Transfer Dehydrogenation Research Articles

Access to α,β-Unsaturated Carboxylic Acids by Iridium-Catalyzed Transfer Dehydrogenation

Access to α,β-Unsaturated Carboxylic Acids by Iridium-Catalyzed Transfer Dehydrogenation

Iridium-Catalyzed Oxidant-Free Transfer Dehydrogenation of Carboxylic Acids.

Direct dehydrogenation of carboxylic acids to their unsaturated counterparts represents a valuable transformation for complex molecule synthesis, which, however, has been challenging to achieve. In addition, the current carbonyl desaturation methods are almost all based on oxidative conditions. Here we report an Ir-catalyzed redox-neutral transfer dehydrogenation approach to directly convert carboxylic acids to either α,β- or β,γ-unsaturated counterparts. These reactions avoid using oxidants or strong bases, thus, tolerating various functional groups. The combined experimental and computational mechanistic studies suggest that this transfer hydrogenation reaction involves directed C-H oxidative addition, β-H elimination, and dihydride transfer to an alkene acceptor with C(sp3)-H reductive elimination as the turnover-limiting step.

Production of Lactic Acid via Catalytic Transfer Dehydrogenation of Glycerol Catalyzed by Base Metal Salt Ferrous Chloride and Its NNN Pincer-Iron Complexes.

NNN-Bis(imino) pyridine-based pincer-Fe(II) complexes with an expected trigonal bipyramidal (TBP) geometry equilibrated to a rearranged ion pair of an octahedral dicationic Fe complex containing two bis(imino)pyridine ligands that are neutralized by a tetrahedral dianionic-[FeCl4]2-. Single-crystal X-ray diffraction (SCXRD), high-resolution mass spectrometry (HRMS), and UV-visible (UV-vis) studies suggested that the equilibrium was dictated by the sterics of the R group on the imine N, with the less-crowded groups tilting the equilibrium to the ion pair and the bulky ones favoring the TBP geometry. Electron paramagnetic resonance (EPR) and Evan's magnetic moment measurements indicated that the complexes were paramagnetic with Fe(II) in a high-spin state. In solution, over a period of 7 days, these Fe(II) complexes oxidized to a mixture of low-spin and high-spin Fe(III) species. These pincer-Fe(II) were found to be highly active toward the transformation of biodiesel waste glycerol to value-added lactic acid (LA). Particularly, (Ph2NNN)FeCl2 (0.1 mol %) gave 91% LA with a 99% selectivity at 140 °C using 1.2 equiv of NaOH. With 0.0001 mol % (Ph2NNN)FeCl2, very high turnovers (74% LA, 98% selectivity, 740 000 turnover number (TON) at 4405 turnovers per hour (TOs/h)) were obtained after 7 days. EPR indicated Fe(III) species to be the active catalyst, a few of which were detected by HRMS. Experiments with Hg are suggestive of the mostly homogeneous molecular nature of the catalyst with a minor contribution from heterogeneous Fe nanoparticles.

Stability of Medium-Ring Cyclic Unsaturated Carbonyl Compounds: Direct Access to Unsaturated Ketones with C-C Double Bonds at Distal Positions via Transfer Dehydrogenation of Alicyclic Ketones.

Medium-ring cyclic compounds have characteristic distortions. For alicyclic enones, conjugated enones are known to be less stable than unconjugated enones. In this study, the relative stability of cycloalkenones with varied C-C double bond positions with C6-C12 rings was theoretically investigated to reveal that C8-C12 cycloalkenones prefer the unconjugated isomer. Furthermore, direct access to distal unsaturated cycloalkenones has been accomplished by transfer dehydrogenation of cycloalkanones catalyzed by iridium complexes.

