Aryl Backbone Research Articles

Operando Raman Spectroscopy Reveals Degradation Byproducts from Ionomer Oxidation in Anion Exchange Membrane Water Electrolyzers.

This work showcases the discovery of degradation mechanisms for nonplatinum group metal catalyst (PGM free) based anion exchange membrane water electrolyzers (AEMWE) that utilize hydroxide ion conductive polymer ionomers and membranes in a zero gap configuration. An entirely unique and customized test cell was designed from the ground up for the purposes of obtaining Raman spectra during potentiostatic operation. These results represent some of the first operando Raman spectroscopy explorations into the breakdown products that are generated from high oxidative potential conditions with carbonate electrolytes. We provide a unique design and fabrication method for three-dimensional (3D) printable flow cells that enable spatially resolved Raman spectra collection from the electrode surface into the bulk electrolyte. It is proposed that the generation of breakdown products from the hydroxide-conductive ionomers and membranes originates from a multistep, free radical reaction pathway resulting in chain scission of the poly aryl backbone. This hypothesis is backed by the detection of carboxylic and aromatic functional group Raman signals from small molecules that had dissolved and diffused into the bulk electrolyte.

Bridging Dehydrogenation and Hydrogenation in Heterogeneous Catalysis: A Demonstration of a Unified Catalytic Approach.

In the pursuit of enhancing the applications of hydrogen as an energy carrier, this research delved into the utilization of a singular hybrid catalyst capable of performing both dehydrogenation and hydrogenation processes for Liquid Organic Hydrogen Carriers (LOHCs). This study presents the synthesis and characterization of a hybrid catalyst, combining an organometallic pincer complex with Pd-Ru heterostructures supported on γ-alumina. Unlike conventional transition and noble metal nanoparticles, the use of a pincer complex offers exceptional thermal stability due to its aryl backbone, which is advantageous for various endothermic dehydrogenation reactions of hydrocarbons in LOHCs. This pioneering hybrid catalyst is a novel approach, demonstrating a proof of concept. In this study, we utilized the hybrid catalyst to investigate the dehydrogenation and hydrogenation of a lower enthalpic system, specifically the cyclooctane-cyclooctene system. The dehydrogenation of cyclooctane was conducted at 443 K using tertiary butyl ethylene as a sacrificial hydrogen acceptor, while the hydrogenation of cyclooctene reaction catalyzed by Pd-Ru nanostructures occurred at 298 K and 1 atm H2. The results showed successful tandem dehydrogenation-hydrogenation reactions. However, challenges were noted in terms of catalytic activity and recyclability, providing valuable insights for further refinement and optimization.

Regulating the microphase separation structure of poly(aryl-alkylene) anion exchange membranes through the spatial structure of the main chain

Improving the microphase separation structure of membranes is considered an effective strategy to solve the trade-off problem, which can increase the conductivity and reduce the swelling. This article designs poly(aryl alkylene) AEMs with different aryl backbone chains (biphenyl, p-terphenyl and m-terphenyl), with the goal of examining the effect of spatial structure main chains on the membrane performance. It is found that the membrane (m-QTPNP) has a better microphase separation structure and a conductivity of 143.5 mS cm−1 (80 °C) via SAXS, TEM and AFM. Due to the lower IEC (2.23 meq g−1), m-QTPNP has a good anti-swelling performance. During tensile testing, the elongation at break of m-QTPNP can reach 105.5% (wet state). Moreover, due to the obvious microphase separation, the attack of OH− on the quaternized groups is reduced, and m-QTPNP has a conductivity retention rate of 95.5% (80 °C, 2 M NaOH, 1560 h). In addition, the peak power density (PPD) of the H2/O2 single cell equipped with m-QTPNP can reach 749 mW cm−2 (80 °C). After 90 h of durability testing, the voltage decayed to 92.2%, with a decay rate of 0.54 mV h−1.

Self-assembly of chiral diketopyrrolopyrrole chromophores giving supramolecular chains in monolayers and twisted microtapes.

