Electron‐Deficient Alloxazinium Salts: Efficient Organocatalysts of Mild and Chemoselective Sulfoxidations with Hydrogen Peroxide

A series of substituted alloxazinium perchlorates has been prepared and tested as catalysts for the oxidation of sulfides to sulfoxides with hydrogen peroxide. The logarithms of the observed rate constants of thioanisole oxidation correlate with the Hammett σ constants of the substituents on the alloxazinium catalysts, as well as with their reduction potentials E^0′ and their pK_R+ values, representing the alloxazinium salt/pseudobase equilibrium. The stronger the electron‐withdrawing substituent, the more efficient is the alloxazinium catalyst. The alloxazinium salts with a cyano or trifluoromethyl group in position 8 proved to be the most efficient, operating at room temperature at small loadings, down to 0.1 mol%, achieving turnover number values of up to 640 and acceleration by a factor of 350 relative to the non‐catalyzed oxidation. The 8‐cyanoalloxazinium perchlorate was evaluated as the best catalyst; however, due to its relatively good accessibility, the 8‐(trifluoromethyl)alloxazinium perchlorate seems to be the catalyst of choice for sulfoxidations with hydrogen peroxide. It was successfully tested for the sulfoxidation of a series of aliphatic and aromatic sulfides on a preparative scale. It produced the corresponding sulfoxides in quantitative conversions and with high isolated yields (87–98%). No over‐oxidation to sulfone was ever observed.

     

Supported Sulfonic Acid as Green and Efficient Catalyst for Baeyer–Villiger Oxidation with 30% Aqueous Hydrogen Peroxide

Silica‐supported propylsulfonic acid is a very good heterogeneous catalyst for the Baeyer–Villiger oxidation of cyclic ketones to lactones with stoichiometric 30% aqueous hydrogen peroxide in 1,1,1,3,3,3‐hexafluoro‐2‐propanol as solvent.     

Environmentally friendly liquid phase oxidation: enhanced selectivity in the aerial oxidation of alkyl aromatics,epoxidations and the Baeyer–Villiger oxidation using novel silica supported transition metal ions

Active transition metal species (Co, Cu, Cr, Ni or Mn) supported on a chemically modified silica gel are used as heterogeneous catalysts in a range of liquid phase oxidation reactions: alkyl aromatic side chain oxidations, epoxidations of alkenes and Baeyer–Villiger oxidations of linear ketones to esters and cyclic ketones to lactones. The catalyst employs metal centres bound to the silica surface via a hydrophobic spacer chain and is thus chemically robust and has a relatively high loading for a supported reagent (c 0.4 mmol g⁻¹). The Cr version of the catalyst promotes the oxidation of ethylbenzene to acetophenone in a solvent‐free system at a rate of 5.5% h⁻¹ (>370 turnover h⁻¹). It is also active for the oxidation of p‐chlorotoluene and p‐xylene to p‐chlorobenzoic acid and p‐toluic acid respectively. Cyclohexene is converted to its oxide at room temperature at a rate of c 28% h⁻¹ (c 12 turnover h⁻¹) using either the Ni or Cu versions of the catalyst. The room temperature Baeyer–Villiger oxidation of cyclohexanone is achieved at a rate of 44% h⁻¹ (49 turnover h⁻¹) using the Ni‐containing catalyst. The same material also promotes the Baeyer–Villiger oxidation of linear aliphatic ketones and aromatic side chains. All the above systems use either air or molecular oxygen as the oxidant rather than peroxides or peracids. © 1999 Society of Chemical Industry     

A New Type of Stereoselectivity in Baeyer–Villiger Reactions: Access to E‐ and Z‐Olefins

A new concept for accessing configurationally defined trisubstituted olefins has been developed. Starting from a common ketone precursor of the type 4‐ethylidenecyclohexanone, Baeyer–Villiger monooxygenases are employed as catalysts in diastereoselective Baeyer–Villiger reactions leading to the corresponding E‐ or Z‐configurated lactones. Wild‐type cyclohexanone monooxygenase (CHMO) as catalyst delivers the E‐isomers and a directed evolution mutant the opposite Z‐isomers. Subsequent transition metal‐catalyzed chemical transformations of a key product containing a vinyl bromide moiety provide a variety of different trisubstituted E‐ or Z‐olefins. A model based on QM/MM sheds light on the origin of this unusual type of diastereoselectivity. In contrast to this biocatalytic approach, traditional Baeyer–Villiger reagents such as m‐CPBA fail to show any selectivity, 1:1 mixtures of E‐ and Z‐olefins being formed.     

