Efficient and highly aldehyde selective Wacker oxidation

A method for efficient and aldehyde-selective Wacker oxidation of aryl-substituted olefins using PdCl(2)(MeCN)(2), 1,4-benzoquinone, and t-BuOH in air is described. Up to a 96% yield of aldehyde can be obtained, and up to 99% selectivity can be achieved with styrene-related substrates.     

In situ formation of allyl ketones via Hiyama-Nozaki reactions followed by a chromium-mediated Oppenauer oxidation

In Hiyama-Nozaki reactions of allylchromium with aldehydes the expected products are homoallylalcohols. However, oxidation products derived from these, predominantly allyl ketones, can be common side products. This can be explained by an Oppenauer-(Meerwein-Ponndorf-Verley)-type mechanism (OMPV-reaction). The amount of oxidation is strongly dependent on the substitution pattern of the reaction partners and the reaction conditions. An appropriate choice of these can lead to preferential formation of ketones instead of the alcohols. In addition to its synthetic usefulness, the oxidation-reduction equilibrium is of the utmost importance for the design of enantioselective Hiyama-Nozaki reactions because it is also a potential racemization pathway.     

Tishchenko reactions and Oppenauer oxidation reactions of aldehydes promoted by diisobutylaluminum hydride

The aliphatic aldehydes react with catalytic amount of Dibal-H in n-pentane to give the corresponding Tishchenko products in good to excellent yields. Contrary, α-silyloxyaldehydes give α-silyloxyketones via Oppenauer oxidation under similar condition.     

Acidic iridium hydrides: implications for aerobic and Oppenauer oxidation of alcohols

[Cp*Ir(H)(bpym)]+ and [Cp*Ir(H)(bpy)]+ are the first examples of iridium based catalysts for the aerobic oxidation of alcohols; the catalytic cycle proceeds via acidic hydrides. Deprotonation of the hydride leads to a highly oxygen sensitive Ir I species that regenerate the Ir III complexes upon oxidation with dioxygen.     

Oppenauer oxidation of secondary alcohols with 1,1,1-trifluoroacetone as hydride acceptor

1,1,1-Trifluoroacetone (2a) reacts as a hydride-acceptor in the Oppenauer oxidation of secondary alcohols (1) in the presence of diethylethoxyaluminum. The oxidant allows for selective oxidation of secondary alcohols in the presence of primary alcohols.     

Practical Oppenauer (OPP) oxidation of alcohols with a modified aluminum catalyst

[reaction: see text] Modified aluminum catalyst 2 was found to be highly effective for Oppenauer (OPP) oxidation of alcohols under mild conditions. For example, oxidation of carveol in toluene with t-BuCHO (1.2 equiv) as a hydride acceptor in the presence of 2 (1 mol %) proceeded smoothly at 21 degrees C to give carvone in 94% yield after 1 h. A practical aspect of this system was highlighted by the facile OPP oxidation of terpenoids and steroids with 1.2-3.0 equiv of acetone and 2 (3-5 mol %).     

Efficient oxidation of benzyl alcohol with heteropolytungstate as reaction-controlled phase-transfer catalyst

A series of heteropolytungstates has been synthesized and utilized as catalysts to catalyze oxidation of benzyl alcohol with aqueous hydrogen peroxide.The results indicated that three of these catalysts showed the properties of reaction-controlled phase- transfer catalysis,and they had excellent catalytic ability to the oxidation of benzyl alcohol.No other by-products were detected by gas chromatography.Once the hydrogen peroxide was consumed completely,the catalyst precipitated from solvent,and the results of the catalyst recycle showed that the catalyst had high stability.     

Biocatalytic oxidation of benzyl alcohol to benzaldehyde via hydrogen transfer

Various types of biocatalysts like oxidases, alcohol dehydrogenases, and microbial cells were tested for the oxidation of benzyl alcohol. Oxidases in combination with molecular oxygen led to low conversion. Alcohol dehydrogenases and microbial cells were tested in a hydrogen transfer reaction employing acetaldehyde, chloroacetone, and acetone as hydrogen acceptor. Excellent conversion (95%) could be achieved employing lyophilised cells of Janibacter terrae DSM 13953 at a substrate concentration of 97 mM.     

Efficient synthesis of multifunctional polymers via thiol-epoxy "click" chemistry

We demonstrate high efficiency and simplicity of the thiol-epoxy reaction towards preparation of a wide range of main-chain as well as end-chain multifunctional polymers.     

Stabilization of G-quadruplex DNA by highly selective ligands via click chemistry

A series of G-quadruplex stabilizing compounds have been prepared via click chemistry employing the Cu(I)-catalyzed Huisgen reaction. These compounds were shown to bind tightly to G-quadruplex DNA even in the presence of competing high concentrations of duplex DNA. Furthermore, a modified TRAP assay has shown that some of these compounds also inhibit telomerase at low micromolar concentration.     

Efficient selective oxidation of alcohols to aldehydes catalyzed by a morpholinone nitroxide

Efficient chemoselective oxidation of primary alcohols to the corresponding aldehydes is described. The transformation is promoted by a catalytic morpholinone nitroxide radical catalyst which can be easily synthesized. A broad range of substrates including aromatic and aliphatic primary alcohols are converted with excellent yields under mild conditions. The control experiments reveal that the morpholinone nitroxide can be used as a selective oxidant for primary alcohols in the presence of secondary alcohols.     

