Catalytic transformation of alcohols to carboxylic acid salts and H2 using water as the oxygen atom source
TL;DR: An alternative dehydrogenative pathway effected by water and base with the concomitant generation of hydrogen gas is described, which could be a safer and cleaner process for the synthesis of carboxylic acids and their derivatives at both laboratory and industrial scales.
Abstract: The development of a catalytic, mild and atom-economical transformation of alcohols to carboxylic acid salts and hydrogen gas is described. The reaction uses water as a source of oxygen, with a homogenous Ru catalyst at low (0.2 mol%) catalyst loadings in basic aqueous solution.
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TL;DR: In this paper, the potential of lignocellulosic biomass as an alternative platform to fossil resources has been analyzed and a critical review provides insights into the potential for LBS.
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TL;DR: In this article, the potential of lignocellulosic biomass as an alternative platform to fossil resources has been analyzed and a critical review provides insights into the potential for LBS.
Abstract: The demand for petroleum dependent chemicals and materials has been increasing despite the dwindling of their fossil resources. As the dead-end of petroleum based industry has started to appear, today's modern society has to implement alternative energy and valuable chemical resources immediately. Owing to the importance of lignocellulosic biomass being the most abundant and bio-renewable biomass on earth, this critical review provides insights into the potential of lignocellulosic biomass as an alternative platform to fossil resources. In this context, over 200 value-added compounds, which can be derived from lignocellulosic biomass by various treatment methods, are presented with their references. Lignocellulosic biomass based polymers and their commercial importance are also reported mainly in the frame of these compounds. This review article aims to draw the map of lignocellulosic biomass derived chemicals and their synthetic polymers, and to reveal the scope of this map in today's modern chemical and polymer industry.
1,089 citations
TL;DR: Acceptorless dehydrogenation and related dehydrogenative coupling reactions have the potential for redirecting synthetic strategies to the use of sustainable resources, devoid of toxic reagents and deleterious side reactions, with no waste generation.
Abstract: Conventional oxidations of organic compounds formally transfer hydrogen atoms from the substrate to an acceptor molecule such as oxygen, a metal oxide, or a sacrificial olefin. In acceptorless dehydrogenation (AD) reactions, catalytic scission of C-H, N-H, and/or O-H bonds liberates hydrogen gas with no need for a stoichiometric oxidant, thereby providing efficient, nonpolluting activation of substrates. In addition, the hydrogen gas is valuable in itself as a high-energy, clean fuel. Here, we review AD reactions selectively catalyzed by transition metal complexes, as well as related transformations that rely on intermediates derived from reversible dehydrogenation. We delineate the methodologies evolving from this recent concept and highlight the effect of these reactions on chemical synthesis.
1,088 citations
TL;DR: Examples of MLC in which both the metal and the ligand are chemically modified during bond activation and 2) Bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligands is not directly bound to the metal.
Abstract: Metal-ligand cooperation (MLC) has become an important concept in catalysis by transition metal complexes both in synthetic and biological systems. MLC implies that both the metal and the ligand are directly involved in bond activation processes, by contrast to "classical" transition metal catalysis where the ligand (e.g. phosphine) acts as a spectator, while all key transformations occur at the metal center. In this Review, we will discuss examples of MLC in which 1) both the metal and the ligand are chemically modified during bond activation and 2) bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligand is not directly bound to the metal (e.g. via tautomerization). The role of MLC in enabling effective catalysis as well as in catalyst deactivation reactions will be discussed.
846 citations
TL;DR: The heterolytic activation of dihydrogen by NH/H2 MLC was reported by Fryzuk and co-workers in 1987 with pincer Ir and Rh complexes, and complex 73 was found to be an active dehydrogenation catalyst for ammoniaborane and hydrogenation reactions as discussed below.