Organoselenium‐Catalyzed Baeyer–Villiger Oxidation of α,β‐Unsaturated Ketones by Hydrogen Peroxide to Access Vinyl Esters

By carefully screening the organoselenium pre‐catalysts and optimizing the reaction conditions, simple dibenzyl diselenide was found to be the best pre‐catalyst for Baeyer–Villiger oxidation of (E)‐α,β‐unsaturated ketones with the green oxidant hydrogen peroxide at room temperature. The organoselenium catalyst used in this reaction could be recycled and reused several times. This new method was suitable not only for methyl unsaturated ketones, but also for alkyl and aryl unsaturated ketones. Therefore, it provided a direct, mild, practical, highly functional group‐tolerant process for the chemoselective preparation of the versatile (E)‐vinyl esters from the readily available (E)‐α,β‐unsaturated ketones. A possible mechanism was also proposed to rationalize the activity of the organoselenium catalyst in the presence of hydrogen peroxide in this Baeyer–Villiger oxidation reaction.

     

Supported hydrogensulfate ionic liquid catalysis in Baeyer–Villiger reaction

We report here the method of synthesis and characterisation of ionic liquid: 1-methyl-3-(triethoxysilylpropyl)imidazolium hydrogensulfate, anchored onto solid support via cation–anion stays unbounded to the surface. As a support, we used a silica material with the extensive system of meso- and macropores. The structure of the support was not destroyed after the immobilisation of the ionic liquid. The new material was tested as acidic catalyst with the Baeyer–Villiger reaction. Cyclic ketones were readily oxidised with 68% hydrogen peroxide in dry dichloromethane to their corresponding lactones in high yields (60–91%) at 50 °C within a short time (5–20 h). The use of hydrogen peroxide and a heterogeneous catalyst allows a clean oxidation and the possibility of easy catalyst removal by simple filtration and reuse three times in another oxidation process without lost of catalyst activity.     

Sodium Perborate—A Convenient Baeyer–Villiger Oxidation Reagent in the Synthesis of the Corey Aldehyde

Sodium perborate tetrahydrate is a cheap, safe and readily available alternative to the commonly used peracetic acid for the Baeyer–Villiger oxidation step of the Corey aldehyde synthesis. Chloroketo acid 1 is smoothly converted by sodium perborate tetrahydrate in formic acid to the chloroketolactone 2 in 66% isolated yield. In contrast to previously reported reactions using other oxidants, the formation of the lactone is completely regioselective in favour of the “bridgehead‐migrated” isomer 2 .     

Supramolecular Activation of Hydrogen Peroxide in the Selective Sulfoxidation of Thioethers by a Self‐Assembled Hexameric Capsule

An efficient metal‐free organocatalytic activation of hydrogen peroxide (H₂O₂) towards thioethers leading to the corresponding sulfoxides in high yields at room temperature within hours was promoted by the hexameric capsule formed by the self‐assembly of resorcin[4]arene units. The capsule plays a dual role of activating the oxidant through hydrogen bonding and favouring the oxidation reaction inside the cavity. Inactivation of the supramolecular organocatalyst was observed by using competitive ammonium guests, mimicking the inactivation of enzymes by competitive inhibitors.

     

Insights into Sequence–Activity Relationships amongst Baeyer–Villiger Monooxygenases as Revealed by the Intragenomic Complement of Enzymes from Rhodococcus jostii RHA1

The Rhodococcus jostii RHA1 genome encodes a number of enzymes that can be exploited as biocatalysts. Study of the substrate spectrum and enantioselectivity of Baeyer–Villiger monooxygenases from R. jostii allowed the identification of short amino acid sequences specific to groups displaying certain catalytic characteristics. The gel illustrates the substrate acceptance spectra and selectivities of the different proteins.

