Acceptorless Dehydrogenative Oxidation of Secondary Alcohols Catalysed by Cp*IrIII–NHC Complexes

A series of new Ir^III complexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X‐ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2‐trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron‐rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H₂‐formation step.     

Hydrogen Production from a Methanol–Water Solution Catalyzed by an Anionic Iridium Complex Bearing a Functional Bipyridonate Ligand under Weakly Basic Conditions

An efficient catalytic system for the production of hydrogen from a methanol–water solution has been developed using a new anionic iridium complex bearing a functional bipyridonate ligand as a catalyst. This system can be operated under mild conditions [weakly basic solution (0.046 mol L^?1 NaOH) below 100 °C] without the use of an additional organic solvent. Long‐term continuous hydrogen production from a methanol–water solution catalyzed by the anionic iridium complex was also achieved.     

Water Oxidation with Molecularly Defined Iridium Complexes: Insights into Homogeneous versus Heterogeneous Catalysis

Molecularly defined Ir complexes and different samples of supported IrO₂ nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant. By comparing the activity of nano‐scaled supported IrO₂ particles to the one of organometallic complexes it is shown that the overall activity of the homogeneous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir^IV ‐ oxo nanoparticles. In the first phase of the reaction the activity is dominated by the homogeneous active species. With increasing reaction time, the influence of nano‐sized Ir ‐ oxo particles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir‐oxo nanoparticles and the different particle sizes have a significant influence on the overall activity. In addition to the homogeneous systems, IrO₂@MCM‐41 has also been synthesized, which contains stabilized nanoparticles of between 1 and 3 nm in size. This latter system shows a similar activity to IrCl₃ ? xH₂O and complexes 4 and 5 . Mechanistic insights were obtained by in situ X‐ray absorption spectroscopy and scanning transmission electron microscopy.     

Oxydifluoromethylation of Alkenes by Photoredox Catalysis: Simple Synthesis of CF2H‐Containing Alcohols

We have developed a novel and simple protocol for the direct incorporation of a difluoromethyl (CF₂H) group into alkenes by visible‐light‐driven photoredox catalysis. The use of fac‐[Ir(ppy)₃] (ppy=2‐pyridylphenyl) photocatalyst and shelf‐stable Hu's reagent, N‐tosyl‐S‐difluoromethyl‐S‐phenylsulfoximine, as a CF₂H source is the key to success. The well‐designed photoredox system achieves synthesis of not only β‐CF₂H‐substituted alcohols but also ethers and an ester from alkenes through solvolytic processes. The present method allows a single‐step and regioselective formation of C(sp³)–CF₂H and C(sp³)?O bonds from C=C moiety in alkenes, such as hydroxydifluoromethylation, regardless of terminal or internal alkenes. Moreover, this methodology tolerates a variety of functional groups.     

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH₃)(cod)(MeIm(2‐ methoxybenzyl))][BF₄] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF₀) of 1283 h^?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde.     

Highly Effective Water Oxidation Catalysis with Iridium Complexes through the Use of NaIO4

Exceptional water oxidation (WO) turnover frequencies (TOF=17 000 h^?1), and turnover numbers (TONs) close to 400 000, the largest ever reported for a metal‐catalyzed WO reaction, have been found by using [Cp*Ir^III(NHC)Cl₂] (in which NHC=3‐methyl‐1‐(1‐phenylethyl)‐imidazoline‐2‐ylidene) as the pre‐catalyst and NaIO₄ as oxidant in water at 40 °C. The apparent TOF for [Cp*Ir^III(NHC)X₂] ( 1 X , in which X stands for I ( 1 I ), Cl ( 1 Cl ), or triflate anion ( 1 OTf )) and [(Cp*‐NHCMe)Ir^IIII₂] ( 2 ) complexes, is kept constant during almost all of the O₂ evolution reaction when using NaIO₄ as oxidant. The TOF was found to be dependent on the ligand and on the anion (TOF ranging from ≈600 to ≈1100 h^?1 at 25 °C). Degradation of the complexes by oxidation of the organic ligands upon reaction with NaIO₄ has been investigated. ¹H NMR, ESI‐MS, and dynamic light‐scattering measurements (DLS) of the reaction medium indicated that the complex undergoes rapid degradation, even at low equivalents of oxidant, but this process takes place without formation of nanoparticles. Remarkably, three‐month‐old solution samples of oxidized pre‐catalysts remain equally as active as freshly prepared solutions. A UV/Vis feature band at λ_max=405 nm is observed in catalytic reaction solutions only when O₂ evolves, which may be attributed to a resting state iridium speciation, most probably Ir–oxo species with an oxidation state higher than IV.     

Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7–6–7 Ring‐Based Pincer Iridium Complexes

A series of Ir–PCP pincer precatalysts [(7–6–7‐^RPCP)Ir(H)(Cl)] and [(7–6–7‐^ArPCP)Ir(H)(Cl)(MeCN)] bearing a novel “7–6–7” fused‐ring skeleton have been synthesized based upon the postulate that the catalytic species would have durability due to their rather rigid structure and high activity owing to the low but sufficient flexibility of their backbones, which are not completely fixed. Treatment of these precatalysts with NaOtBu gave rise to the active 14 electron (14e) species [(7–6–7‐^iPrPCP)Ir] and [(7–6–7‐^PhPCP)Ir], which can trap hydrogen and were spectroscopically characterized as the tetrahydride complexes. Both [(7–6–7‐^iPrPCP)Ir] and [(7–6–7‐^PhPCP)Ir] were found to be highly effective in the transfer dehydrogenation of cyclooctane with tert‐butylethylene as the hydrogen acceptor, the initial reaction rate at high temperature (230 °C) being higher for [(7–6–7‐^iPrPCP)Ir] than [(7–6–7‐^PhPCP)Ir], and the turnover number (TON) of the overall hydrogen transfer being higher for the latter. Nonetheless, the estimated TONs were as high as 4600 and 4820 for the two complexes at this temperature, respectively, which are unprecedented absolute values. In terms of durability, the [(7–6–7‐^PhPCP)Ir] complex is the catalyst of choice for this reaction. Structural analysis and computational studies support the importance of the low flexibility of the ligand core.     

Stepwise Metal–Ligand Cooperation by a Reversible Aromatization/Deconjugation Sequence in Ruthenium Complexes with a Tetradentate Phenanthroline‐Based Ligand

The synthesis and reactivity of ruthenium complexes containing the tetradentate phenanthroline‐based phosphine ligand 2,9‐bis((di‐tert‐butylphosphino)methyl)‐1,10‐phenanthroline (PPhenP) is described. The hydrido chloro complex [RuHCl(PPhenP)] ( 2 ) undergoes facile dearomatization upon deprotonation of the benzylic position, to give [RuH(PPhenP‐H)] ( 4 ). Addition of dihydrogen to 4 causes rearomatization of the phenanthroline moiety to trans‐[Ru(H)₂(PPhenP)] ( 5 ), followed by hydrogenation of an aromatic heterocycle in the ligand backbone, to give a new dearomatized and deconjugated complex [RuH(PPhenP*‐H)] ( 6 ). These aromatization/deconjugation steps of the coordinated ligand were demonstrated to be reversible and operative in the dehydrogenation of primary alcohols without the need for a hydrogen acceptor. This aromatization/deconjugation sequence constitutes an unprecedented mode of a stepwise cooperation between the metal center and the coordinated ligand.     

