Deeper Mechanistic Insight into Ru Pincer-Mediated Acceptorless Dehydrogenative Coupling of Alcohols: Exchanges, Intermediates, and Deactivation Species

Highly Enantiomerically Enriched Secondary Alcohols via Epoxide Hydrogenolysis

In this article, we report the development of ruthenium-catalyzed hydrogenolysis of epoxides to selectively give the branched (Markovnikov) alcohol products. In contrast to previously reported catalysts, the use of Milstein's PNN-pincer-ruthenium complex at room temperature allows the conversion of enantiomerically enriched epoxides to secondary alcohols without racemization of the product. The catalyst is effective for a range of aryl epoxides, alkyl epoxides, and glycidyl ethers and is the first homogeneous system to selectively promote hydrogenolysis of glycidol to 1,2-propanediol, without loss of enantiomeric purity. A detailed mechanistic study was conducted, including experimental observations of catalyst speciation under catalytically relevant conditions, comprehensive kinetic characterization of the catalytic reaction, and computational analysis via density functional theory. Heterolytic hydrogen cleavage is mediated by the ruthenium center and exogenous alkoxide base. Epoxide ring opening occurs through an opposite-side attack of the ruthenium hydride on the less-hindered epoxide carbon, giving the branched alcohol product selectively.

Mechanistic Insight into Acceptorless Dehydrogenation of Methanol to Syngas Catalyzed by MACHO-Type Ruthenium and Manganese Complexes: A DFT Study

The acceptorless dehydrogenation of methanol to produce carbon monoxide (CO) and dihydrogen (H2) mediated by MACHO-type 1-Ru and 1-Mn complexes was theoretically investigated via density functional theory calculations. The 1-Ru-catalyzed process involves the formation of active species 4-Ru through a methanol-bridged H2 release pathway. Methanol dehydrogenation by 4-Ru yields formaldehyde and 1-Ru, followed by H2 release to regenerate 4-Ru (rate-determining step, ΔG‡ = 32.5 kcal/mol). Formaldehyde further reacts with methanol via nucleophilic attack of the MeO- ligand in the Ru complex (ΔG‡ = 9.6 kcal/mol), which is more favorable than the traditional methanol-to-formaldehyde nucleophilic attack (ΔG‡ = 33.8 kcal/mol) due to the higher nucleophilicity of MeO-. CO is ultimately produced through the methyl formate decarbonylation reaction. Accelerated H2 release in the early reaction stage compared to CO results from the initial methanol dehydrogenation and condensation of formaldehyde with methanol. In contrast, CO generation occurs later via methyl formate decarbonylation. The 1-Mn-catalyzed reaction has reduced efficiency compared to 1-Ru for the higher Gibbs energy barrier (ΔG‡ = 34.1 kcal/mol) of the rate-determining step. Excess NaOtBu promotes the reaction of CO and methanol, forming methyl formate, significantly reducing the CO/H2 ratio as the catalyst amount decreases. These findings deepen our understanding of the methanol-to-syngas transformation and can drive progress in this field.

Double Intramolecular 1,2 C-H Addition of o-Methyl Groups To Form Ruthenium Pincer Double Tuck-In Complexes

Ruthenium pincer complexes have a rich history of coordination and reaction chemistries. In this work, we report our discoveries of previously unreported Ru pincer coordination geometries. We found that mono tuck-in κ4-ArPNHPRuLCl complexes react with NaN(SiMe3)2 producing double tuck-in mer-κ5-ArPNHPRuL complexes. Interestingly, when κ4-MesPNHPRuCl is dehydrohalogenated, the resulting double tuck-in complex binds N2, forming the nitrogen complex κ5-MesPNHPRuN2. The mer-κ5-ArPNHPRuL complexes thermally isomerize to the fac-κ5-ArPNHPRuL isomers, which is an uncommon reaction for pincer complexes. The mer-κ5-ArPNHPRuL complexes react with CO and CO2 to form amide κ4-ArPNHPRu(CO)L or carbamate κ5-ArPN(CO2)PRuL complexes, respectively, supporting the hypothesis that the κ4-ArPNPRuL amide intermediates are accessible and reactive.