     

Recent developments in the application of Baeyer-Villiger monooxygenases as biocatalysts

Baeyer-Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer-Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin-containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio- and enantioselective Baeyer-Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO-mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.     

Incorporation of tin in different types of pores in SBA-15: Synthesis, characterization and catalytic activity

Mesoporous Sn-SBA-15 has been synthesized by three different methods such as conventional hydrothermal route, using cocatalyst NH₄F and in the presence of organosilane precursor. All the materials are thoroughly characterized by powder X-ray diffraction (XRD), SEM, TEM, N₂ sorption and surface area measurements, diffuse-reflectance UV–visible and FTIR spectroscopy, TG–DTA and elemental analysis through ICP. Nitrogen adsorption data, XRD patterns, and TEM observations suggests that the textural properties are retained during the isomorphous substitution of silicon by tin. ICP chemical analysis indicates that tin can be substituted in the range of Si/Sn = 69–162. UV–visible spectra of samples synthesized by the cocatalytic approach exhibit unique absorption band at 213 nm characteristics of tin atom substituted in the smaller pores (2–3 nm) located inside the walls of mesopores. Further, an additional band at 224 nm can be assigned to Sn atoms located in the distorted tetrahedral position along the primary mesopores. In contrary, only one absorption band centered at 224 nm is observed for all the samples synthesized by conventional hydrothermal as well as in the presence of organosilane precursor. ¹⁹F NMR spectra confirmed (no signal) the absence of occluded F⁻ ions in the samples made with NH₄F. Observed high catalytic activity in Baeyer–Villiger oxidation and Meerwin–Pondorf–Verly reduction under the liquid-phase conditions suggest the incorporation of a portion of tin in the smaller pores for the Sn-SBA-15 materials synthesized through cocatalyst method.     

Controlling the Regioselectivity of Baeyer–Villiger Monooxygenases by Mutation of Active‐Site Residues

Baeyer–Villiger monooxygenase (BVMO)‐mediated regiodivergent conversions of asymmetric ketones can lead to the formation of “normal” or “abnormal” lactones. In a previous study, we were able to change the regioselectivity of a BVMO by mutation of the active‐site residues to smaller amino acids, which thus created more space. In this study, we demonstrate that this method can also be used for other BVMO/substrate combinations. We investigated the regioselectivity of 2‐oxo‐Δ³‐4,5,5‐trimethylcyclopentenylacetyl‐CoA monooxygenase from Pseudomonas putida (OTEMO) for cis‐bicyclo[3.2.0]hept‐2‐en‐6‐one ( 1 ) and trans‐dihydrocarvone ( 2 ), and we were able to switch the regioselectivity of this enzyme for one of the substrate enantiomers. The OTEMO wild‐type enzyme converted (?)‐ 1 into an equal (50:50) mixture of the normal and abnormal products. The F255A/F443V variant produced 90 % of the normal product, whereas the W501V variant formed up to 98 % of the abnormal product. OTEMO F255A exclusively produced the normal lactone from (+)‐ 2 , whereas the wild‐type enzyme was selective for the production of the abnormal product. The positions of these amino acids were equivalent to those mutated in the cyclohexanone monooxygenases from Arthrobacter sp. and Acinetobacter sp. (CHMO_Arthro and CHMO_Acineto) to switch their regioselectivity towards (+)‐ 2 , which suggests that there are hot spots in the active site of BVMOs that can be targeted with the aim to change the regioselectivity.     

