A convenient palladium-catalyzed aerobic oxidation of alcohols at room temperature

A simple Pd-catalyzed aerobic oxidation of benzylic and aliphatic alcohols to the corresponding aldehydes and ketones at room temperature is described.     

Using mechanistic and computational studies to explain ligand effects in the palladium-catalyzed aerobic oxidation of alcohols

The experimental and computational mechanistic details of the Pd(OAc)(2)/TEA-catalyzed aerobic alcohol oxidation system are disclosed. Measurement of various kinetic isotope effects and the activation parameters as well as rate law derivation support rate-limiting deprotonation of the palladium-coordinated alcohol. Rate-limiting deprotonation of the alcohol is contrary to the majority of related kinetic studies for Pd-catalyzed aerobic oxidation of alcohols, which propose rate-limiting beta-hydride elimination. This difference in the rate-limiting step is supported by the computational model, which predicts the activation energy for deprotonation is 3 kcal/mol higher than the activation energy for beta-hydride elimination. The computational features of the similar Pd(OAc)(2)/pyridine system were also elucidated. Details of the study illustrate that the use of TEA results in an active catalyst that has only one ligand bound to the Pd, resulting in a significant lowering of the activation energy for beta-hydride elimination and, therefore, a catalyst that is active at room temperature.     

Na-promoted aerobic oxidation of alcohols to ketones

Aerobic oxidation of a number of diaryl and arylalkyl carbinols to ketones was promoted by Na in THF at room temperature with up to 99% yield. This new oxidation method is also selective with good efficiency for the oxidation of benzylic secondary alcohols but not for a primary alcohol or nonbenzylic secondary alcohols. Under nitrogen, a catalytic amount of Ni or transition metal halides such as CoCl₃, FeCl₃, and NiCl₃ in combination with Na was also found to conduct a dehydrogenation of a secondary alcohol to the corresponding ketone in high yield at room temperature.     

Oxovanadium complex-catalyzed aerobic oxidation of propargylic alcohols

A catalytic system consisting of vanadium oxyacetylacetonate [VO(acac)(2)] and 3 A molecular sieves (MS3A) in acetonitrile works effectively for the aerobic oxidation of propargylic alcohols [R(1)CH(OH)Ctbd1;CR(2)] to the corresponding carbonyl compounds under an atmospheric pressure of molecular oxygen. Although the reactivity of alpha-acetylenic alkanols (R(1) = alkyl) is lower compared to that of the alcohols of R(1) = aryl, alkenyl, and alkynyl, the use of VO(hfac)(2) as a catalyst and the addition of hexafluoroacetylacetone improve the product yield in these cases. A catalytic cycle involving a vanadium(V) alcoholate species and beta-hydrogen elimination from it has been proposed for this oxidation.     

Unexpected roles of molecular sieves in palladium-catalyzed aerobic alcohol oxidation

Methods for palladium-catalyzed aerobic oxidation of alcohols often benefit from the presence of molecular sieves. This report explores the effect of molecular sieves on the Pd(OAc)2/pyridine and Pd(OAc)2/DMSO (DMSO = dimethyl sulfoxide) catalyst systems by performing kinetic studies of alcohol oxidation in the presence and absence of molecular sieves. Molecular sieves enhance the rate of the Pd(OAc)2/pyridine-catalyzed oxidation of alcohols, and the effect is attributed to the ability of molecular sieves to serve as a Br?nsted base. In contrast, no rate enhancement is observed for the Pd(OAc)2/DMSO-catalyzed reaction. Both catalyst systems exhibit improved catalyst stability in the presence of molecular sieves, manifested by higher catalytic turnover numbers. Control experiments indicate that neither of these beneficial effects is associated with the ability of molecular sieves to absorb water, a stoichiometric byproduct of these reactions. Finally, the use of simultaneous gas-uptake and in-situ IR spectroscopic studies reveal that molecular sieves inhibit the disproportionation of H2O2, an observation that contradicts a previous suggestion that the beneficial effect of molecular sieves may arise from their ability to promote H2O2 disproportionation.     

