Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism

We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir­(2-PyCH₂(C₄H₅N₂))­(COD)]­OTf (<b>1</b>) or [Ir­(2-PyCH₂PBu₂^t)­(COD)]­OTf (<b>2</b>). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex <b>2</b> reacts with alcohols to form the crystallographically characterized catalytic intermediates [IrH­(η¹,η³-C₈H₁₂)­(2-PyCH₂P^tBu₂)] (<b>2a</b>) and [Ir₂H₃(CO)­(2-PyCH₂P^tBu₂)­{μ-(C₅H₃N)­CH₂P^tBu₂}] (<b>2c</b>). The unexpected similarities in reactivities of <b>1</b> and <b>2</b> in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of <b>1</b> and <b>2</b>, respectively, in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation

Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism

We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir­(2-PyCH₂(C₄H₅N₂))­(COD)]­OTf (<b>1</b>) or [Ir­(2-PyCH₂PBu₂^t)­(COD)]­OTf (<b>2</b>). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex <b>2</b> reacts with alcohols to form the crystallographically characterized catalytic intermediates [IrH­(η¹,η³-C₈H₁₂)­(2-PyCH₂P^tBu₂)] (<b>2a</b>) and [Ir₂H₃(CO)­(2-PyCH₂P^tBu₂)­{μ-(C₅H₃N)­CH₂P^tBu₂}] (<b>2c</b>). The unexpected similarities in reactivities of <b>1</b> and <b>2</b> in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of <b>1</b> and <b>2</b>, respectively, in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation

Di(carbene)-Supported Nickel Systems for CO₂ Reduction Under Ambient Conditions

Di­(carbene)-supported nickel species <b>1</b> and <b>2</b> are efficient catalysts for the room-temperature reduction of CO₂ to methanol in the presence of sodium borohydride. The catalysts feature unusual stability, particularly for a base metal catalyst, enabling >1.1 million turnovers of CO₂. Moreover, while other systems involve more expensive reducing reagents, sodium borohydride is inexpensive and easily handled. Furthermore, effecting reduction in the presence of water enables direct access to methanol. Preliminary mechanistic data collected are most consistent with a mononuclear nickel active species that mediates rate-determining reduction of a boron formate

Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism

We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir­(2-PyCH₂(C₄H₅N₂))­(COD)]­OTf (<b>1</b>) or [Ir­(2-PyCH₂PBu₂^t)­(COD)]­OTf (<b>2</b>). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex <b>2</b> reacts with alcohols to form the crystallographically characterized catalytic intermediates [IrH­(η¹,η³-C₈H₁₂)­(2-PyCH₂P^tBu₂)] (<b>2a</b>) and [Ir₂H₃(CO)­(2-PyCH₂P^tBu₂)­{μ-(C₅H₃N)­CH₂P^tBu₂}] (<b>2c</b>). The unexpected similarities in reactivities of <b>1</b> and <b>2</b> in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of <b>1</b> and <b>2</b>, respectively, in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation

Di(carbene)-Supported Nickel Systems for CO₂ Reduction Under Ambient Conditions

Di­(carbene)-supported nickel species <b>1</b> and <b>2</b> are efficient catalysts for the room-temperature reduction of CO₂ to methanol in the presence of sodium borohydride. The catalysts feature unusual stability, particularly for a base metal catalyst, enabling >1.1 million turnovers of CO₂. Moreover, while other systems involve more expensive reducing reagents, sodium borohydride is inexpensive and easily handled. Furthermore, effecting reduction in the presence of water enables direct access to methanol. Preliminary mechanistic data collected are most consistent with a mononuclear nickel active species that mediates rate-determining reduction of a boron formate

A Three-Stage Mechanistic Model for Ammonia–Borane Dehydrogenation by Shvo’s Catalyst

