Meerwein Ponndorf Verley Oppenauer Research Articles

The ‘Open’ Site of Lewis Acid Zeolites for Meerwein‐Ponndorf‐Verley‐Oppenauer Reactions in Biomass Catalysis

AbstractHeteroatom zeolites are efficient catalysts for activating oxygenates in Meerwein‐Ponndorf‐Verley‐Oppenauer (MPVO) reactions, due to the unique Lewis acid character of their tetrahedral heteroatom metal sites. Such Lewis acid zeolites, especially heteroatom Beta zeolites, are intensively investigated with increasing attention in the valorization of biomass‐derived oxygenates. Herein, we first briefly review the recent development and advantage of Lewis acid zeolites for MPVO reactions involved in biomass valorization, majorly based on the selected case studies of the most representative heteroatom Beta zeolites. Increasing recognization of the open framework sites as the intrinsic active sites for Lewis acid zeolites in MPVO reactions has been discussed, with static and dynamic viewpoints. Furthermore, the recent progress on the development of characterization technologies applied in recent the state‐of‐the‐art showcases of identification and quantification of open sites, has been assessed, with a special focus on indirect detection using probe molecules and direct detection approaches. Future opportunities and challenges regarding the development of complimentary in situ characterization technologies and in‐depth understanding of the true nature of active sites and correlative mechanisms are examined for the ultimate goal of providing stable MPVO catalysts in biomass valorization.

Tunable Solid Acid Catalyst Thin Films Prepared by Atomic Layer Deposition.

Solid acid catalysts, including zeolites and amorphous silica-aluminas (ASAs), are industrially important materials widely used in the fuel and petrochemical industries. The versatility of zeolites is due to the Brønsted acidity of the bridging hydroxyl and shape selectivity that can be tailored during and after synthesis. This is in contrast to amorphous silica-alumina, where tailoring acidity is a major challenge as the Brønsted acid structure in ASA is still debated. In both cases, however, the pore size and acidity cannot be tuned independently, and this is particularly limiting in the application of biomass conversion, where zeolite pores are too small for the molecules of interest. Herein, we present a method using atomic layer deposition (ALD) to prepare thin films of solid acid materials where the ratio of Brønsted to Lewis acid sites can be tuned precisely. This capability, combined with the sub-nm pore size control afforded by ALD yields a powerful and flexible method for synthesizing solid acid catalysts inside virtually any mesoporous host. We demonstrate the utility of these materials in two acid-catalyzed reactions relevant to biomass conversion: (1) Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction and dehydration of fructose and (2) cascade reaction of glucose to 5-hydroxymethylfurfural. Finally, we propose a plausible structure for the Brønsted acid sites in our materials based on infrared spectroscopy and solid-state nuclear magnetic resonance measurements and density functional theory calculations and argue that this same structure might apply to conventional ASAs as well.

Gem‐Diol‐Type Intermediate in the Activation of a Ketone on Sn‐β Zeolite as Studied by Solid‐State NMR Spectroscopy

AbstractLewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl‐containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem‐diol‐type species in the activation of acetone on Sn‐β zeolite. 13C, 119Sn, and 13C–119Sn double‐resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)3Sn‐OH) on Sn‐β is responsible for the formation of the surface species. 13C MAS NMR experiments together with density functional theory calculations suggest that the gem‐diol‐type species exhibits high reactivity and can serve as an active intermediate in the Meerwein—Ponndorf–Verley–Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem‐diol‐type species offers an energy‐preferable pathway for the direct carbon‐to‐carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl‐containing molecules catalyzed by Lewis acid zeolites.

Gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy.

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. 13 C, 119 Sn, and 13 C-119 Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)3 Sn-OH) on Sn-β is responsible for the formation of the surface species. 13 C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.

Effect of Doping Metal Oxide in ZnO/SBA‐15 on Its Acid‐Base Properties and Performance in Ethanol‐to‐Butadiene Process

AbstractThe Ethanol‐to‐Butadiene (ETB) process is experiencing a renaissance with the increasing shortage of petroleum‐based resources, however, the development of active catalysts for this process still remains sluggish due to the lack of fundamental understanding of the properties of catalysts, especially acidity and basicity. We present here that individual reaction step in the ETB process requires specific acid or/and basic sites with different strengths and densities. Weak acid and moderate basic sites are beneficial for ethanol dehydrogenation, while moderate basic sites with optimal strength are also active for condensation and Meerwein‐Ponndorf‐Verley‐Oppenauer (MPVO) reactions. In contrast, moderate and strong acid sites cause the ethanol dehydration, lowering the selectivity towards 1,3‐butadiene (BD) production. Moreover, the selectivity and yield of BD are more dependent on the properties of sites for both condensation and MPVO reactions than for dehydrogenation reaction.