An Iminophosphinite Pincer Iridium Complex: Synthesis and Catalytic Tool for Alkane Transfer Dehydrogenations

AbstractA [(POCN)Ir(H)(OAc)] complex bearing an iminophosphinite pincer ligand was synthesized. The complex showed catalytic transfer dehydrogenation of various linear and cyclic alkanes with reasonable conversions and a maximum TON of 400 for n‐octane. A catalytic isomerization of 1‐octene to yield internal octenes was also possible. It is notable that the catalyst could perform catalytic transfer dehydrogenation even at a temperature of 100 °C with a TON of 100.

The Transfer Dehydrogenation Method Enables a Family of High Crystalline Benzimidazole‐linked Cu (II)‐phthalocyanine‐based Covalent Organic Frameworks Films

AbstractHeterocycle‐linked phthalocyanine‐based COFs with close‐packed π–π conjugated structures are a kind of material with intrinsic electrical conductivity, and they are considered to be candidates for photoelectrical devices. Previous studies have revealed their applications for energy storage, gas sensors, and field‐effect transistors. However, their potential application in photodetector is still not fully studied. The main difficulty is preparing high‐quality films. In our study, we found that our newly designed benzimidazole‐linked Cu (II)‐phthalocyanine‐based COFs (BICuPc‐COFs) film can hardly formed with a regular aerobic oxidation method. Therefore, we developed a transfer dehydrogenation method with N‐benzylideneaniline (BA) as a mild reagent. With this in hand, we successfully prepared a family of high crystalline BICuPc‐COFs powders and films. Furthermore, both of these new BICuPc‐COFs films showed high electrical conductivity (0.022–0.218 S/m), higher than most of the reported COFs materials. Due to the broad absorption and high conductivity of BICuPc‐COFs, synaptic devices with small source‐drain voltage (VDS=1 V) were fabricated with response light from visible to near‐infrared. Based on these findings, we expect this study will provide a new perspective for the application of conducting heterocycle‐linked COFs in synaptic devices.

The Transfer Dehydrogenation Method Enables a Family of High Crystalline Benzimidazole-linked Cu (II)-phthalocyanine-based Covalent Organic Frameworks Films.

Heterocycle-linked phthalocyanine-based COFs with close-packed π-π conjugated structures are a kind of material with intrinsic electrical conductivity, and they are considered to be candidates for photoelectrical devices. Previous studies have revealed their applications for energy storage, gas sensors, and field-effect transistors. However, their potential application in photodetector is still not fully studied. The main difficulty is preparing high-quality films. In our study, we found that our newly designed benzimidazole-linked Cu (II)-phthalocyanine-based COFs (BICuPc-COFs) film can hardly formed with a regular aerobic oxidation method. Therefore, we developed a transfer dehydrogenation method with N-benzylideneaniline (BA) as a mild reagent. With this in hand, we successfully prepared a family of high crystalline BICuPc-COFs powders and films. Furthermore, both of these new BICuPc-COFs films showed high electrical conductivity (0.022-0.218 S/m), higher than most of the reported COFs materials. Due to the broad absorption and high conductivity of BICuPc-COFs, synaptic devices with small source-drain voltage (VDS=1 V) were fabricated with response light from visible to near-infrared. Based on these findings, we expect this study will provide a new perspective for the application of conducting heterocycle-linked COFs in synaptic devices.

Mechanistic Insights of the Ir‐bipyridonate Catalyzed Aqueous Methanol Dehydrogenation and Transfer Dehydrogenation to Acetophenone: Experimental and DFT Study

