Chemoselective Catalytic Oxidation Research Articles

Application of Pd-Catalyzed C-H Alkylation Reaction in Total Syntheses of Twelve Amicoumacin-Type Natural Products.

Enantioselective total syntheses of 12 amicoumacin-type natural products are accomplished with a palladium(II)-catalyzed C-H alkylation as the key step to furnish the 3,4-dihydroisocoumarin scaffold. The target chemicals are assembled in a convergent protocol by merging 3,4-dihydroisocoumarin derived amine part with categories of acid segments that are efficiently prepared by chemoselective catalytic oxidation of chiral 1,2-dihydroxyethylfuran-2(5H)-ones. Afterward, the cytotoxicity of amicoumacins on five cancer cell lines and one normal cell line is investigated.

Chemoselective Catalytic Oxidation of Olefin Derivatives with Co–Salen Immobilized SBA-15

Chemoselective catalytic oxidation of olefin derivatives containing alcoholic and olefinic groups is important in organic synthesis. In the present study, Co–Salen immobilized on SBA-15 was synthesized and characterized to study its chemoselective catalytic activity towards the oxidation of cinnamyl alcohol and cinnamaldehyde, with tert-butyl hydroperoxide as the oxidant at 60, 70 and 80 oC. Cinnamyl alcohol selectively yielded cinnamaldehyde at the reactant ratio of cinnamyl alcohol tert-butyl hydroperoxide equal to 1:0.5. Cinnamaldehyde selectively yielded 2,3-epoxy-cinnamaldehyde at the ratio of cinnamaldehyde tert-butyl hydroperoxide equal to 1:0.5. The catalyst yielded epoxide with many styrene derivatives and benzaldehyde with benzyl alcohol, but only cinnamaldehyde with cinnamyl alcohol, thus proving its applicability for chemoselective catalytic oxidation of olefin derivatives. The cinnamaldehyde conversion and selectivity to 2,3-epoxy-cinnamaldehyde increased with the increase in the content of tert-butyl hydroperoxide. So, this study establishes that the catalyst can be applied to selective oxidation of primary alcoholic function without affecting olefinic double bonds. The conversion was maximum at 70 oC. The conversion and product selectivity remained the same for 5 cycles thus endorsing stability of the catalysts. Based on the activity and selectivity of the catalysts, it is inferred that the same catalyst can be exploited for selective oxidation of multifunctional olefin derivatives.

Discovery of novel enzyme genes involved in the conversion of an arylglycerol-β-aryl ether metabolite and their use in generating a metabolic pathway for lignin valorization

Microbial conversions known as “biological funneling” have attracted attention for their ability to upgrade heterogeneous mixtures of low-molecular-weight aromatic compounds obtained by chemical lignin depolymerization. β-hydroxypropiovanillone (HPV) and its analogs can be obtained by chemoselective catalytic oxidation of lignin using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone/tert-butyl nitrite/O2, followed by cleavage of arylglycerol-β-aryl ether with zinc. Sphingobium sp. strain SYK-6 can degrade HPV generated by the catabolism of arylglycerol-β-aryl ether through 2-pyrone-4,6-dicarboxylate (PDC), a promising platform chemical. Therefore, production of PDC from HPV can be achieved using the HPV catabolic pathway. However, the pathway and genes involved in the catabolism of vanilloyl acetic acid (VAA) generated during HPV catabolism have not been investigated. In the present study, we isolated SLG_24960 (vceA), which encodes an enzyme that converts VAA into a coenzyme A (CoA) derivative of vanillate (vanilloyl-CoA) from SYK-6, by shotgun cloning. The analysis of a vceA mutant indicated that this gene is not required for VAA conversion in vivo, but it encodes a major enzyme catalyzing CoA-dependent VAA conversion in vitro. We also identified SLG_12450 (vceB), whose product can convert vanilloyl-CoA to vanillate. Enzyme genes besides vceA and vceB, which are necessary for the conversions of HPV to VAA and of vanillate to PDC, were introduced and expressed in Pseudomonas putida. The resulting engineered strain completely converted 1 mM HPV into PDC after 24 h. Our results suggest that the enzyme genes that are not required for the catabolic pathway in microorganisms but can be used for the conversion of target substrates are buried in microbial genomes. These genes are, thus, useful for designing metabolic pathways to produce value-added metabolites.