Abstract: ed by an internal site in intramolecular heterolytic splitting, intermolecular activation requires an external base. Such intramolecular heterolytic splitting is also applicable to C−H or other heteroatom−H bond activation as is frequently observed with ruthenium pincer complexes and discussed in this section. 3.1. Activation of the H−H Bond As shown in Scheme 4b, formation of a dihydride complex such as [(PNP)Ru(H)2(H2)] 20 is a result of dihydrogen activation by oxidative addition due to the electron-rich ruthenium center as discussed above. Gunnnoe and co-workers reported a five-coordinate amido complex (PCP)Ru(CO)(NH2) 71, which heterolytically activates H2, as 71 has the required vacant coordination site for H2 coordination and a basic amido ligand, which abstracts intramolecularly a proton from coordinated H2, followed by dissociation of the formed ammonia (BDERu−NH3 = 12.6 kcal/mol), providing the complex (PCP)RuH(CO) 72 (Scheme 23). The dearomatized pincer complexes 3a−5a developed by Milstein react with molecular hydrogen at room temperature, undergoing rearomatization to provide the trans-dihydride complexes of 3b−5b, respectively, upon heterolysis of dihydrogen (Scheme 24). In the H NMR spectra, the magnetically equivalent trans-dihydrides of 3b and 4b resonate as a triplet at δ = −4.96 ppm (JPH = 20.0 Hz) and δ = −4.90 ppm (JPH = 17.0 Hz), respectively, whereas they display a doublet at δ = −4.06 ppm (JPH = 17.0 Hz) for complex 5b. Further, the structure of the trans-dihydride complex 5b was determined by a single-crystal X-ray analysis (Figure 2). The trans-dihydride complexes 3b−5b slowly lose H2 at room temperature to regenerate complexes 3a−5a, respectively, setting the stage for several catalytic processes (see below) including the production of molecular hydrogen from biorenewable sources. As discussed in the MLC section, the acridine pincer complex 9 also heterolytically activates dihydrogen (Scheme 16). The amido ruthenium pincer complex 73 is in equilibrium with complexes 74 and 75. Formation of amino trans-dihydride complex 74 results from the reversible heterolysis of dihydrogen (Scheme 25). The trans-dihydrides of 74, which resonate at δ = −8.00 and δ = −8.52 ppm, are magnetically inequivalent (JPH = 16.7−24.3 Hz), due to the proximity of one of the hydrides to the backbone amino proton. As observed in heterolysis of hydrogen by Ir pincer complexes, DFT studies suggest that proton transfer from ruthenium to the amido nitrogen is facilitated by the presence of water, which forms a hydrogen bond with both amido nitrogen and H2, leading to a six-membered transition state and lowering the barrier for proton transfer. Complex 73 was found to be an active dehydrogenation catalyst for ammoniaborane and hydrogenation reactions as discussed below. The heterolytic activation of dihydrogen by NH/H2 MLC was reported by Fryzuk and co-workers in 1987 with pincer Ir and Rh complexes. Using the pincer complex [RuCl(PPh3)Scheme 22. Oxidative Addition versus Heterolytic Cleavage of H−H and H−X Bonds Scheme 23. Heterolytic Activation of Hydrogen Scheme 24. Heterolytic Activation of Hydrogen by Aromatization−Dearomatization Process Figure 2. X-ray structure of complex 5b (50% probability level). Hydrogen atoms (except hydrides) are omitted for clarity. Reproduced with permission from ref 100. Copyright 2011 Nature Publishing Group. Chemical Reviews Review dx.doi.org/10.1021/cr5002782 | Chem. Rev. XXXX, XXX, XXX−XXX K (N(SiMe2CH2PPh2)2)], they reported in 1991 that reaction with H2 resulted in a mixture of Ru−H complexes, albeit the reversibility of the reaction was not observed. Recently, Koridze and co-workers reported that a metallocene-derived PCP pincer ruthenium complex also activates molecular hydrogen heterolytically. 3.2. Activation of C−H Bonds While many PCP-type ruthenium pincer complexes were formed as a result of C−H activation or even double C−H activation of the ligand, there are only a few examples of intraor intermolecular C−H activations by ruthenium pincer complexes, as discussed in this section. C−H activation of arenes is widespread with Ir-pincer PNP and PCP complexes. However, such sp C−H activation by Rupincer complexes is rarely encountered. An example of intramolecular C−H activation in a Ru pincer complex was reported by Fryzuk and co-workers. Gunnnoe and co-workers reported that the five-coordinate complexes [(PCP)Ru(CO)(NH2)] 71 and [(PCP)Ru(CO)(CH3)] 76 liberate ammonia and methane, respectively, to generate the cyclometalated PCP ruthenium pincer complex 77 as a result of intramolecular sp C−H activation (Scheme 26). The conversion rate of the methyl complex 76 to 77 is approximately 5 times faster (Kobs = 3.2(1) × 10 −4 s−1 at 50 °C) than the analogous conversion with the amido complex 71 (Kobs = 6.0(3) × 10 −5 s−1 at 50 °C). However, attempted intermolecular C−H activation of methane by complex 71 was not successful and led only to intramolecular C−H activation to provide 77. Gusev and co-workers reported a double C−H activation reaction in a pincer complex. Caulton’s square-planar Ru(II) pincer