Selective Catalytic Oxidation of Alcohols,Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts and Hydrogen Peroxide

The manganese‐containing catalytic system [Mn^IV,IV₂O₃(tmtacn)₂]²⁺ ( 1 )/carboxylic acid (where tmtacn=N,N′,N′′‐trimethyl‐1,4,7‐triazacyclononane), initially identified for the cis‐dihydroxylation and epoxidation of alkenes, is applied for a wide range of oxidative transformations, including oxidation of alkanes, alcohols and aldehydes employing H₂O₂ as oxidant. The substrate classes examined include primary and secondary aliphatic and aromatic alcohols, aldehydes, and alkenes. The emphasis is not primarily on identifying optimum conditions for each individual substrate, but understanding the various factors that affect the reactivity of the Mn‐tmtacn catalytic system and to explore which functional groups are oxidised preferentially. This catalytic system, of which the reactivity can be tuned by variation of the carboxylato ligands of the in situ formed [Mn^III,III₂(O)(RCO₂)₂(tmtacn)₂]²⁺ dimers, employs H₂O₂ in a highly atom efficient manner. In addition, several substrates containing more than one oxidation sensitive group could be oxidised selectively, in certain cases even in the absence of protecting groups.

     

过氧化氢催化氧化载体的研究进展

本文介绍有关过氧化氢的催化氧化反应,并重点介绍了其反应过程中的各种催化剂载体的研究进展,比如硅胶、分子筛、活性炭等载体。     

Biosynthesis of Hygrocins,Antitumor Naphthoquinone Ansamycins Produced by Streptomyces sp. LZ35

Hygrocins are naphthoquinone ansamycins with significant antitumor activities. Here, we report the identification and characterization of the hygrocin biosynthetic gene cluster (hgc) in Streptomyces sp. LZ35. A biosynthetic pathway is proposed based on bioinformatics analysis of the hgc genes and intermediates accumulated in selected gene disruption mutants. One of the steps during the biosynthesis of hygrocins is a Baeyer–Villiger oxidation between C5 and C6, catalyzed by luciferase‐like monooxygenase homologue Hgc3. Hgc3 represents the founding member of a previously uncharacterized family of enzymes acting as Baeyer–Villiger monooxygenases.     

Ionic Liquids as Solvents for Catalyzed Oxidations of Organic Compounds

This review summarizes the oxidation methods of organic compounds carried out in the presence of a catalyst using ionic liquids or molten ammonium salts as solvents. From an ecological point of view, these procedures are particularly interesting when the catalyst is efficiently immobilized in the solvent, thus leading to a reusable system.     

Switch in Cofactor Specificity of a Baeyer–Villiger Monooxygenase

Baeyer–Villiger monooxygenases (BVMOs) catalyze the oxidation of ketones to esters or lactones by using molecular oxygen and a cofactor. Type I BVMOs display a strong preference for NADPH. However, for industrial purposes NADH is the preferred cofactor, as it is ten times cheaper and more stable. Thus, we created a variant of the cyclohexanone monooxygenase from Acinetobacter sp. NCIMB 9871 (CHMO_Acineto); this used NADH 4200‐fold better than NADPH. By combining structure analysis, sequence alignment, and literature data, 21 residues in proximity of the cofactor were identified and targeted for mutagenesis. Two combinatorial variants bearing three or four mutations showed higher conversions of cyclohexanone with NADH (79 %) compared to NADPH (58 %) as well as specificity. The structural reasons for this switch in cofactor specificity of a type I BVMO are especially a hydrogen‐bond network coordinating the two hydroxy groups of NADH through direct interactions and bridging water molecules.     

四丁基溴化铵催化过氧化氢氧化香紫苏醇合成香紫苏内酯的研究

在有机溶剂中,以质量分数30%过氧化氢作氧源,四丁基溴化铵为相转移催化剂,钨酸钠、硫酸氢钠为酸性催化剂,催化氧化香紫苏醇合成香紫苏内酯,考察了硫酸氢钠、钨酸钠、四丁基溴化铵用量和反应时间对反应的影响。当n（香紫苏醇）：n（双氧水）：n（四丁基溴化铵）：n（钨酸钠）：n（硫酸氢钠）=100：300：2：1：1,在回流温度下,反应6h,产率可达72%。     

Aerobic Oxidation of Glucose with Gold Catalyst: Hydrogen Peroxide as Intermediate and Reagent

Careful analytical determinations show that the gold‐catalysed aerobic oxidation of glucose occurs through a two‐electrons mechanism leading to gluconate and hydrogen peroxide. This latter decomposes before reaching the critical concentration for competing with O₂ in glucose oxidation. A mechanism of glucose oxidation on gold nanoparticles is presented.