Bacterial Catabolism of β-Hydroxypropiovanillone and β-Hydroxypropiosyringone Produced in the Reductive Cleavage of Arylglycerol-β-Aryl Ether in Lignin.

Sphingobium sp. strain SYK-6 converts four stereoisomers of arylglycerol-β-guaiacyl ether into achiral β-hydroxypropiovanillone (HPV) via three stereospecific reaction steps. Here, we determined the HPV catabolic pathway and characterized the HPV catabolic genes involved in the first two steps of the pathway. In SYK-6 cells, HPV was oxidized to vanilloyl acetic acid (VAA) via vanilloyl acetaldehyde (VAL). The resulting VAA was further converted into vanillate through the activation of VAA by coenzyme A. A syringyl-type HPV analog, β-hydroxypropiosyringone (HPS), was also catabolized via the same pathway. SLG_12830 (hpvZ), which belongs to the glucose-methanol-choline oxidoreductase family, was isolated as the HPV-converting enzyme gene. An hpvZ mutant completely lost the ability to convert HPV and HPS, indicating that hpvZ is essential for the conversion of both the substrates. HpvZ produced in Escherichia coli oxidized both HPV and HPS and other 3-phenyl-1-propanol derivatives. HpvZ localized to both the cytoplasm and membrane of SYK-6 and used ubiquinone derivatives as electron acceptors. Thirteen gene products of the 23 aldehyde dehydrogenase (ALDH) genes in SYK-6 were able to oxidize VAL into VAA. Mutant analyses suggested that multiple ALDH genes, including SLG_20400, contribute to the conversion of VAL. We examined whether the genes encoding feruloyl-CoA synthetase (ferA) and feruloyl-CoA hydratase/lyase (ferB and ferB2) are involved in the conversion of VAA. Only FerA exhibited activity toward VAA; however, disruption of ferA did not affect VAA conversion. These results indicate that another enzyme system is involved in VAA conversion.IMPORTANCE Cleavage of the β-aryl ether linkage is the most essential process in lignin biodegradation. Although the bacterial β-aryl ether cleavage pathway and catabolic genes have been well documented, there have been no reports regarding the catabolism of HPV or HPS, the products of cleavage of β-aryl ether compounds. HPV and HPS have also been found to be obtained from lignin by chemoselective catalytic oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone/tert-butyl nitrite/O2, followed by cleavage of the β-aryl ether with zinc. Therefore, value-added chemicals are expected to be produced from these compounds. In this study, we determined the SYK-6 catabolic pathways for HPV and HPS and identified the catabolic genes involved in the first two steps of the pathways. Since SYK-6 catabolizes HPV through 2-pyrone-4,6-dicarboxylate, which is a building block for functional polymers, characterization of HPV catabolism is important not only for understanding the bacterial lignin catabolic system but also for lignin utilization.

ChemInform Abstract: Chemoselective Catalytic Oxidation of 1,2‐Diols to α‐Hydroxy Acids Controlled by TEMPO—ClO2 Charge‐Transfer Complex.

AbstractA broad range of diversely substituted 1,2‐diols, including optically active ones, are efficiently oxidized to the corresponding α‐hydroxy acids.

Chemoselective Catalytic Oxidation of 1,2-Diols to α-Hydroxy Acids Controlled by TEMPO–ClO2 Charge-Transfer Complex

Chemoselective catalytic oxidation from 1,2-diols to α-hydroxy acids in a cat. TEMPO/cat. NaOCl/NaClO2 system has been achieved. The use of a two-phase condition consisting of hydrophobic toluene and water suppresses the concomitant oxidative cleavage. A study of the mechanism suggests that the observed selectivity is derived from the precise solubility control of diols and hydroxy acids as well as the active species of TEMPO. Although the oxoammonium species TEMPO(+)Cl(-) is hydrophilic, the active species dissolves into the organic layer by the formation of the charge-transfer (CT) complex TEMPO-ClO2 under the reaction conditions.