Chiral diketopyrrolopyrroles appended with enantiomeric ethyl lactate functions through an ether linkage to the aryl backbone of the chromophore were synthesized via the Mitsunobu reaction. The molecules have good solubility and excellent optical properties, high molar absorption coefficients, and fluorescence quantum yields. Helical aggregates with circular dichroism arising from the supramolecular arrangement are seen in both solution and thin films, and the aggregates also display circularly polarized luminescence (glum ≈ ±0.1). The molecules assemble to give monolayers on graphite and precipitate from solution forming supramolecular twisted tapes hundreds of microns long.

Physically and Chemically Stable Anion Exchange Membranes with Hydrogen-Bond Induced Ion Conducting Channels.

Anion exchange membranes (AEMs) with desirable properties are the crucial components for numerous energy devices such as AEM fuel cells (AEMFCs), AEM water electrolyzers (AEMWEs), etc. However, the lack of suitable AEMs severely limits the performance of devices. Here, a series of physically and chemically stable AEMs have been prepared by the reaction between the alkyl bromine terminal ether-bond-free aryl backbone and the urea group-containing crosslinker. Morphology analyses confirm that the hydrogen bonding interaction between urea groups is capable of driving the ammonium cations to aggregate and further form continuous ion-conducting channels. Therefore, the resultant AEM demonstrates remarkable OH- conductivity (59.1 mS cm-1 at 30 °C and 122.9 mS cm-1 at 90 °C) despite a moderate IEC (1.77 mmol g-1). Simultaneously, due to the adoption of ether-bond-free aryl backbone and alkylene chain-modified trimethylammonium cation, the AEM possesses excellent alkaline stability (87.3% IEC retention after soaking in 1 M NaOH for 1080 h). Moreover, the prepared AEM shows desirable mechanical properties (tensile stress > 25 MPa) and dimensional stability (SR = 20.3% at 90 °C) contributed by the covalent-bond and hydrogen-bond crosslinking network structures. Moreover, the resulting AEM reaches a peak power density of 555 mW cm-2 in an alkaline H2/O2 single fuel cell at 70 °C without back pressure. This rational structural design presented here provides inspiration for the development of high-performance AEMs, which are crucial for membrane technologies.

Novel fluorinated aromatic polymers with ether-bond-free aryl backbones for pure and mixed gas separation

Three novel fluorinated polymers with ether-bond-free aryl backbones were obtained by one-pot, metal-free, room temperature superacid catalyzed non-stoichiometric step polymerization of 1,1,1-trifluoromethyl acetone with carbazole (CBZ), fluorene (FLN) and 9,9-dimethylfluorene (DFL). 1H and 13C NMR analyses of the polymers revealed their linear structure and high regioselectivity of the polymer forming reaction. The polymers also possess high thermostability with decomposition temperature up to 387 °C for CBZ and up to 530 °C for FLN and DFL as well as reasonable mechanical properties. The gas transport properties of six pure gases and binary gas mixture CO2/CH4 (10/90 mol%) are reported. These polymers present a good combination of permeability and selectivity up to 10 bar at 35 °C for CO2/CH4 mixed-gas feed. DFL exhibits the best pure-gas permeability and selectivity (PCO2 = 78.0, PH2 = 151.5, CO2/CH4 = 22.3 and H2/CH4 = 43.3) and mixed-gas permeability and selectivity CO2/CH4 (PCO2 = 71.3, CO2/CH4 = 23.7) at 35 °C and 2 bar with a slight decrease in selectivity at 10 bar. The highest permselectivity is obtained for FLN (CO2/CH4 = 36.7 and H2/CH4 = 88.4) with a separation performance close to the 2008 upper bound trade off.