Arene Ruthenium(II)-Catalyzed Sustainable Synthesis of 2,4-Disubstituted Quinazolines via Acceptorless Dual Dehydrogenative Coupling of Alcohols

We demonstrate an efficient and sustainable strategy for the direct synthesis of 2,4-disubstituted quinazolines by arene Ru(II)benzhydrazone complex via the eco-friendly sequential acceptorless dehydrogenative coupling of 2-aminobenzhydrol derivatives and benzyl alcohols for the first time. The new ruthenium(II) complex of the general formula [(η6-p-cymene)Ru(L1)Cl] (L1-acenaphthenequinone hydrazone) has been synthesized and characterized by analytical, spectroscopic, and single-crystal X-ray diffraction techniques. A broad spectrum of 2,4-disubstituted quinazolines have been successfully derived (25 examples) from 2-aminobenzhydrol derivatives with various benzyl alcohols using 1 mol % of catalyst loading in the presence of NH4OAc. The present protocol is highly selective and produces a maximum yield of 95% under mild reaction conditions. The different reaction intermediates detected through control experiments such as aldehyde, 2-aminobenzophenone, benzylidene(amino)phenylmethanone, and 1,2-dihydroquinazoline are isolated and authenticated by the NMR study. Gratifyingly, the coupling reaction is a simple and atom economic with the release of water and hydrogen gas as the only byproducts. A gram-scale synthesis of 2-(4-methoxyphenyl)-4-phenylquinazoline illustrates the synthetic utility of the present protocol.

Ruthenium complexes of 1,4-diazabutadiene ligands with a cis-RuCl2 moiety for catalytic acceptorless dehydrogenation of alcohols: DFT evidence of chemically non-innocent ligand participation

The acceptorless dehydrogenative coupling (ADC) of primary alcohols to esters by diazabutadiene-coordinated ruthenium compounds is reported. Treatment of cis-Ru(dmso)4Cl2 in acetone at 56 °C with different 1,4-diazabutadienes [p-XC6H4N[double bond, length as m-dash]C(H)(H)C[double bond, length as m-dash]NC6H4X-p; X = H, CH3, OCH3, and Cl; abbreviated as DAB-X], gives trans-Ru[κ2-N,N-DAB-X]2Cl2 as the kinetic product of substitution. Heating these products in o-xylene at 144 °C gives the thermodynamically favored cis-Ru[κ2-N,N-DAB-X]2Cl2 isomers. Electronic structure calculations confirm the greater stability of the cis diastereomer. The molecular structures for each pair of geometric isomers have been determined by X-ray diffraction analyses. Cyclic voltammetry experiments on the complexes show an oxidative response and a reductive response within 0.50 to 0.93 V and -0.76 to -1.24 V vs. SCE respectively. The cis-Ru[κ2-N,N-DAB-X]2Cl2 complexes function as catalyst precursors for the acceptorless dehydrogenative coupling of primary alcohols to H2 and homo- and cross-coupled esters. When 1,4-butanediol and 1,5-pentanediol are employed as substrates, lactones and hydroxyaldehydes are produced as the major dehydrogenation products, while secondary alcohols afforded ketones in excellent yields. The mechanism for the dehydrogenation of benzyl alcohol to benzyl benzoate and H2 using cis-Ru[κ2-N,N-DAB-H]2Cl2 (cis-1) as a catalyst precursor was investigated by DFT calculations. The data support a catalytic cycle that involves the four-coordinate species Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-OCH2Ph) whose protonated κ1-diazabutadiene moiety functions as a chemically non-innocent ligand that facilitates a β-hydrogen elimination from the κ1-O-benzoxide ligand to give the corresponding hydride HRu[κ2-N,N-DAB-H][κ1-N-DAB-H](κ2-O,C-benzaldehyde). H2 production follows a Noyori-type elimination to give (H2)Ru[κ2-N,N-DAB-H][κ1-N-DAB-H](κ1-O-benzaldehyde) as an intermediate in the catalytic cycle.