Elucidating the significance of beta-hydride elimination and the dynamic role of acid/base chemistry in a palladium-catalyzed aerobic oxidation of alcohols

The mechanistic details of aerobic alcohol oxidation with catalytic Pd(IiPr)(OAc)(2)(H(2)O) (IiPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) are disclosed. Under optimal conditions, beta-hydride elimination is rate-limiting supported by kinetic studies including a high primary kinetic isotope effect (KIE) value of 5.5 +/- 0.1 and a Hammett rho value of -0.48 +/- 0.04. On the basis of these studies, a late transition state is proposed for beta-hydride elimination, which is further corroborated by theoretical calculations using density functional theory. Additive acetic acid modulates the rates of both the alcohol oxidation sequence and regeneration of the Pd catalyst. With no additive [HOAc], turnover-limiting reprotonation of intermediate palladium peroxo is kinetically competitive with beta-hydride elimination, allowing for reversible oxygenation and decomposition of Pd(0). With additive [HOAc] (>2 mol %), reprotonation of the palladium peroxo is fast and beta-hydride elimination is the single rate-controlling step. This proposal is supported by an apparent decomposition pathway modulated by [HOAc], a change in alcohol concentration dependence, a lack of [O(2)] dependence at high [HOAc], and significant changes in the KIE values at different HOAc concentrations.     

Productive utilization of chlorobenzene: palladium-catalyzed selective oxidation of alcohols

[reaction: see text] The palladium/ligand-catalyzed activation of chlorobenzene provides a general, efficient, and functional group friendly method for the selective oxidation of alcohols to carbonyl compounds.     

Biomimetic aerobic oxidation of amino alcohols to lactams

The right path: N-Protected amino alcohols undergo aerobic and biomimetic oxidation to the corresponding lactams in the presence of a ruthenium catalyst and a combination of electron-transfer mediators under air. The reaction was used for the synthesis of five-, six-, and seven-, membered lactams and showed good tolerance to a number of N-protecting groups.     

Pseudohalide assisted aerobic oxidation of alcohols in the presence of visible-light

Pseudohalides are well known to do similar chemistry like halides. Thiocyanate, a pseudohalide acts like halides in many ways. Thiocyanate radicals (SCN) are generated from readily available thiocyanate salts using Rose Bengal through single electron transfer (SET) in the presence of visible light. Thiocyanate radicals abstract hydrogen like other halide radicals, so this aspect of chemistry was used for the oxidation of alcohols to their corresponding aldehydes using oxygen as the terminal oxidant. This method shows a broad scope and well tolerance towards various functional groups.     

Unique gold chemoselectivity for the aerobic oxidation of allylic alcohols

Gold nanoparticles supported on nanocrystalline ceria has been found to be more active and chemoselective than palladium and gold(core)-palladium(shell) nanoparticles for the aerobic oxidation of allylic alcohols.     

pH-Sensitive gold nanoparticle catalysts for the aerobic oxidation of alcohols

Gold nanoparticles (NPs) stabilized by carboxylate modified polyvinylpyrrolidone have been prepared and fully characterized. The gold NPs efficiently catalyze the aerobic oxidation of benzyl alcohol in water at ambient temperature and are easily separated from the reaction mixture by lowering the pH of the solution, causing the NPs to precipitate. The mechanism of the precipitation process has been studied. Due to the efficiency of this process, the NPs may be reused as catalysts by readjusting their pH.     

Promoting gold nanocatalysts in solvent-free selective aerobic oxidation of alcohols

A trace amount of metal carbonate, acetate or borate significantly boosts gold nanocatalysts in selective aerobic oxidation of alcohols under mild solvent-free conditions.     

Cesium hydroxide-promoted aerobic oxidation of sec-aromatic alcohols

A CsOH-promoted aerobic oxidation of sec-aromatic alcohols has been developed, using air as a free and clean oxidant, and providing aryl ketones in good to excellent yields.     

Heterocycle-substituted tetrazole ligands for copper-catalysed aerobic oxidation of alcohols

An efficient copper-catalysed aerobic oxidation of alcohols has been established, which employed heterocycle-substituted tetrazoles as ligands. The commercially available (S)-5-(pyrrolidin-2-yl)-1H-tetrazole proved as the best ligand for this oxidation. Under optimized conditions, the substrate scope was broadened. A plausible mechanism was also proposed.