complex 63a reacted with 1 equiv of MeLi at −78 °C to afford the hydrido−carbene complex 79 as a result of double C−H activation of a single methyl group (Scheme 27). Apart from the characteristic spectroscopic features, DFT calculations indicate that the Ru−C bond length (2.08 Å) in complex 78 decreases to 1.84 Å in 79, in line with formation of a Ru− carbene. The intermediate complex 78 facilitates the C−H agostic interaction and the observed loss of methane. The hydrido−carbene complex 79 reacts with excess of pyridine, immediately forming the diamagnetic η-pyridyl complex 80 as a result of ortho sp C−H activation. Recently, Milstein reported the intramolecular sp C−H activation of expanded PNN ruthenium pincer complexes. Upon deprotonation of the saturated Ru(II) PNN pincer complexes (7 and 8), containing alkyl or cycloalkyl groups on one of the methylene arms, the dearomatized pincer complexes 7a and 8a were formed (Scheme 28). Attempted synthesis of 7a also resulted in intramolecular C−H activation and quantitative formation of the cyclometalated 7b after 4 h. Independent synthesis of 7a was possible using the base KHMDS (potassium hexamethyldisilazide) at −35 °C in toluene-d8, although upon workup only 7b was obtained as a single diastereoisomer, implying that 7a is indeed an intermediate that undergoes cyclometalation and concomitant aromatization to provide 7b. The X-ray structure of 7b exhibits a highly distorted octahedron with the PNC donors coordinated in a pseudomeridional manner (Figure 3). The PNN complex 8a, analogous to 7a, was relatively stable due to steric rigidity, and the conversion to cyclometalated aromatized complex 8b occurred slowly over 5 days. Dearomatized pincer complexes, particularly the bipyridine derived [(PNN)Ru(H)(CO)] 6a and its parent complex 6, are involved in the sp CH activation/exchange of αand βpositions of alcohols with D2O, as will be discussed in section 4.3.1. 3.3. Activation of N−H Bonds As in H2 and C−H activation reactions, MLC of pyridinederived pincer complexes also enables N−H activation of amines and ammonia. As described by Milstein, upon reaction of the dearomatized PNP complex 4a with electron-poor anilines, the unsaturated ligand arm was protonated, and the pyridine ring underwent rearomatization as a result of N−H activation (Scheme 29). The saturated amide complexes 82a Scheme 25. Heterolytic Activation of Hydrogen by Amide−Amine Interconversion Scheme 26. Intramolecular Activation of sp C−H Bonds Scheme 27. Double C−H Activation Leading to the Formation of a Hydrido Carbene Complex Chemical Reviews Review dx.doi.org/10.1021/cr5002782 | Chem. Rev. XXXX, XXX, XXX−XXX L and 82b were obtained in pure form from the reaction of complex 4a with 4-nitroaniline and 2-chloro-4-nitroaniline, respectively, and their symmetrical structures were evident from the P NMR (single signal for each complex) and H NMR spectra. However, the reaction of 2-bromoaniline and 3,4dichloroaniline with complex 4a provided equilibrium mixtures of aromatized N−H activated amide complexes 82c and 82d, and the starting 4a, despite the presence of excess of haloanilines. This observation of reversible N−H bond activation at room temperature was unique, and indicated the low barrier for these reactions. For electron-rich amines and ammonia, the amine-coordinated unsaturated complexes of type 81 are thermodynamically favored. Upon reaction of complex 4a with ND3, formation of the deuterated complex 83 was observed after 5 min at room temperature, showing N−D activation (Scheme 30). The H NMR spectrum of 83 confirmed that deuteration at the methylene arm is stereospecific as only one of the two CH2 arm protons disappeared. No exchange occurred with vinylic protons, and such high selectivity indicated that the activation process on these types of pincer systems occurs only intramolecularly on one face of the ligand with the coordinated ND3 ligand. This observation indicated that other activation processes could also be stereoselective. The trend observed in the experiment was in agreement with DFT studies carried out on the N−H activation reactions (Figure 4). In the reaction of 2-bromoaniline with 4a, the unbound state (I) and the activated state (III) have similar energies with a connecting barrier of 20 kcal/mol. The N−H activated complex of isopropylamine is 16.8 kcal mol−1 above the unbound state, and experimentally it reacted with complex 4a upon warming to 80 °C, whereas in the case of the amines that underwent irreversible N−H activation at room temperature the calculated activated complexes (III) are energetically below the unbound state (I) as expected; barriers for the exchange between amine coordinated (II) and activated (III) states are accessible at room temperature. PNP Rh(I) pincer complexes also activate N−H bonds by similar MLC. N−H bond acti
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TL;DR: Increasing research efforts are carried out to design and develop more efficient anode electrocatalysts for DAFCs, which are attracting increasing interest as power sources for portable applications.