Meerwein–Ponndorf–Verley–Oppenauer Non-catalytic Reaction of Monoterpenoids in Supercritical Fluids

Abstract—The Meerwein–Ponndorf–Verley–Oppenauer non-catalytic reaction of cyclic monoterpenic ketones and alcohols (menthone, camphor, menthol, and borneol) was studied under supercritical conditions. It has been showed that the ketones can be reduced in supercritical isopropanol with high selectivity (89–98%) without any catalyst or base. At the same time, the conversion of menthone was 63% at 350°C, and that of camphor was 11% at 300°C for 5 h reaction. The conversions of menthol and borneol in the reverse dehydrogenation reaction in supercritical acetone were 34 and 22%, respectively, and the selectivities were 95 and 88%. Thermal dehydration is a main side reaction for all the reactions.

Direct Transformation of HMF into 2,5-Diformylfuran and 2,5-Dihydroxymethylfuran without an External Oxidant or Reductant.

The selective transformation of 5-hydroxymethylfurfural (HMF) to valuable 2,5-diformylfuran (DFF) and 2,5-dihydroxymethylfuran (DHMF) is highly desirable but remains a great challenge owing to its tendency to over-oxidation and over-reduction. In this work, HMF is directly converted into DFF and DHMF without external oxidant or reductant through a Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction. In such a MPVO process, HMF is used as both oxidant and reductant and DFF and DHMF are simultaneously produced with a 1:1 molar ratio in the presence of a Lewis acid catalyst. Under high initial HMF concentration, a HMF conversion of up to 44.7 % can be reached within 1 h. Moreover, this atom-efficient transformation route for HMF also provides a promising protocol for the crude separation of DHMF products from DFF products, owing to the lower solubility of DHMF compared to DFF in acetonitrile.

Synthesis, Characterization, and Catalytic Activity of a Series of Aluminium–Amidate Complexes

The aluminium complexes {[κ2-N,O-(t-BuNCOPh)]AlMe2}2 (2), [κ2-N,O-(t-BuNCOPh)]2AlMe (3), and [κ2-N,O-(t-BuNCOPh)]3Al (4) were prepared through the protonolysis reaction between trimethylaluminium and one, two, or three equivalents, respectively, of N-tert-butylbenzamide. Complex 2 was also prepared via a salt metathesis reaction between K(t-BuNCOPh) and dimethylaluminium chloride. Complexes 2–4 were characterized using 1H and 13C NMR spectroscopy. Single-crystal X-ray diffraction analysis of the complexes corroborated ligand : metal stoichiometries and revealed that all the amidate ligands coordinate to the aluminium ion in a κ2 fashion. The Al–amidate complexes 2–4 were viable catalyst precursors for the Meerwein–Ponndorf–Verley–Oppenauer reduction–oxidation manifold, successfully interconverting several classes of carbonyl and alcohol substrates.

Meerwein–Ponndorf–Verley–Oppenauer reaction of crotonaldehyde with ethanol over Zr-containing catalysts

A series of Zr-containing catalysts including bulk ZrO2, ZrO2 supported on silica, titania and ceria, Zr-BEA zeolite and Zr-MCM-41 mesoporous material was prepared and characterized by X-ray diffraction, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, 29Si MAS NMR, TPD NH3, and FTIR of adsorbed CO. The catalytic materials were tested in the MPVO reaction of crotonaldehyde and ethanol in the temperature range of 473–573K. The results showed that the activity of Zr-based catalysts in the MPVO reduction of crotonaldehyde correlates with the amount of Zr4+ Lewis acid sites in the catalysts. Tetrahedral Zr atom sites isolated within the crystalline structure of zeolite BEA were found to be the most efficient. The best catalyst performance in terms of selectivity was observed over ZrBEA: 72% selectivity to crotyl alcohol was achieved at crotonaldehyde conversion of 15%.

Ketone–Alcohol Hydrogen‐Transfer Equilibria: Is the Biooxidation of Halohydrins Blocked?