AbstractThe mechanisms of the Cp*IrIII(bpyOO)‐catalyzed (bpyOO=bidentate (NN) doubly deprotonated 2,2′‐bipyridine‐6,6′‐diol) acceptorless methanol dehydrogenation and acetophenone transfer hydrogenation by methanol under basic conditions have been explored by the combination of 1H NMR, kinetics, and DFT computational studies. During dehydrogenation of methanol and of its dehydrogenated derivatives, the presence of two iridium hydride species (anionic [Cp*Ir(bpyOO)H]−, C* and neutral [Cp*Ir(bpyOOH)H], D*), which interconvert depending on pH, was detected. The DFT studies on a Cp model system highlighted three interrelated catalytic cycles of methanol, formaldehyde and formic acid dehydrogenation, all leading to the same hydride intermediates C and D. The dehydrogenation of methanol prefers a direct β‐hydride transfer pathway from the methoxide ion to Ir, rather than the classical β‐hydride elimination pathway from a coordinated methoxide ligand, but an alternative bifunctional H+/H− transfer with involvement of a ligand O atom may become competitive at lower pH. The transfer hydrogenation of acetophenone using methanol as hydrogen source features species C* as resting state, with the acetophenone reduction being rate‐determining and following the reverse pathway of methanol oxidation, with a first‐order acetophenone decay and a kinetic isotope effect of 2.36±0.09.

Chloroiridium Complexes of Biaryl-Based Diphosphines for Thermal Catalytic Transfer Dehydrogenation of Hindered 1,1-Disubstituted Ethanes

Chloroiridium Complexes of Biaryl-Based Diphosphines for Thermal Catalytic Transfer Dehydrogenation of Hindered 1,1-Disubstituted Ethanes

Efficient net transfer-dehydrogenation of glycerol: NNN pincer-Mn and manganese chloride as a catalyst unlocks the effortless production of lactic acid and isopropanol.

Herein, a series of pincer-Mn complexes based on bis(imino)pyridine ligands of the type R2NNN (R = tBu, iPr, Cy and Ph) were synthesized and characterized using various spectroscopic techniques. SCXRD studies revealed a trigonal bipyramidal geometry around the metal center in all the complexes. EPR spectroscopy confirmed the presence of high-spin Mn(II) centers with the consistent observation of sextets in EPR spectra. Additionally, solution magnetic moment measurement exhibited values ranging from 5.8 to 6.2 BM for all the complexes, which are in accordance with the theoretical value of 5.92 BM. HRMS analysis complemented structural characterization, showing fragments corresponding to various solvent adducts and derivatives of the complexes. Subsequently, the synthesized complexes were investigated for their catalytic activity in the transfer dehydrogenation of glycerol to lactic acid in the presence of acetone. Among the considered complexes, the catalyst Ph2NNNMnCl2 was found to be highly efficient. Remarkably, a yield of 92% LA was observed with >99% selectivity at 0.5 mol% loading of Ph2NNNMnCl2 in the presence of 1 equivalent of NaOH at 140 °C in 24 h, surpassing the yield obtained from its precursor MnCl2·4H2O, where a yield of 72% LA was observed with 96% selectivity under similar reaction conditions. This catalytic system was further investigated with a range of acceptors, and good to moderate yields were observed in most cases. Moreover, several control experiments, including reaction with PPh3, CS2 and Hg, highlighted the major involvement of molecular species in the reaction medium. Deuterium labelling studies indicated the involvement of C-H bond activation in the catalytic cycle but not in the rate-determining step (RDS), with a secondary kinetic isotope effect (KIE) of 1.25.

Catalyst-Free Transfer Hydrogenation from Amine-Borane Small Oligomers.

Amine-borane dimers and oligomers with varied steric and electronic profiles were prepared via capping agent-controlled AA/BB polycondensations. They were used for transfer hydrogenations to aldehydes, ketones, imines as well as electron-poor alkene/alkyne moieties. The amine-borane Lewis-paired oligomers and the congested bis(amine-borane)s provided the highest yields. This was likely helped by facilitated dissociation (oligomers) or H-bond assistance. In the case of the oligomers, the second equivalent of H2 present was also engaged in the reaction. Solid-state NMR characterization provides evidence that the boron-containing materials obtained after transfer dehydrogenation are highly similar to those obtained from thermal dehydrogenation. The oligomers bridge the gap between simple amine-borane molecular reductants and the poly-amine-boranes and provide a full picture of the reactivity changes at the different scales.

Acceptorless or Transfer Dehydrogenation of Glycerol Catalyzed by Base Metal Salt Cobaltous Chloride - Facile Access to Lactic Acid and Hydrogen or Isopropanol.