Electronic Ligand Modifications on Cobalt Complexes and Their Application toward the Semi-Hydrogenation of Alkynes and Para-Hydrogenation of Alkenes

The effect of the electronic modification of a bis(carbene) pincer ligand, (MesCCCR), on cobalt catalysis has been investigated. The pincer ligand was modified in the para position of the aryl backbone with a tert-butyl and trifluoromethyl moiety to yield the electronic variants that were applied toward the synthesis and characterization of several cobalt complexes, (MesCCCR)Co. The application of the (MesCCCR)CoI(N2)PPh3 complexes toward the semihydrogenation of alkynes revealed that while the tert-butyl group does not impact reactivity, the loss of electron density at the metal center, by the installation of the CF3 group, does affect product ratios. Further inspection of the proposed mechanism suggested that the installation of the trifluoromethyl group slows down olefin hydrogenation. This finding was further supported in the application of the (MesCCCR)CoI-py (py = pyridine) complexes toward the parahydrogenation of ethyl acrylate, which demonstrated that the electron-withdrawing ligand variant produced less polarization.

Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells

One promising approach to reduce the cost of fuel cell systems is to develop hydroxide exchange membrane fuel cells (HEMFCs), which open up the possibility of platinum-group-metal-free catalysts and low-cost bipolar plates. However, scalable alkaline polyelectrolytes (hydroxide exchange membranes and hydroxide exchange ionomers), a key component of HEMFCs, with desired properties are currently unavailable, which presents a major barrier to the development of HEMFCs. Here we show hydroxide exchange membranes and hydroxide exchange ionomers based on poly(aryl piperidinium) (PAP) that simultaneously possess adequate ionic conductivity, chemical stability, mechanical robustness, gas separation and selective solubility. These properties originate from the combination of the piperidinium cation and the rigid ether-bond-free aryl backbone. A low-Pt membrane electrode assembly with a Ag-based cathode using PAP materials showed an excellent peak power density of 920 mW cm−2 and operated stably at a constant current density of 500 mA cm−2 for 300 h with H2/CO2-free air at 95 °C. A key challenge for hydroxide exchange membrane fuel cells is the development of membranes with both high ionic conductivity and mechanical strength. Here the authors report a high-performance family of poly(aryl piperidinium) membranes enabling promising durability and power density.

Environmental photochemistry by plasmonic semiconductor decorated GO nanocomposites: SERS detection and visible light driven degradation of aromatic dyes

Environmental process such as toxic dye sensing and degradation can be stepped up through photochemical process such as surface enhanced Raman scattering (SERS) and llight induced catalytic degradation. In this work, Titanium dioxide/silver/graphene oxide nanocomposites (TAG) were studied as ideal dual substrates for SERS sensing and visible light driven degradation of Congo red (CR) and crystal violet (CV). Solvothermal route of synthesize used here resulted in a narrow size distribution (6–12 nm) of TAG. A shift towards the visible spectra observed in absorption spectroscopy along with the anatase phase titanium dioxide illustrated in the X-ray diffraction pattern and Raman spectra aided the activation of TAG under visible light irradiation. On addition of TAG, typical SERS bands corresponding to the azo nature of CR were clearly exhibited by C-N symmetric bending modes and C-N stretching modes in the 1160 cm−1–1200 cm−1 region. For CV, the aryl backbone represented by C-C bridge between two phenyl molecules at 1298 cm−1 was recorded in the SERS spectra. Visible light driven degradation of CV and CR in the presence of TAG resulted in high rate constants of 1.09 × 10−2/min and 1.26 × 10−2/min respectively. The reactive oxidative radical species in the photo-degradation reaction of dye was studied by using various scavengers. The observed augmented SERS detection and photocatalytic degradation arises from the synergistic effects of plasmonic properties Ag NPs, anatase TiO2 and π-π stacking nature of GO. This study suggests that TAG can offer the right 00path way towards an ideal commercial pollutant detection and degradation system.

Transition‐Metal‐Free Approach for the Direct Arylation of Thiophene: Experimental and Theoretical Investigations towards the (Het)‐Aryne Route

This paper presents the results of our investigations on the arylation of thiophene using the transition‐metal‐free “aryne coupling” methodology. The reaction was studied by both experiment and computation (density functional theory) and comparison with phenyllithium was established. In parallel, the effects of the ligand and the salt on the coupling reaction were examined. The results underline the remarkable effect of such additives on the coupling reaction and the potency of the method to construct hetaryl–aryl backbones which open up promising access to a wide range of heterobiaryl structures using the novel “Het‐Aryne” route.