Isolation of Functionalized Phenolic Monomers through Selective Oxidation and CO Bond Cleavage of the β‐O‐4 Linkages in Lignin

AbstractFunctionalized phenolic monomers have been generated and isolated from an organosolv lignin through a two‐step depolymerization process. Chemoselective catalytic oxidation of β‐O‐4 linkages promoted by the DDQ/tBuONO/O2 system was achieved in model compounds, including polymeric models and in real lignin. The oxidized β‐O‐4 linkages were then cleaved on reaction with zinc. Compared to many existing methods, this protocol, which can be achieved in one pot, is highly selective, giving rise to a simple mixture of products that can be readily purified to give pure compounds. The functionality present in these products makes them potentially valuable building blocks.

Isolation of functionalized phenolic monomers through selective oxidation and C-O bond cleavage of the β-O-4 linkages in lignin.

Functionalized phenolic monomers have been generated and isolated from an organosolv lignin through a two-step depolymerization process. Chemoselective catalytic oxidation of β-O-4 linkages promoted by the DDQ/tBuONO/O2 system was achieved in model compounds, including polymeric models and in real lignin. The oxidized β-O-4 linkages were then cleaved on reaction with zinc. Compared to many existing methods, this protocol, which can be achieved in one pot, is highly selective, giving rise to a simple mixture of products that can be readily purified to give pure compounds. The functionality present in these products makes them potentially valuable building blocks.

First insights of the Pdpt/C and Pdptru/C Electrocatalysts Towards Glycerol Electro-Oxidation in Alkaline Medium

Platinum is a powerful and widely catalyst used for oxidation of various molecules like glycerol [i] . Compared to others catalysts (example Au), Pt has a lower oxidation potential in alkaline medium, however shows low kinetics for the reaction [ii] ,[iii] . Furthermore several studies have shown that the glycerol electrooxidation on Pt electrodes leads to the formation of various by-products causing decrease in the efficiency of the systemiv. Another disadvantage of Pt as catalysts is that platinum is an expensive noble metal and has low natural abundance.In this study we used PdPt and PdPtRu based electrocatalysts for glycerol electro-oxidation in alkaline medium. The carbon-supported materials were prepared by formic acid medium[iv]. After thorough physic-chemical characterizations, they were evaluated with regards to the glycerol electro-oxidation reaction. The experiments were conducted in 1.0 M NaOH solutions at 25°C, in absence/presence of 0.1 M of glycerol.In absence of glycerol, the cyclic voltammograms (CV) for all electrocatalysts show typical behavior of Pd, Pt and Ru materials. In presence of glycerol, Pd25Pt25Ru50/C electrocatalyst presented better performance; the onset potential for this material was close to 0.4 V vs. RHE, similar to Pt/C and the maximum peak current obtained was 107 mA cm-2, against 76 mA cm-2obtained for Pt/C electrocatalyst.FTIR measurements for all electrocatalysts were assessed to investigate the possible by-products from the glycerol electro-oxidation reaction (Fig. 1). For all materials, Pt/C, Pd/C, Pd50Pt50/C and Pd25Pt25Ru50/C, was possible identify four main absorption bands from glycerol electro-oxidation, which clearly evidence that the main products from glycerol electro-oxidation reaction are the same for any of these catalysts. The absorption band at 1308 cm-1 is typical of oxalate ions and it is presented in all electrocatalysts. The double band at 1349 and 1382 cm-1 are typical of formate ions standard spectra. The double band was only seen for Pd/C and Pd25Pt25Ru50/C electrocatalysts.As glycerol electro-oxidation leads to the formation of various by-products, formic acid can be one, however platinum electrocatalysts is able to oxidize formate ion to CO2. These results are consistent, since for Pt/C (Fig. 1B) electrocatalysts was not identified the double band feature of formic acid, however a single band was identified at 1387 cm-1, feature of carbonate from CO2. Pd50Pt50/C (Fig. 1C) electrocatalysts exhibited similar behavior for Pt/C.Then, according to results obtained from FTIR spectroscopy, the electro-oxidation of glycerol on Pt/C and Pd50Pt50/C electrocatalysts leads to the formation of by-products such as oxalate, carbonate and other carboxylates (1572 cm-1), whereas Pd/C and Pd25Pt25Ru50/C (Fig. 1A and B, respectively) leads to the formation of oxalate, formate and carboxylates ions. Probably, carbonate also may be produced but the presence of double band in the range of 1349 – 1382 cm-1 may be overlapped the carbonate band. This seems to mean that the higher amount of Pt in the catalysts contribute to the oxidize glycerol directly to CO2.The authors thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (particularly, process n. 2012/10856-7) [[i]]Garcia, R. Besson, M. Gallezot, P. Chemoselective catalytic oxidation of glycerol with air on platinum metals. Appl Catal A: Gen . v. 127, p. 165-176, 1995. [[ii]]Gomes, J. F. Tremiliosi-Filho, G. Spectroscopic studies of the glycerol electro-oxidation on polycrystalline Au and Pt surfaces in acidic and alkaline media. Electrocatalysis. v. 2, p. 96- 105. 2011.[[iii]]Kwon, Y. Schouten, P. J. Koper, M. T. M. Mechanism of the catalytic oxidation of glycerol on polycrystalline gold and platinum electrodes. ChemCatChem. v. 3, p. 1176-1185. 2011.[[iv]]Pinheiro, A.L.N. Oliveira-Neto, A. de Souza, E.C. Perez, J. Paganin, V.A. Ticianelli, E.A. Gonzalez, E.R. Electrocatalysis on noble metal and noble metal alloys dispersed on high surface area carbon. J. New Mater. Electrochem. Syst. 6 (2003)