The dehydrogenation of glycerol to lactic acid (LA) under both acceptorless and transfer dehydrogenation conditions using readily available, inexpensive, environmentally benign and earth-abundant base metal salt CoCl2 is reported here. The CoCl2 (0.5 mol %) catalyzed acceptorless dehydrogenation of glycerol at 160 °C in the presence of 0.75 equiv. of KOH, gave up to 33 % yield of LA in 44 % selectivity apart from hydrogen. Alternatively, with acetone as a sacrificial hydrogen acceptor, the CoCl2 (0.5 mol %) catalyzed dehydrogenation of glycerol at 160 °C in the presence of 1.1 equiv. of NaOt Bu resulted in up to 93 % LA with 96 % selectivity along with another value-added product isopropanol. Labelling studies revealed a modest secondary KIE of 1.68 which points to the involvement of C-H bond activation as a part of the catalytic cycle but not as a part of the rate-determining step. Catalyst poisoning experiments with PPh3 and CS2 are indicative of the homogeneous nature of the reaction mixture involving molecular species that are likely to be in-situ formed octahedral Co(II) as inferred from EPR, HRMS and Evans magnetic moment studies. The net transfer dehydrogenation activity is attributed to exclusive contribution from the alcoholysis step.

Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations.

(Cyclopentadienone)iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4'-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

Chemical Modification of Oxidized Polyethylene Enables Access to Functional Polyethylenes with Greater Reuse.

Polyethylene is a commodity material that is widely used because of its low cost and valuable properties. However, the lack of functional groups in polyethylene limits its use in applications that include adhesives, gas barriers, and plastic blends. The inertness of polyethylene makes it difficult to install groups that would enhance its properties and enable programmed chemical decomposition. To overcome these deficiencies, the installation of pendent functional groups that imbue polyethylene with enhanced properties is an attractive strategy to overcome its inherent limitations. Here, we describe strategies to derivatize oxidized polyethylene that contains both ketones and alcohols to monofunctional variants with bulk properties superior to those of unmodified polyethylene. Iridium-catalyzed transfer dehydrogenation with acetone furnished polyethylenes with only ketones, and ruthenium-catalyzed hydrogenation with hydrogen furnished polyethylenes with only alcohols. We demonstrate that the ratio of these functional groups can be controlled by reduction with stoichiometric hydride-containing reagents. The ketones and alcohols serve as sites to introduce esters and oximes onto the polymer, thereby improving surface and bulk properties over those of polyethylene. These esters and oximes were removed by hydrolysis to regenerate the original oxygenated polyethylenes, showing how functionalization can lead to materials with circularity. Waste polyethylenes were equally amenable to oxidative functionalization and derivatization of the oxidized material, showing that this low- or negative-value feedstock can be used to prepare materials of higher value. Finally, the derivatized polymers with distinct solubilities were separated from mechanically mixed plastic blends by selective dissolution, demonstrating that functionalization can lead to novel approaches for distinguishing and separating polymers from a mixture.

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

AbstractSynthesis, 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.

A precise method to monitor hydroxyl radical in natural waters based on a fluoride-containing fluorescence probe

In natural waters, hydroxyl radical (OH) can initiate many free radical-induced reactions, oxidizing various inorganic and organic compounds through electron transfer reactions, dehydrogenation reactions, addition reactions, and self-quenching reactions. However, due to its extremely low concentration and short lifetime in natural waters, studies on the quantitative measurement of OH levels are insufficient. In this work, we developed the first quinolinium-based fluorescence probe containing fluoride substituted donor that could detect hydroxyl radicals in the water system. This probe exhibits excellent selectivity towards OH with a large Stokes shift (114 nm) and 23-fold enhancement in fluorescence. Additionally, this probe has been proven to be low toxicity and applied to detect OH in living cells, zebrafish, and natural water samples with good recovery (over 92 %).

Photoredox Catalysis with Visible Light for Synthesis of Thioxanthones Derivatives.