Performance of homogeneous catalysts viewed in dynamics

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

Theoretical Study on the Hydrogenolysis of Polyurethanes to Improve the Catalytic Activities

The metal-catalyzed hydrogenolysis of polymers is important in waste recycling; however, it is limited by the harsh reaction conditions and the low activities of catalysts, especially for earth-abundant metal-based catalysts. Herein, we perform a comprehensive study on the hydrogenolysis of polyurethane model catalyzed by Fe-, Mn-, Ru-, and Ir-^iPrMACHO pincer complexes and propose a cascade mechanism comprising two-level hydrogenolysis and the hydrogenation of formaldehyde. In addition, the substrates and ligands are modulated to improve the activities of chemical recycling to monomer. It is found that the pincer ligands could dissociate from the metal centers at high reaction temperatures and further result in the deactivation of catalysts. The rigid Fe and Mn catalysts with tetradentate cyclic ligands are designed following the guidance, and the computations suggest that those designed catalysts could have high stabilities and activities.

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH₆(PⁱPr₃)₂ promotes the C-H bond activation of the 1,3-disubstituted phenyl group of the [BF₄]^- and [BPh₄]^- salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ²-C,N- or κ²-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ³-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ²-C,N- and κ²-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF₄]^- inhibits the azolium reactions, [BPh₄]^- favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C-H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Bulky PNP ligands blocking metal-ligand cooperation allow for isolation of Ru(0), and lead to catalytically active Ru complexes in acceptorless alcohol dehydrogenation

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H 2 allows for stabilization and extensive characterization of bis CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2)-H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Fatty Acids and their Derivatives as Renewable Platform Molecules for the Chemical Industry

Oils and fats of vegetable and animal origin remain an important renewable feedstock for the chemical industry. Their industrial use has increased during the last 10 years from 31 to 51 million tonnes annually. Remarkable achievements made in the field of oleochemistry in this timeframe are summarized herein, including the reduction of fatty esters to ethers, the selective oxidation and oxidative cleavage of C-C double bonds, the synthesis of alkyl-branched fatty compounds, the isomerizing hydroformylation and alkoxycarbonylation, and olefin metathesis. The use of oleochemicals for the synthesis of a great variety of polymeric materials has increased tremendously, too. In addition to lipases and phospholipases, other enzymes have found their way into biocatalytic oleochemistry. Important achievements have also generated new oil qualities in existing crop plants or by using microorganisms optimized by metabolic engineering.

Direct synthesis of polyureas from the dehydrogenative coupling of diamines and methanol

We report here the first example of the direct synthesis of polyureas from the dehydrogenative coupling of diamines and methanol using a ruthenium pincer catalyst. The present methodology replaces the use of toxic diisocyanates, conventionally used for the production of polyureas, with methanol, which is renewable, less toxic, and cheaper, making the overall process safer and more sustainable. Further advantages of the current method have been demonstrated by the synthesis of a renewable, a chiral, and the first 13C-labelled polyurea.

Mechanistic Investigations of Ruthenium Catalyzed Dehydrogenative Thioester Synthesis and Thioester Hydrogenation

We have recently reported the previously unknown synthesis of thioesters by coupling thiols and alcohols (or aldehydes) with liberation of H₂, as well as the reverse hydrogenation of thioesters, catalyzed by a well-defined ruthenium acridine-9H based pincer complex. These reactions are highly selective and are not deactivated by the strongly coordinating thiols. Herein, the mechanism of this reversible transformation is investigated in detail by a combined experimental and computational (DFT) approach. We elucidate the likely pathway of the reactions, and demonstrate experimentally how hydrogen gas pressure governs selectivity toward hydrogenation or dehydrogenation. With respect to the dehydrogenative process, we discuss a competing mechanism for ester formation, which despite being thermodynamically preferable, it is kinetically inhibited due to the relatively high acidity of thiol compared to alcohol and, accordingly, the substantial difference in the relative stabilities of a ruthenium thiolate intermediate as opposed to a ruthenium alkoxide intermediate. Accordingly, various additional reaction pathways were considered and are discussed herein, including the dehydrogenative coupling of alcohol to ester and the Tischenko reaction coupling aldehyde to ester. This study should inform future green, (de)hydrogenative catalysis with thiols and other transformations catalyzed by related ruthenium pincer complexes.