Iridium‐Catalyzed Dehydrogenative Decarbonylation of Primary Alcohols with the Liberation of Syngas

A new iridium ‐ catalyzed reaction in which molecular hydrogen and carbon monoxide are cleaved from primary alcohols in the absence of any stoichiometric additives has been developed. The dehydrogenative decarbonylation was achieved with a catalyst generated in situ from [Ir(coe)₂Cl]₂ (coe=cyclooctene) and racemic 2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (rac‐BINAP) in a mesitylene solution saturated with water. A catalytic amount of lithium chloride was also added to improve the catalyst turnover. The reaction has been applied to a variety of primary alcohols and gives rise to products in good to excellent yields. Ethers, esters, imides, and aryl halides are stable under the reaction conditions, whereas olefins are partially saturated. The reaction is believed to proceed by two consecutive organometallic transformations that are catalyzed by the same iridium(I)–BINAP species. First, dehydrogenation of the primary alcohol to the corresponding aldehyde takes place, which is then followed by decarbonylation to the product with one less carbon atom.     

Enhanced Hydrogen Generation from Formic Acid by Half‐Sandwich Iridium(III) Complexes with Metal/NH Bifunctionality: A Pronounced Switch from Transfer Hydrogenation

By switching the catalytic function from transfer hydrogenation based on the metal/NH bifunctionality, facile dehydrogenation of formic acid was achieved by amido‐ and hydrido(amine)–Ir complexes derived from N‐triflyl‐1,2‐diphenylethylenediamine (TfDPEN) at ambient temperature even in the absence of base additives. Further acceleration was observed by the addition of water, leading to a maximum turnover frequency above 6000 h^?1. A proton‐relay mechanism guided by the protic amine ligand and water is postulated for effective protonation of metal hydrides.     

Combined Heterogeneous Metal/Chiral Amine: Multiple Relay Catalysis for Versatile Eco‐Friendly Synthesis

Herein is described a versatile and broad synergistic strategy for expansion of chemical space and the synthesis of valuable molecules (e.g. carbocycles and heterocycles), with up to three quaternary stereocenters, in a highly enantioselective fashion from simple alcohols (31 examples, 95:5 to >99.5:0.5 e.r.) using integrated heterogeneous metal/chiral amine multiple relay catalysis and air/O₂ as the terminal oxidant. A novel highly 1,4‐selective heterogeneous metal/amine co‐catalyzed hydrogenation of enals was also added to the relay catalysis sequences.     

Synthesis,Structure, and Catalytic Activity of Cyclometalated Iridium Complexes with a Bidentate POC Ligand

Synthesis, characterization and catalytic activity of cyclometalated iridium complexes with a bidentate POC ligand is presented. Metalation of POC-H (di-tert-butyl(phenoxy)phosphane) with [Ir(COD)Cl]₂ proceeded rapidly at room temperature and afforded mixture of (POC)(POC-H)IrHCl ( 1 a ) and (POC)(COD)IrHCl ( 1 b ), from which complexes (POC)(L)IrHCl where L=PPh₃ ( 1 c ), bipyridine ( 1 d ) and [2,2′-bipyridine]-6,6′-diol ( 1 e ) were prepared through ligand exchange. The compounds were tested in acceptorless dehydrogenation of 1-phenylethanol and transfer dehydrogenation of ethanol in a context of comparison with pincer counterparts (POCOP)IrHCl and (PCN)IrHCl. An attempt to prepare a dihydride complex from 1 e led to dimeric complex [(POC)(bipy-diol−)IrH]₂ ( 3 ) that could explain the low activity of 1 e . DFT studies provided insight into POC-H vs POCOP-H metalation mechanism.     

The Synthesis of Benzimidazoles and Quinoxalines from Aromatic Diamines and Alcohols by Iridium‐Catalyzed Acceptorless Dehydrogenative Alkylation

Benzimidazoles and quinoxalines are important N‐heteroaromatics with many applications in pharmaceutical and chemical industry. Here, the synthesis of both classes of compounds starting from aromatic diamines and alcohols (benzimidazoles) or diols (quinoxalines) is reported. The reactions proceed through acceptorless dehydrogenative condensation steps. Water and two equivalents of hydrogen are liberated in the course of the reactions. An Ir complex stabilized by the tridentate P^N^P ligand N²,N⁶‐bis(di‐isopropylphosphino)pyridine‐2,6‐diamine revealed the highest catalytic activity for both reactions.     