Halogen Bonding in Ring‐Substituted Group 10 POCOP Iodido Complexes with Iodine and Its Possible Role in Oxidative Addition

Halogen bonding (XB) of an isostructural series of complexes of the type [3,5‐tBu(tBuPOCOPtBu)MI] [3,5‐tBu2(tBuPOCOPtBu) = κ3‐C6H‐3,5‐(tBu)2‐2,6‐(OPtBu2)2; M = Ni, Pd, Pt] with iodine as XB donor was studied in the solid state. Co‐crystallization experiments afforded single crystals for all three XB systems [3,5‐tBu(tBuPOCOPtBu)NiI] (1‐I2), [3,5‐tBu(tBuPOCOPtBu)PdI] (2‐I2), and [3,5‐tBu(tBuPOCOPtBu)PtI] (3‐I2). Whereas in the case of1‐I2, two molecules of I2bind to the iodide ligand, complexes2‐I2and3‐I2are isostructural 1:1 adducts with almost orthogonal binding of I2to the metal iodido acceptor. All adducts form zig‐zag chain structures where the pincer complexes act as bifunctional XB acceptors, forming XB interactions between metal iodide ligand and I2as well as between the aryl backbone of the pincer ligand and the I2XB donor. DFT studies were performed to analyze the electronic structure of the XB adducts. Halogen bonded adducts and related products of I2coordination were examined as formal intermediates in oxidative addition of I2to a metal(II) center. Computational data suggest that the herein described complexes do not play a role in this context.

Promoting Intermolecular C-N Bond Formation under the Auspices of Iodine(III).

The quest for the development of new protocols that provide general conditions for oxidative carbon-nitrogen bond formation has grown over recent years. Within this context, due to feasibility and benignity considerations in biochemical sciences, reactions that rely on main group oxidants as the only promoters have received particular interest. We have recently found that simple protonolysis events enable the incorporation of nitrogenated groups of the bissulfonimide family into the coordination sphere of common iodine(III) complexes such as diacetoxy iodobenzene. The products of the type ArI(OAc)(NTs2) represent rare examples of iodine(III) compounds displaying reactive iodine-nitrogen single bonds. Further protonolysis furnishes the corresponding iodine(III) compounds ArI(NTs2)2 containing two defined iodine-nitrogen single bonds for unprecedented dual transfer of both nitrogenated groups. It is of great synthetic importance that these new compounds contain iodine-nitrogen entities, which upon dissociation in solution lead to electrophilic iodine centers and nucleophilic nitrogen groups. This has enabled the development of a body of conceptually new amination reactions, which do not rely on conventional electrophilic nitrogen reagents but rather employ iodine(III) as an electrophilic activator and bissulfonimides as the source of subsequent nucleophilic amination. Additional diversification arises from the ambident nature of bissulfonimines enabling oxygenation pathways. The exciting chemistry covered in this Account comprises structural features of the reagents (including X-ray analysis), scope and limitation in synthetic amination of different hydrocarbons (including sp-, sp2-, and sp3-hybridized centers as in acetylenes, alkenes, enols, butadienes, allenes, arenes, and alkylketones), and physical-organic and theoretical analysis of the underlying reaction mechanisms. The oxidative transformations with all their rich diversifications originate from the versatile redox chemistry of the iodine(III) and iodine(I) pair, which shares several aspects of transition metal high oxidation state chemistry. For the present aryliodine(III) reagents, steric and electronic fine-tuning is possible through accurate engineering of the arene substituent. In addition to the general reactivity of the I-N bond, chiral aryliodine(III) reagents with defined stereochemical information in the aryl backbone are conceptually compatible with this approach. Thus, the development of enantioselective amination reactions with up to 99% ee was also successful. Several of the active enantioselective reagents have been isolated and structurally characterized. Following this approach for the important class of chiral vicinal diamines, an unprecedented direct diamination of alkenes could be conducted in an enantioselective catalytic manner under full intermolecular reaction control. This latter reaction is based on the precise engineering of a chiral aryliodine(III) catalyst in combination with bismesylimide as nitrogen source. It is the consequence of the precise understanding of the reaction behavior of structurally defined bisimidoiodine(III) reagents.