A visible-light-promoted and transition-metal-free photoredox-catalysis strategy is developed for the synthesis of thioxanthone derivatives (TXs). The mechanistic study and density functional theory calculations suggest that visible-light-promoted intramolecular cyclization can be divided into hydrogen atom transfer, C-C formation, and oxidative dehydrogenation with high regioselectivity and reactivity. Significantly, this C-C formation strategy can be used in TXs-based commercial photoinitiators and drugs at the gram level.

Regulation of Redox Molecular Junctions in Covalent Organic Frameworks for H2O2 Photosynthesis Coupled with Biomass Valorization

AbstractH2O2 photosynthesis coupled with biomass valorization can not only maximize the energy utilization but also realize the production of value‐added products. Here, a series of COFs (i.e. Cu3‐BT‐COF, Cu3‐pT‐COF and TFP‐BT‐COF) with regulated redox molecular junctions have been prepared to study H2O2 photosynthesis coupled with furfuryl alcohol (FFA) photo‐oxidation to furoic acid (FA). The FA generation efficiency of Cu3‐BT‐COF was found to be 575 mM g−1 (conversion ≈100 % and selectivity >99 %) and the H2O2 production rate can reach up to 187 000 μM g−1, which is much higher than Cu3‐pT‐COF, TFP‐BT‐COF and its monomers. As shown by theoretical calculations, the covalent coupling of the Cu cluster and the thiazole group can promote charge transfer, substrate activation and FFA dehydrogenation, thus boosting both the kinetics of H2O2 production and FFA photo‐oxidation to increase the efficiency. This is the first report about COFs for H2O2 photosynthesis coupled with biomass valorization, which might facilitate the exploration of porous‐crystalline catalysts in this field.

Thermocatalytic Dehydrogenation‐Enabled Arene‐Alkane Couplings†

Comprehensive SummaryThe direct use of non‐prefunctionalized arene and alkane as the starting materials to construct Caryl−Calkyl bond is an unfulfilled target and approaches to this challenge rarely surface in methodology studies. Current methods for thermocatalytic arene‐alkane couplings (AAC) occur with specific substrates and/or inconvenient reagents. Herein, we report a one‐pot relay bicatalysis system for AAC involving (pincer)Ir‐catalyzed alkane transfer dehydrogenation and Fe(OTf)3‐catalyzed olefin hydroarylation. This system exhibits broad scope and is particularly effective for alkylation of arenes with arylalkanes to form 1,1‐diarylalkanes with high chemo‐ and regioselectivity, making it potentially useful for late‐stage alkylation of complex molecules. Experimental mechanistic data provide a view into the factors controlling the regioselectivity. Finally, the strategy of dehydrogenation‐enabled arene‐alkane couplings has been successfully extended to a tandem catalysis by using a heterogeneous olefin hydroarylation catalyst.

Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand

PNCNP pincer iridium complexes possessing arylnitrene linkers (−N(Ar)−) catalyze transfer dehydrogenation of cyclooctane (COA) under low-temperature conditions, giving 200 turnovers at 125 °C using tert-butylethylene (TBE) and 60 turnovers at 50 °C using cyclohexene (CHE) as a sacrificial hydrogen acceptor. Despite prior reports dealing with the synthetic difficulty of pincer iridium complexes bearing di(isopropyl)phosphoramine sidearms, our design using chemically inert (3,5-disubstituted-aryl)nitrene linkers allowed for the successful complexation with iridium. The structural characterization and computational study revealed that steric congestion provided by arylnitrene linkers destabilizes the resting internal alkene adducts, thereby reducing the overall activation barrier of the rate-limiting C–H addition. The steric congestion also hinders undesirable disproportionation of CHE, allowing for its use as an effective hydrogen acceptor. With their straightforward synthesis coupled with fine tunability through the 3,5-substituents on the aryl groups, the N,N′-diarylated PNCNP ligands presented in this work will be a valuable addition to the highly successful PCP pincer framework.