We propose a mechanistic model for three-stage dehydrogenation of ammonia–borane (AB) catalyzed by Shvo’s cyclopentadienone-ligated ruthenium complex. We provide evidence for a plausible mechanism for catalyst deactivation and the transition from fast catalysis to slow catalysis and relate those findings to the invention of a second-generation catalyst that does not suffer from the same deactivation chemistry. The primary mechanism of catalyst deactivation is borazine-mediated hydroboration of the ruthenium species that is the active oxidant in the fast catalysis case. This transition is characterized by a change in the rate law for the reaction and changes in the apparent resting state of the catalyst. Also, in this slow catalysis situation, we see an additional intermediate in the sequence of boron, nitrogen species, aminodiborane. This occurs with concurrent generation of NH₃, which itself does not strongly affect the rate of AB dehydrogenation

Recycling Benzoxazine–Epoxy Composites via Catalytic Oxidation

Carbon fiber-reinforced polymers (CFRPs) are structural composites used in the aerospace and sporting goods industries. Their chief appeal lies in their high specific properties, which generally outperform metallic counterparts. There is a contemporary need for viable methods for recycling CRFPs at the end of their lifecycles and for utilizing the considerable production waste (ca. 30%) of CFRP part manufacturing. The cost associated with these waste streams is a principal economic driver inhibiting the penetration of CRFPs into larger-scale manufacturing, particularly in the automotive industry. Reported techniques for CRFP degradation involve pyrolysis or mechanical grinding of the CFRP, processes which are outlawed in some jurisdictions and can reduce the thermomechanical properties of the recycled products. In this study, we report a conceptually different approach to degrading a commercial blended benzoxazine/epoxy resin under mild, oxidative conditions. The thermosetting resin is polymerized, characterized, and then catalytically depolymerized via hydride abstraction with a ruthenium catalyst. These results demonstrate a concept for sustainable recycling of CFRP composites

Upgrading Biodiesel from Vegetable Oils by Hydrogen Transfer to Its Fatty Esters

Conversion of vegetable-derived triglycerides to fatty acid methyl esters (FAMEs) is a popular approach to the generation of biodiesel fuels and the basis of a growing industry. Drawbacks of the strategy are that (a) the glycerol backbone of the triglyceride is discarded as waste, and (2) most available natural triglycerides in the U.S. are multiunsaturated or fully saturated, giving inferior fuel performance and causing engine problems. Here we show that catalysis by iridium complex <b>1</b> can address both of these problems through selective reduction of triglycerides high in polyunsaturation. This is realized using hydrogen from methanol or those imbedded in the triglyceride backbone, concurrently generating lactate as a value-added C3 product. Additional methanol or glycerol as a hydrogen source enables reduction of corn and soybean oils to >80% oleate. The cost of the iridium catalyst is mitigated by its recovery through aqueous extraction. The process can be further driven with a supporting iron-based catalyst for the complete saturation of all olefins. Preparative procedures are established for synthesis and separation of methyl esters of the hydrogenated fatty acids, enabling instant access to upgraded biofuels

Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst

We previously reported that quantitative poisoning, a test for homogeneous catalysis, behaves oddly in the dehydrogenation of ammonia borane (AB) by Shvo’s catalyst, whereas the “poison” 1,10-phenanthroline (phen) accelerates catalysis and apparently prevents catalyst deactivation. Thus, we proposed a protective role for phen in the catalysis. Herein we account for the mechanistic origin of this accelerated AB dehydrogenation in the presence of phen and define the relevance boundaries of our prior proposal. In so doing, we present syntheses for novel amine- and pyridine-ligated homologues of the Shvo catalyst and show their catalytic efficacy in AB dehydrogenation. These catalysts are synthetically easy to access, air stable, and rapidly release over 2 equiv of H₂. The mechanisms of these reactions are also discussed

Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst

We previously reported that quantitative poisoning, a test for homogeneous catalysis, behaves oddly in the dehydrogenation of ammonia borane (AB) by Shvo’s catalyst, whereas the “poison” 1,10-phenanthroline (phen) accelerates catalysis and apparently prevents catalyst deactivation. Thus, we proposed a protective role for phen in the catalysis. Herein we account for the mechanistic origin of this accelerated AB dehydrogenation in the presence of phen and define the relevance boundaries of our prior proposal. In so doing, we present syntheses for novel amine- and pyridine-ligated homologues of the Shvo catalyst and show their catalytic efficacy in AB dehydrogenation. These catalysts are synthetically easy to access, air stable, and rapidly release over 2 equiv of H₂. The mechanisms of these reactions are also discussed