Poly(aryl piperidinium) Based Hydroxide Exchange Membranes and Ionomers

The first commercial sales of fuel cell automobiles based on high-performance proton exchange membrane fuel cells (PEMFCs) mark an important milestone for the field, bringing to the market a zero-emission vehicle with the advantages of long driving range and short refuelling time.1 Chiefly due to the use of Pt electrocatalysts in PEMFCs, they are not yet cost competitive with gasoline engines. Thus, further cost reductions are needed to facilitate PEMFCs’ wider adoption. Current efforts have been focused on the reduction of the Pt loading in membrane electrode assemblies (MEAs), however, the acidic environment has been a significant barrier.2 A fundamentally different approach to lower the cost of fuel cells is to switch from the acidic proton exchange membranes (PEMs) to the alkaline hydroxide exchange membranes (HEMs) in HEM fuel cells (HEMFCs), which could enable the employment of platinum-group-metal (PGM)-free anode and cathode catalysts.3 , 4 A key challenge in the development of high performance HEMFCs is the lack of chemically stable, ionically conductive, and mechanically robust HEMs. HEMs are typically composed of organic cations tethered to a polymer backbone, with the charge balanced by free OH− anions. The alkaline stability of both the organic cation and the polymer backbone limit the overall stability of the HEM.5 Recently, Marino and Kreuer reported that piperidinium exhibited a 21-fold increase in alkaline stability as compared to benzyl trimethyl ammonium, the benchmark ammonium.6 Meanwhile, a reliable and scalable synthetic route to tether piperidinium to a stable polymer backbone remains elusive. It is generally accepted that functionalized ether-bond-containing engineering plastics, e.g., poly(arylene ether) and poly(phenylene oxide), are susceptible to chemical degradation via ether-bond scission and the subsequent decomposition.7 Another key challenge in the development of HEMs is the tradeoff between ionic conductivity and mechanical strength. In this work, we report a new family of poly(aryl piperidinium) (PAP) HEMs/HEIs in which alkaline stable piperidinium cations were tethered to an ether-bond-free, rigid and hydrophobic aryl backbone. PAP’s excellent properties originate from the combination of the stable piperidinium cation and the rigid ether-bond-free aryl backbone which enables an unprecedented combination of a high ion exchange capacity (IEC) and a low water uptake/swelling ratio. PAP HEMs have demonstrated excellent alkaline stability (e.g., no degradation in 1 M KOH for 2000 h at 100 °C), high OH− conductivity (e.g., 200 mS cm−1 at 95 °C in water), and high mechanical strength (140 MPa). PAP also has the selective solubility to make ionomer solutions and can be cast into membranes with thicknesses down to 5 µm. MEAs using PAP HEM and the corresponding ionomer showed a peak power density of 860 mW cm−2 at 95 °C with minimal optimization. Further, discharge at 90 °C revealed no voltage degradation at constant current density of 200 mA cm−2 for 60 h, in contrast to the rapid degradation of a commercial membrane/ionomer under identical conditions (within minutes). (1) Yoshida, T.; Kojima, K. Electrochem Soc Inte 2015, 24, 45. (2) Setzler, B. P.; Zhuang, Z. B.; Wittkopf, J. A.; Yan, Y. S. Nat Nanotechnol 2016, 11, 1020. (3) Varcoe, J. R.; Slade, R. C. T. Fuel Cells 2005, 5, 187. (4) Dekel, D. R. J Power Sources 2017. (5) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T. W.; Zhuang, L. Energ Environ Sci 2014, 7, 3135. (6) Marino, M. G.; Kreuer, K. D. Chemsuschem 2015, 8, 513. (7) Arges, C. G.; Ramani, V. P Natl Acad Sci USA 2013, 110, 2490.

High-valent nitridorhenium(v) complexes containing PNP ligands: implications of ligand flexibility.