Abstract: Direct alcohol fuel cells (DAFCs) are attracting increasing interest as power sources for portable applications due to some unquestionable advantages over analogous devices fed with hydrogen.1 Alcohols, such as methanol, ethanol, ethylene glycol, and glycerol, exhibit high volumetric energy density, and their storage and transport are much easier as compared to hydrogen. On the other hand, the oxidation kinetics of any alcohol are much slower and still H2-fueled polymer electrolyte fuel cells (PEMFCs) exhibit superior electrical performance as compared to DAFCs with comparable electroactive surface areas.2,3 Increasing research efforts are therefore being carried out to design and develop more efficient anode electrocatalysts for DAFCs.
1,427 citations
TL;DR: A reaction in which primary amines are directly acylated by equimolar amounts of alcohols to produce amides and molecular hydrogen in high yields and high turnover numbers is reported.
Abstract: Given the widespread importance of amides in biochemical and chemical systems, an efficient synthesis that avoids wasteful use of stoichiometric coupling reagents or corrosive acidic and basic media is highly desirable. We report a reaction in which primary amines are directly acylated by equimolar amounts of alcohols to produce amides and molecular hydrogen (the only products) in high yields and high turnover numbers. This reaction is catalyzed by a ruthenium complex based on a dearomatized PNN-type ligand [where PNN is 2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine], and no base or acid promoters are required. Use of primary diamines in the reaction leads to bis-amides, whereas with a mixed primary-secondary amine substrate, chemoselective acylation of the primary amine group takes place. The proposed mechanism involves dehydrogenation of hemiaminal intermediates formed by the reaction of an aldehyde intermediate with the amine.
1,098 citations
TL;DR: The fundamental concepts underlying the principles of green and sustainable chemistry--atom and step economy and the E factor--are presented, within the general context of efficiency in organic synthesis, and the transition from fossil-based chemicals manufacture to a more sustainable biomass-based production is discussed.
Abstract: In this tutorial review, the fundamental concepts underlying the principles of green and sustainable chemistry - atom and step economy and the E factor - are presented, within the general context of efficiency in organic synthesis. The importance of waste minimisation through the widespread application of catalysis in all its forms – homogeneous, heterogeneous, organocatalysis and biocatalysis – is discussed. These general principles are illustrated with simple practical examples, such as alcohol oxidation and carbonylation and the asymmetric reduction of ketones. The latter reaction is exemplified by a three enzyme process for the production of a key intermediate in the synthesis of the cholesterol lowering agent, atorvastatin. The immobilisation of enzymes as cross-linked enzyme aggregates (CLEAs) as a means of optimizing operational performance is presented. The use of immobilised enzymes in catalytic cascade processes is illustrated with a trienzymatic process for the conversion of benzaldehyde to (S)-mandelic acid using a combi-CLEA containing three enzymes. Finally, the transition from fossil-based chemicals manufacture to a more sustainable biomass-based production is discussed.
1,095 citations
TL;DR: A water-soluble palladium(II) bathophenanthroline complex is a stable recyclable catalyst for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water-alcohol system.
Abstract: Alcohol oxidations are typically performed with stoichiometric reagents that generate heavy-metal waste and are usually run in chlorinated solvents. A water-soluble palladium(II) bathophenanthroline complex is a stable recyclable catalyst for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water-alcohol system. The use of water as a solvent and air as the oxidant makes the reaction interesting from both an economic and environmental point of view.
1,024 citations
TL;DR: This tutorial review briefly discusses organic synthesis in water with a Green Chemistry perspective.
Abstract: The use of water as solvent features many benefits such as improving reactivities and selectivities, simplifying the workup procedures, enabling the recycling of the catalyst and allowing mild reaction conditions and protecting-group free synthesis in addition to being benign itself. In addition, exploring organic chemistry in water can lead to uncommon reactivities and selectivities complementing the organic chemists' synthetic toolbox in organic solvents. Studying chemistry in water also allows insight to be gained into Nature's way of chemical synthesis. However, using water as solvent is not always green. This tutorial review briefly discusses organic synthesis in water with a Green Chemistry perspective.
923 citations