A General and Mild Catalytic α‐Alkylation of Unactivated Esters Using Alcohols

Catalytic α‐alkylation of esters with primary alcohols is a desirable process because it uses low‐toxicity agents and generates water as the by‐product. Reported herein is a NCP pincer/Ir catalyst which is highly efficient for α‐alkylation of a broad scope of unactivated esters under mild reaction conditions. For the first time, alcohols alkylate unactivated α‐substituted acyclic esters, lactones, and even methyl and ethyl acetates. This method can be applied to the synthesis of carboxylic acid derivatives with diverse structures and functional groups, some of which would be impossible to access by conventional enolate alkylations with alkyl halides.     

From Surface‐Inspired Oxovanadium Silsesquioxane Models to Active Catalysts for the Oxidation of Alcohols with O2—The Cinnamic Acid/Metavanadate System

Silsesquioxane dioxovanadate(V) complexes were investigated with respect to their potential as a catalyst for the oxidative dehydrogenation of alcohols with O₂ as an oxidant. The turnover frequencies determined were comparatively low, but during the oxidation of cinnamic alcohol an increase in activity was observed in the course of the process, which was inspected more closely. It turned out that during the oxidation of cinnamic alcohol, not only was the aldehyde formed but also cinnamic acid, which in turn reacts with the silsesquioxane complex employed to give NBu₄[O₂V(O₂CC₂H₂Ph)₂], which can also be obtained from NBu₄VO₃ and cinnamic acid and represents a far more active catalyst, not only for cinnamic alcohol but also for other activated alcohols and hydrocarbons. The rate‐determining step of the conversion corresponds to an hydrogen‐atom abstraction from the C? H units, as shown by the determination of the kinetic isotope effect in case of 9‐hydroxyfluorene, and the reoxidation of the reduced catalyst proceeds via a peroxo intermediate, which is also capable of oxidizing one alcohol equivalent. Furthermore the influence of the organic residues at the carboxylate ligands on the catalyst performance was investigated, which showed that the activity increases with decreasing pK_s value. Moreover, it was found that during the oxidation the catalyst slowly decomposes, but can be regenerated by addition of excessive carboxylic acid.     

Asymmetric Hydrogenation of α,β‐Unsaturated Nitriles with Base‐Activated Iridium N,P Ligand Complexes

Although many chiral catalysts are known that allow highly enantioselective hydrogenation of a wide range of olefins, no suitable catalysts for the asymmetric hydrogenation of α,β‐unsaturated nitriles have been reported so far. We have found that Ir N,P ligand complexes, which under normal conditions do not show any reactivity towards α,β‐unsaturated nitriles, become highly active catalysts upon addition of N,N‐diisopropylethylamine. The base‐activated catalysts enable conjugate reduction of α,β‐unsaturated nitriles with H₂ at low catalyst loadings, affording the corresponding saturated nitriles with high conversion and excellent enantioselectivity. In contrast, alkenes lacking a conjugated cyano group do not react under these conditions, making it possible to selectively reduce the conjugated C?C bond of an α,β‐unsaturated nitrile, while leaving other types of C?C bonds in the molecule intact.     

Hydrogenation of Esters to Alcohols Catalyzed by Defined Manganese Pincer Complexes

The first manganese‐catalyzed hydrogenation of esters to alcohols has been developed. The combination of Mn(CO)₅Br with [HN(CH₂CH₂P(Et)₂)₂] leads to a mixture of cationic and neutral Mn PNP pincer complexes, which enable the reduction of various ester substrates, including aromatic and aliphatic esters as well as diesters and lactones. Notably, related pincer complexes with isopropyl or cyclohexyl substituents showed very low activity.     

Hydrogenation of Esters to Alcohols with a Well‐Defined Iron Complex

We present the first base‐free Fe‐catalyzed ester reduction applying molecular hydrogen. Without any additives, a variety of carboxylic acid esters and lactones were hydrogenated with high efficiency. Computations reveal an outer‐sphere mechanism involving simultaneous hydrogen transfer from the iron center and the ligand. This assumption is supported by NMR experiments.