The synthesis of (PNP)Re(N)X (PNP = [2-P(CHMe2)2-4-MeC6H3]2N, X = Cl and Me) complexes is described. The methylnitridorhenium complex 3 was found to react differently with CO and isocyanides, leading to the isolation of a Re(v) acyl complex 4 and an isocyanide adduct 6. Two parallel pathways were observed for the reaction of 3 with CO: (1) CO inserts into the Re-Me bond to afford 4, and (2) 3 isomerizes by distortion of the aryl backbone of the PNP ligand to afford the isomer 3'. This is followed by the reaction of 3' with CO to afford the tricarbonyl complex 5, which was fully characterized. The contrasting reaction of 3 with 2,6-dimethylphenyl isocyanide lends further support for the proposed isomerization pathway. DFT (M06) calculations suggest that insertion of CNR into the Re-Me bond (27.2 kcal mol-1) is inaccessible at room temperature. Instead the substrate adds to the metal center via the most accessible face i.e. syn to the rhenium-nitrido bond, to afford 6. The addition of CO to isomer 3' is proposed to proceed with a similar mechanism to 2,6-dimethylphenyl isocyanide.

Iridium(iii) hydrido complexes for the catalytic dehydrogenation of hydrazine borane.

The synthesis of 3,5-disubstituted cyclometalated iridium(iii) hydrido complexes of the type [3,5-R2(POCOP)IrHX] (3,5-R2(POCOP) = κ3-C5HR2-2,6-(OPtBu2)2 with R = t-Bu, COOMe; X = Cl, H) is described. All complexes were investigated in the catalytic dehydrogenation of hydrazine borane and compared with the unsubstituted compounds [(POCOP)IrHX] (X = Cl, H). All catalysts are highly active and recyclable, clearly maintaining hydrogen production activity. The dehydrogenation products were structurally characterised by solid state NMR and FTIR spectroscopy. Experimental observations were complemented by a dispersion-corrected DFT study to rationalise the mechanism of hydrazine borane dehydrogenation.

Aluminium salophen and salen initiators in the ring-opening polymerisation of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerisation rates

Three aluminium salophen and two aluminium salen complexes were synthesised, characterised and screened in the ring-opening polymerisation (ROP) of rac-lactide and rac-β-butyrolactone. The focus was on controlling the apparent polymerisation rate (kp) and stereoselectivity of poly(lactic acid) and poly(3-hydroxybutyrate) by modulating the electron density at the aluminium centre or by switching from an alkyl backbone (salen complex) to an aryl backbone (salophen complex). The salen complexes generally showed higher kp as well as isoselectivity compared to the salophen complexes. For instance, salophen and salen complexes biased the microstructure of poly(3-hydroxybutyrate) towards syndiotacticity and isotacticity, respectively. Electron-withdrawing or electron-donating backbones on a salophen complex tuned kp, with electron-donating backbones offering faster kp.

Phenol-derived chiral phosphine–phosphite ligands in the rhodium-catalyzed enantioselective hydrogenation of functionalized olefins

A set of 15 chiral Taddol- and Binol-based phosphine–phosphite ligands were tested in the Rh-catalyzed asymmetric hydrogenation of three olefins, methyl 2-hydroxymethyl-acrylate, 1-phenylvinyl acetate, and α-methyl cinnamic acid. The best enantioselectivities (up to 92% ee) were observed in the hydrogenation of methyl 2-hydroxymethyl-acrylate using Binol-based ligands. As previously observed in other applications of this class of modular chiral ligand in enantioselective catalysis, the stereochemical outcome of the reactions greatly depended on the substituents at the ligand aryl backbone in the ortho-position to the chiral phosphite unit.