Isonitrile ruthenium and iron PNP complexes: synthesis, characterization and catalytic assessment for base-free dehydrogenative coupling of alcohols

Neutral and ionic ruthenium and iron aliphatic PNHP-type pincer complexes (PNHP = NH(CH2CH2PiPr2)2) bearing benzyl, n-butyl or tert-butyl isocyanide ancillary ligands have been prepared and characterized. Reaction of [RuCl2(PNHP)]2 with one equivalent CN-R per ruthenium center affords complexes [RuCl2(PNHP)(CNR)] (R = benzyl, 1a, R = n-butyl, 1b, R = t-butyl, 1c), with cationic [RuCl(PNHP)(CNR)2]Cl 2a-c as side-products. Dichloride species 1a-c react with excess NaBH4 to afford [RuH(PNHP)(BH4)(CN-R)] 3a-c, analogues to benchmark Takasago catalyst [RuH(PNHP)(BH4)(CO)]. Reaction of 1a-c with a single equivalent of NaBH4 results in formation of [RuHCl(PNHP) (CN-R)] (4a-c), from which 3a-c can be prepared upon reaction with excess NaBH4. Use of one equivalent of NaHBEt3 with 4a and 4c affords bishydrides [Ru(H)2(PNHP)(CN-R)] 5a and 5c. Deprotonation of 4c by KOtBu generates amido derivative [RuH(PNP)(CN-t-Bu)] (6, PNP = -N(CH2CH2PiPr2)2), unstable in solution. Addition of excess benzylisonitrile to 4a provides cationic hydride [RuH(PNHP) (CN-CH2Ph)2]Cl (7). Concerning iron chemistry, [Fe(PNHP)Br2] reacts with one equivalent of benzylisonitrile to afford [FeBr(PNHP)(CNCH2Ph)2]Br (8). The outer-sphere bromide anion can be exchanged by salt metathesis with NaBPh4 to generate [FeBr(PNHP) (CNCH2Ph)2](BPh4) (9). Cationic hydride species [FeH(PNHP) (CN-t-Bu)2](BH4) (10) is prepared from consecutive addition of excess CN-t-Bu and NaBH4 on [Fe(PNPH)Br2]. Ruthenium complexes 3a-c are active in acceptorless alcohol dehydrogenative coupling into ester under base-free conditions. From kinetic follow-up, the trend in initial activity is 3a ≈ 3b > [RuH(PNHP)(BH4)(CO)] ≫ 3c; for robustness, [RuH(BH4)(CO)(PNHP)] > 3a > 3b ≫ 3c. Hypotheses are given to account for the observed deactivation. Complexes 3b, 3c, 4a, 4c, 5c, 7, cis-8 and 9 were characterized by X-ray crystallography.

Direct synthesis of ring-fused quinolines and pyridines catalyzed by NNHY-ligated manganese complexes (Y = NR2 or SR)

A robust NNS-manganese(i) complex has been developed that can efficiently catalyze the dehydrogenation coupling of γ-amino alcohols with ketones or secondary alcohols in a one-step synthesis of quinolines and pyridines with high yields.

Dehydrogenative alcohol coupling and one-pot cross metathesis/dehydrogenative coupling reactions of alcohols using Hoveyda–Grubbs catalysts

Grubbs catalysts are shown to catalyze one-pot cross-metathesis/dehydrogenative alcohol coupling reactions in an efficient manner.

Alcohols as Fluoroalkyl Synthons: Ni-catalyzed Dehydrogenative Approach to Access Polyfluoroalkyl Bis-indoles

An acceptorless dehydrogenative strategy for the synthesis of polyfluoroalkylated bis-indoles is described by employing an earth-abundant nickel-based catalytic system under air. The notable feature of the present transformation is the use of bench stable and easily affordable polyfluorinated alcohols without any pre-functionalization for the introduction of precious polyfluoroalkyl groups. The developed straightforward protocol accomplished biologically relevant fluoroalkyl bis-indoles in a sustainable fashion. Extensive DFT study predicts the unique role of indole molecules which stabilizes the transition states during the dehydrogenation process of polyfluorinated alcohols, presumably through non-covalent π⋅⋅⋅π and H-bonding interactions.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Catalytic Disproportionation of Formic Acid to Methanol by using Recyclable Silylformates

A novel strategy to prepare methanol from formic acid without an external reductant is presented. The overall process described herein consists of the disproportionation of silyl formates to methoxysilanes, catalyzed by ruthenium complexes, and the production of methanol by simple hydrolysis. Aqueous solutions of MeOH (>1 mL, >70 % yield) were prepared in this manner. The sustainability of the reaction has been established by recycling of the silicon-containing by-products with inexpensive, readily available, and environmentally benign reagents.