To ensure the quasi-irreversibility of the oxidation of alcohols coupled with the reduction of ketones in a hydrogen-transfer (HT) fashion, stoichiometric amounts of α-halo carbonyl compounds have been employed as hydrogen acceptors. The reason that these substrates lead to quasi-quantitative conversions has been tacitly attributed to both thermodynamic and kinetic effects. To provide a clear rationale for this behavior, we investigate herein the redox equilibrium of a selected series of ketones and 2-propanol by undertaking a study that combines experimental and theoretical approaches. First, the activity of the (R)-specific alcohol dehydrogenase from Lactobacillus brevis (LBADH) with these substrates was studied. The docking of acetophenone/(R)-1-phenyethanol and α-chloroacetophenone/(S)-2-chloro-1-phenylethanol in the active site of the enzyme confirms that there seems to be no structural reason for the lack of reactivity of halohydrins. This assumption is confirmed by the fact that the corresponding aluminum-catalyzed Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions afford similar conversions to those obtained with LBADH, showing that the observed reactivity is independent of the catalyst employed. While the initial rates of the enzymatic reductions and the IR ν(C=O) values contradict the general belief that electron-withdrawing groups increase the electrophilicity of the carbonyl group, the calculated ΔG values of the isodesmic redox transformations of these series of ketones/alcohols with 2-propanol/acetone support the thermodynamic control of the reaction. As a result, a general method to predict the degree of conversion obtained in the HT-reduction process of a given ketone based on the IR absorption band of the carbonyl group is proposed, and a strategy to achieve the HT oxidation of halohydrins is also shown.

The Meerwein–Ponndorf–Verley–Oppenauer type reaction in supercritical or high-temperature alcohols or acetone without catalyst: Effect of oxidation enthalpy and solvent concentrations on yield

In the traditional Meerwein–Ponndorf–Verley (MPV) reduction, it is known that the yield of the produced alcohol is dependent on the oxidation enthalpy and the concentration of the alcohol used as the reducing reagent. In this study, we investigated the reduction of acetophenone to 1-phenylethanol with alcohols under their supercritical or high-temperature conditions without catalyst to confirm the mechanistic resemblance of the MPV type reduction with supercritical or high-temperature alcohols to the traditional MPV reduction. It was found that the yields of the 1-phenylethanol were dependent on the oxidation enthalpies and the concentrations of the alcohols used as the reducing reagents in common with the traditional MPV reduction. The Oppenauer type oxidation of secondary alcohols afforded the corresponding ketones in around 90% yield with excess amounts of supercritical acetone without catalyst. On the other hand, the Oppenauer type oxidation of primary alcohols to aldehydes resulted in low yields. The low yields were assumed to result from the aldol condensation of the resulting aldehydes with the acetone.

Variable Synthesis of Alcohols and Ketones Using the Grignard Method as a Form of MPVO Reaction.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Variable Synthesis of Alcohols and Ketones Using the Grignard Method as a Form of MPVO Reaction*

The unexpected occurrence of ketones 9 and 16 in addition to expected alcohols 5 and 15 in the synthesis of sesquiterpenes salsolenketone 1 and salsolene oxide 2 during Grignard reactions of the corresponding aldehydes with bromoalkenes under air‐ and water‐free conditions by the Meerwein–Ponndorf–Verley–Oppenauer reaction is reported. The product distribution could be influenced by the amounts of aldehyde used. In Memoriam to Prof. Peter Weyerstahl.

Combined epimerisation and acylation: Meerwein–Ponndorf–Verley–Oppenauer catalysts in action

A practical racemisation-epimerisation method for chiral secondary alcohols has been developed. Meerwein-Ponndorf-Verley-Oppenauer catalysts such as neodymium(III) isopropoxide are able to racemise these alcohols with retention of other stereocentres in the molecule. This is particularly useful for the recycling of the undesired products of kinetic resolutions of alcohols. By combination of such a racemisation with an acylation using isopropenyl or ethoxyvinyl esters as acyl donors, a fast straightforward recycling of starting materials may be achieved. The combined epimerisation and acylation process is demonstrated for the steroid estradiol methyl ether.

The Meerwein–Ponndorf–Verley–Oppenauer reaction between 2-hexanol and cyclohexanone on magnesium phosphates

Various magnesium phosphates were tested as catalysts for the vapour-phase Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reaction between 2-hexanol and cyclohexanone. Some of the solids studied are as selective as MgO in this reaction. Others are also active in the dehydration of the alcohols. The activity and selectivity of the catalysts are related to their structure and surface chemical properties.

Eisen(II)-vermittelte Hydrid- und Methanidübertragung auf Carbonylgruppen

Here we report a gas-phase reaction analogous to the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction but with the important differences that (1) the transition-metal ion Fe ○+ serves as metal M and (2), in addition to hydride transfer, methanide transfers have been observed for the first time. For the migration of R=CH 3 ○- , we have found that steric manipulation on the alkoxide moiety leads to a dramatic change in the course of the reaction. Furthermore, kinetic isotope effects for the hydride transfer were determined