Enantioselective Copper‐Catalysed Allylic Alkylation of Cinnamyl Chlorides by Grignard Reagents using Chiral Phosphine‐Phosphite Ligands

AbstractThe copper(I)‐catalysed SN2′‐type allylic substitution of E‐3‐aryl‐allyl chlorides (cinnamyl chlorides) using Grignard reagents represents a powerful method for the synthesis of compounds carrying a benzylic stereocentre. By screening a small library of modular chiral phosphine‐phosphite ligands a new copper(I)‐based catalyst system was identified which allows the performance of such reactions with exceptional high degrees of regio‐ and enantioselectivity. Best results were obtained using TADDOL‐derived ligands (3 mol%), copper(I) bromide⋅dimethyl sulfide (CuBr⋅SMe2) (2.5 mol%) and methyl tert‐butyl ether (MTBE) as a solvent. Various (1‐alkyl‐allyl)benzene derivatives were prepared with up to 99% ee (GC) in isolated yields of up to 99%. In most cases the product contained less than 3% of the linear regioisomer (except for ortho‐substituted substrates). Both electron‐rich and electron‐deficient cinnamyl chlorides were successfully employed. The absolute configuration of the products was assigned by comparison of experimental and calculated CD spectra. The substrates were prepared from the corresponding alcohols by reaction with thionyl chloride. Initially formed mixtures of regioisomeric allylic chlorides were homogenised by treatment with CuBr⋅SMe2 (2.5 mol%) in the presence of triphenyl phosphine (PPh3) (3 mol%) in MTBE at low temperature to give the pure linear isomers. In reactions with methylmagnesium bromide (MeMgBr) an ortho‐diphenylphosphanyl‐arylphosphite ligand with an additional phenyl substituent in ortho′‐position at the aryl backbone proved to be superior. In contrast, best results were obtained in the case of higher alkyl Grignard reagents (such as ethyl‐, n‐butyl‐, isopropyl‐, and 3‐butenylmagnesium bromides) with a related ligand carrying an isopropyl substituent in ortho′‐position. The method was tested on a multi‐mmol scale and is suited for application in natural product synthesis.

Effects of Electron-Deficient β-Diketiminate and Formazan Supporting Ligands on Copper(I)-Mediated Dioxygen Activation

Copper(I) complexes of a diketiminate featuring CF(3) groups on the backbone and dimethylphenyl substituents (4) and a nitroformazan (5) were synthesized and shown by spectroscopy, X-ray crystallography, cyclic voltammetry, and theory to contain copper(I) sites electron-deficient relative to those supported by previously studied diketiminate complexes comprising alkyl or aryl backbone substituents. Despite their electron-poor nature, oxygenation of LCu(CH(3)CN) (L = 4 or 5) at room temperature yielded bis(hydroxo)dicopper(II) compounds and at -80 degrees C yielded bis(mu-oxo)dicopper complexes that were identified on the basis of UV-vis and resonance Raman spectroscopy, spectrophotometric titration results (2:1 Cu/O(2) ratio), electron paramagnetic resonance spectroscopy (silent), and density functional theory calculations. The bis(mu-oxo)dicopper complex supported by 5 exhibited unusual spectroscopic properties and decayed via a novel intermediate proposed to be a metallaverdazyl radical complex, findings that highlight the potential for the formazan ligand to exhibit "noninnocent" behavior.

Synthesis of hydroxypyrone- and hydroxythiopyrone-based matrix metalloproteinase inhibitors: Developing a structure–activity relationship

The zinc(II)-dependent matrix metalloproteinases (MMPs) are associated with a variety of diseases. Development of inhibitors to modulate MMP activity has been an active area of investigation for therapeutic development. Hydroxypyrones and hydroxythiopyrones are alternative zinc-binding groups (ZBGs) that, when combined with peptidomimetic backbones, comprise a novel class of MMP inhibitors (MMPi). In this report, a series of hydroxypyrone- and hydroxythiopyrone-based MMPi with aryl backbones at the 2-, 5-, and 6-positions of the hydroxypyrone ring have been synthesized. Synthetic routes for developing inhibitors with substituents at two of these positions (so-called double-handed inhibitors) are also explored. The MMP inhibition profiles and structure–activity relationship of synthesized hydroxypyrones and hydroxythiopyrones have been analyzed. The results here show that the ZBG, the position of the backbone on the ZBG, and the nature of the linker between the ZBG and backbone are critical for MMPi activities.