Copper–Zirconia Catalysts: Powerful Multifunctional Catalytic Tools to Approach Sustainable Processes

Copper–zirconia catalysts find many applications in different reactions owing to their unique surface properties and relatively easy manufacture. The so-called methanol economy, which includes the CO2 and CO valorization and the hydrogen production, and the emerging (bio)alcohol upgrading via dehydrogenative coupling reaction, are two critical fields for a truly sustainable development in which copper–zirconia has a relevant role. In this review, we provide a systematic view on the factors most impacting the catalytic activity and try to clarify some of the discrepancies that can be found in the literature. We will show that contrarily to the large number of studies focusing on the zirconia crystallographic phase, in the last years, it has turned out that the degree of surface hydroxylation and the copper–zirconia interphase are in fact the two mostly determining factors to be controlled to achieve high catalytic performances.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation

The reversible activation of H₂ via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H₂ via an MLC pathway to the widely used iron catalyst [(^iPrPNP)FeH(CO)] (1) (^iPrPNP = N(CH₂CH₂PⁱPr₂)₂^-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H₂ using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H₂ activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H₂ activation. The results of this study provide guidance about the design of catalysts and additives to promote H₂ activation via an MLC pathway.

A practical method for N-alkylation of phosphinic (thio)amides with alcohols via transfer hydrogenation

This manuscript describes a modular method for preparing N-alkyl phosphinic amides from primary or secondary alcohols and primary phosphinic amide (R¹R²P=ONH₂) nucleophiles via transfer hydrogenation. The transformation typically proceeds in excellent yields, employs conveniently available reagents, and produces water as the only byproduct.

Nonoxidative Dehydrogenation of Methanol to Methyl Formate through Highly Stable and Reusable CuMgO-Based Catalysts

Nonoxidative dehydrogenation of methanol to methyl formate over a CuMgO-based catalyst was investigated. Although the active site is metallic copper (Cu⁰), the best reaction conditions were obtained by tuning the ratio of Cu/Mg and doping the catalyst with 1 wt % of Pd to achieve a very specific activity for methyl formate synthesis. On the basis of the CO₂ temperature-programmed desorption study, the basic strength of the catalyst plays a role in the efficient conversion of methanol to methyl formate via dehydrogenation. These CuMgO-based catalysts show excellent thermal stability during the reaction and the regeneration processes. Approx. 80% methanol conversion with constant selectivity to methyl formate was achieved even after 4 rounds of usage for a total reaction time exceeding 200 h, indicative of their potential for practical applications.

Unusual rearrangement of modified PNP ligand based Ru complexes relevant to alcohol dehydrogenation catalysis

Hangman PNP pincer Ru complexes undergo an unusual sp²–sp³ C–C bond cleavage and formation via a spyrocyclopropane transition state.

Continuous flow aerobic alcohol oxidation using a heterogeneous Ru0 catalyst

Stable Ru⁰ on γ-Al₂O₃ has been used for the selective aerobic oxidation of benzyl alcohol in flow.

Online monitoring by infrared spectroscopy using multivariate analysis – background theory and application to catalytic dehydrogenative coupling of butanol to butyl butyrate

The monitoring of chemical reactions can be arduous especially when sampling is required with highly reactive compounds.

Development and mechanistic investigation of the manganese(iii) salen-catalyzed dehydrogenation of alcohols

Manganese(iii) salen has been developed as a new catalytic motif for alcohol dehydrogenation and the mechanism has been elucidated.

The first example of a manganese(iii) catalyst for the acceptorless dehydrogenation of alcohols is presented. N,N′-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(iii) chloride (2) has been shown to catalyze the direct synthesis of imines from a variety of alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with DFT calculations. The results indicate a metal–ligand bifunctional pathway in which both imine groups in the salen ligand are first reduced to form a manganese(iii) amido complex as the catalytically active species. Dehydrogenation of the alcohol then takes place by a stepwise outer-sphere hydrogen transfer generating a manganese(iii) salan hydride from which hydrogen gas is released.