Efficient synthesis of pyrroles and 4,5,6,7-tetrahydroindoles via palladium-catalyzed oxidation of hydroxy-enamines

Facile and one-pot synthetic route of poly-substituted pyrroles and 4-oxo-4,5,6,7-tetrahydroindoles is established, which consists of three steps: (1) palladium-catalyzed oxidation of hydroxy-enamines by using tetrakis(triphenylphosphine)palladium and mesityl bromide oxidation system, (2) intramolecular cyclization, and (3) dehydration.     

Efficient synthesis of [3H]-sanglifehrin A via selective oxidation/reduction of alcohols at C31 and C35

[Reaction: see text]. Sanglifehrin A is a novel complex natural product showing strong immunosuppressive activity and remarkably high affinity for cyclophilin A. To assess its pharmacokinetic properties in vivo, an efficient synthetic route was developed to introduce a tritium label in position C35 of sangliferin A via an oxidation/reduction strategy. The synthetic approach is particularly attractive, because the C35-oxo intermediate 7 is available in good yield on large scale and the reducing agent, lithium tri-sec-butylborotritide, is readily available. An attempt to apply a similar strategy to the alcohol in position C31 led primarily to C31-epi-hydroxy sanglifehrin A under a variety of conditions.     

A simple, efficient, and selective deuteration via a flow chemistry approach

A simple and efficient deuteration methodology has been established for a wide variety of substrates using a continuous flow hydrogenation reactor. The described procedure is many times faster (1 mg min⁻¹) compared to literature methods and the purity of the crude product can be as high as 99%. The deuterium source is D₂O, the consumption of which is very low.     

Symbiotic reagent activation: Oppenauer oxidation of magnesium alkoxides by silylglyoxylates triggers second-stage aldolization

The treatment of silylglyoxylates with magnesium alkoxides at ambient temperature results in symbiotic Oppenauer oxidation of the alkoxide and Meerwein-Ponndorf-Verley reduction of the silylglyoxylate. The reduced silylglyoxylate undergoes subsequent [1,2]-Brook rearrangement and aldol reaction with the carbonyl oxidation product. The magnesium alkoxide may be accessed via deprotonation of primary or secondary alcohols with EtMgBr, via addition of Grignard reagents to aldehydes, or via CuI-catalyzed alkylation of epoxides. For aliphatic primary alkoxides, moderate levels of anti diastereoselection are observed. A crossover experiment reveals that dissociation of the nascent aldehyde from the magnesium center is faster than [1,2]-Brook rearrangement and aldolization.     

Liquid-phase Oppenauer oxidation of primary allylic and benzylic alcohols to corresponding aldehydes by solid zirconia catalysts

Hydrous zirconia and grafted zirconium 1-propoxide catalysts were found active in the Oppenauer oxidation of cinnamyl alcohol, geraniol and 4-tert-butylcyclohexanol (cis- and trans-). The most active hydrous zirconia catalysts were formed by calcining at 250–300 °C. Grafted zirconium 1-propoxide on silica gel and MCM-41 were active in the Oppenauer oxidation of geraniol with high selectivity to the desired citral product. However, over an acidic support such as AlMCM, the grafted zirconium 1-propoxide catalysed the dehydration and isomerisation of the alcohol, leading to low yield to citral. Also, furfural was found to be an efficient oxidant for the titled Oppenauer oxidation. Other solid catalysts such as γ-Al₂O₃, Na–Al₂O₃, zeolite beta and Mg/Al hydrotalcite showed only moderate catalytic activity and selectivity in the Oppenauer oxidation of geraniol. As compared to other solid catalysts, hydrous zirconia solid catalysts used in this work are active and selective towards the formation of desired carbonyl oxidation products; additionally, these solid zirconia catalysts are easy to prepare and recycle, and applicable to different alcohol substrates.

Graphical abstract

Hydrous zirconia calcined at 250–300 °C and grafted zirconium 1-propoxide solid catalysts were found to be efficient for the Oppenauer oxidation of cinnamyl alcohol, geraniol and 4-tert-butylcyclohexanol in toluene when furfural was used as the oxidant.

Full-size image (4K)

     

Visible-light-activated selective synthesis of sulfoxides via thiol-ene/oxidation reaction cascade

A convenient, highly selective and metal-free synthesis of sulfoxides from alkenes and thiols using NHPI as an inexpensive and reusable organophotoredox catalyst is reported. The protocol involves radical thiol-ene/oxidation reaction cascade and utilizes visible light and air (O₂) as inexpensive, readily available, non-toxic and eco-sustainable reagents to afford up to 96% yields of the product at room temperature.     

Electrophilic activation of hydrogen peroxide: selective oxidation reactions in perfluorinated alcohol solvents

[reaction; see text] The catalytic electrophilic activation of hydrogen peroxide with transition metal compounds toward reaction with nucleophiles is a matter of very significant research and practical interest. We have now found that use of perfluorinated alcoholic solvents such as 1,1, 1,3,3,3-hexafluoro-2-propanol in the absence of catalysts allowed electrophilic activation of hydrogen peroxide toward epoxidation of alkenes and the Baeyer-Villiger oxidation of ketones.     

A potent and highly selective inhibitor of human alpha-1,3-fucosyltransferase via click chemistry

Potent inhibitors of fucosyltransferases, and glycosyltransferases in general, have been elusive due to the inherent barriers surrounding the family of glycosyltransfer reactions. The problems of weak substrate affinity and low catalytic proficiency of fucosyltransferase was offset by recruiting additional binding features, in this case hydrophobic interactions, to produce a high affinity inhibitor, 24, with Ki = 62 nM. The molecule was identified from a GDP-triazole library of 85 compounds, which was produced by the Cu(I)-catalyzed [2 + 3] cycloaddition reaction between azide and acetylene reactants, followed by in situ screening without product isolation.