Catalytic oxidation of sulfides to sulfoxides using sodium perborate and/or sodium percarbonate and silica sulfuric acid in the presence of KBr

Abstract Chemoselective catalytic oxidation of sulfides has been developed. A variety of aliphatic and aromatic sulfides are subjected to the sulfoxidation reaction by treatment of sodium perborate (SPB) and/or sodium percarbonate (SPC), silica sulfuric acid (SSA) and catalytic amounts of KBr in the presence of wet SiO2 (50% w/w) in dichloromethane at room temperature with moderate to good yields.

A one-pot chemoselective synthesis of secondary amines by using a biomimetic electrocatalytic system

A one-pot electrochemically induced oxidation-imine formation-reduction route to secondary amines is described in detail. The key step of the process consists of the o-iminoquinone-mediated chemoselective catalytic oxidation of a primary aliphatic amine substrate, in the presence of a second amine used as the alkylating agent. Through the examination of the scope of the reaction by systematically varying both amine substrate and amine alkylating agent, it can be shown that this reaction sequence, leaving ammonia as the sole by-product, allows the rapid synthesis of various secondary amines in moderate to good yields. This process, that highlights the pre-eminent green advantages of electrochemical synthesis, especially the utilization of electricity as energy instead of chemical reagents, high atom economy as well as ambient temperature and pressure, could be a mild alternative to already reported synthetic methods.

Chemoselective catalytic oxidation of glycerol with air on platinum metals

The transesterification of triglycerides extracted from oilseeds to obtain biodiesel fuel yields up to 14% by weight of glycerol as by-product. The liquid-phase oxidation of glycerol with air on platinum catalysts was investigated to prepare valuable oxidation products such as glyceric acid or dihydroxyacetone. The effect of the pH (pH range 2–11) and of different metal catalysts was studied. The selectivity to glyceric acid can be as high as 70% at 100% conversion on Pd/C at pH 11. On Pt/C catalyst, glyceric acid was still the main product (55% selectivity); but the deposition of bismuth on platinum particles orientates the selectivity towards the oxidation of the secondary hydroxyl group to yield dihydroxyacetone with a selectivity of 50% at 70% conversion.

Chemoselective catalytic oxidation of sulfides to sulfones with tetrapropylammonium perruthenate (TPAP)

Abstract Tetrapropylammonium perruthenate has been found to be an efficient catalyst for the conversion of sulfides to sulfones. The method is highly chemoselective and isolated double bonds are generally unaffected.