Bioactive Molecules from C-Lignin-Derived Propenylcatechol

Catechyl lignin (C-lignin) is a naturally occurring linear homogeneous biopolymer composed solely of caffeyl alcohol subunits with cleavable benzodioxane linkages. The inherent structural features of propenylcatechol, a direct depolymerized product of castor seed coats C-lignin, render it a sustainable and promising platform for the synthesis of bioactive molecules. Herein, diversified transformations of propenylcatechol, including C=C bond difunctionalization, β-modification, β,γ-rearrangement, and γ-methyl derivatization, were reported based on known or developed methods. A series of functional molecular skeletons involved in the current synthetic routes for the preparation of pharmaceuticals and bioactive molecules were obtained. Starting from castor seed coats, annuloline (natural product) and CC-5079 (antitumor) were synthesized using facile and inexpensive reagents in only four- and five-sequence reactions, respectively, thereby demonstrating a superior step-efficiency to that of reported synthetic routes. Almost all atoms in the C-lignin biopolymer were incorporated into the final products owing to the intrinsic structures of naturally occurring C-lignin. Bioactive molecules produced from C-lignin integrate a low-carbon footprint with high-quality and economical manufacture of pharmaceuticals.

Expedient cobalt-catalyzed stereospecific cascade C-N and C-O bond formation of styrene oxides with hydrazones

Cobalt-catalyzed cascade C-N and C-O bond formation of epoxides with hydrazones is described to furnish oxadiazines using air as an oxidant. The catalyst plays a dual role as a Lewis acid followed by a redox catalyst to accomplish the C-H/O-H cyclization. Optically active styrene oxide can be reacted enantiospecifically (>99% ee).

Catalytic Hydrogenation of Epoxides to Alcohols

Atom-economical catalytic reactions are a highly enticing strategy because all atoms of the starting materials are incorporated into the products. Catalytic hydrogenation of epoxides to alcohols is an attractive and alternative protocol to other synthetic methodologies for the synthesis of alcohols from alkenes. In the last two decades, catalytic hydrogenation of epoxides to alcohols has made remarkable progress in chemical synthesis. In this review, an overview of the catalytic hydrogenation of both terminal and internal epoxides to the corresponding alcohols is presented. An outline of both homogeneous and heterogeneous hydrogenation of epoxides to the corresponding alcohols is provided. Moreover, the selectivity, efficiency, and the reaction mechanisms of these epoxide hydrogenation reactions are highlighted.

Regioselective ring-opening of epoxides towards Markovnikov alcohols: A metal-free catalytic approach using abnormal N-heterocyclic carbene

Herein we report the first metal-free regioselective Markovnikov ring-opening of epoxides (selectivity up to 99%) using an abnormal N-heterocyclic carbene (aNHC) to yield secondary alcohols. DFT calculations and X-ray crystallography...

Neutrally Photoinduced MgCl2-Catalyzed Alkenylation and Imidoylation of Alkanes

We report a practical protocol for oxidation of the chloride ion (Cl-) to chlorine radical (Cl.) via a photoinduced MgCl2 catalysis, avoiding the use of strong acid, formal oxidant, and...

Epoxide Electroreduction

Selective hydrogenation of epoxides would be a direct and powerful approach for alcohol synthesis, but it has proven to be elusive. Here, electrochemically epoxide hydrogenation using electrons and protons as reductants is reported. A wide range of primary, secondary, and tertiary alcohols can be achieved through selective Markovnikov or anti-Markovnikov ring opening in the absence of transition metals. Mechanistic investigations revealed that the regioselectivity is controlled by the thermodynamic stabilities of the in situ generated benzyl radicals for aryl-substituted epoxides and the kinetic tendency for Markovnikov selective ring opening for alkyl-substituted epoxides.

Primary Alcohols via Nickel Pentacarboxycyclopentadienyl Diamide Catalyzed Hydrosilylation of Terminal Epoxides

The efficient and regioselective hydrosilylation of epoxides co-catalyzed by a pentacarboxycyclopentadienyl (PCCP) diamide nickel complex and Lewis acid is reported. This method allows for the reductive opening of terminal, monosubstituted epoxides to form unbranched, primary alcohols. A range of substrates including both terminal and nonterminal epoxides are shown to work, and a mechanistic rationale is provided. This work represents the first use of a PCCP derivative as a ligand for transition-metal catalysis.

Photocatalytic Anti-Markovnikov Radical Hydro- and Aminooxygenation of Unactivated Alkenes Tuned by Ketoxime Carbonates

A tunable photocatalytic method is reported for anti-Markovnikov hydro- and aminooxygenation of unactivated alkenes using readily accessible ketoxime carbonates as the diverse functionalization reagents. Mechanistic studies reveal that this reaction is initiated through an energy-transfer-promoted N-O bond homolysis of ketoxime carbonates leading to alkoxylcarbonyloxyl and iminyl radicals under visible-light photocatalysis, followed by the addition of alkoxylcarbonyloxyl radical to alkenes. By taking advantage of the different stability of the iminyl radicals, the generated carbon radical either abstracts a hydrogen atom from the media to form the anti-Markovnikov hydrooxygenation product, or it is trapped by the persistent iminyl radical to furnish the aminooxygenation product. Notably, this is the first example of direct hydrooxygenation of unactivated olefins with anti-Markovnikov regioselectivity involving an oxygen-centered radical.

Synthesis of α-Alkylated Ketones via Selective Epoxide Opening/Alkylation Reactions with Primary Alcohols

A new method for converting terminal epoxides and primary alcohols into α-alkylated ketones under borrowing hydrogen conditions is reported. The procedure involves a one-pot epoxide ring opening and alkylation via primary alcohols in the presence of an N-heterocyclic carbene iridium(I) catalyst, under aerobic conditions, with water as the side product.

A General Regioselective Synthesis of Alcohols by Cobalt-Catalyzed Hydrogenation of Epoxides

A straightforward methodology for the synthesis of anti‐Markovnikov‐type alcohols is presented. By using a specific cobalt triphos complex in the presence of Zn(OTf)₂ as an additive, the hydrogenation of epoxides proceeds with high yields and selectivities. The described protocol shows a broad substrate scope, including multi‐substituted internal and terminal epoxides, as well as a good functional‐group tolerance. Various natural‐product derivatives, including steroids, terpenoids, and sesquiterpenoids, gave access to the corresponding alcohols in moderate‐to‐excellent yields.