Investigations into the selective oxidation of vicinal diols to α-hydroxy ketones with the NaBrO3/NaHSO3 reagent: pH dependence, stoichiometry, substrates and origin of selectivity

The NaBrO₃/NaHSO₃ reagent is one of the few oxidizing agents that chemoselectively oxidizes vicinal diols to α-hydroxy ketones with little overoxidation to the corresponding vicinal-dione or dicarboxylic acid. Oxidation reactions performed with this reagent showed strong pH dependence. cis-Vicinal diols reacted faster than trans-vicinal diols to the α-hydroxy ketone product. Hydroxy functional groups at axial ring positions were more readily oxidized than equatorial hydroxy groups. The application of the NaBrO₃/NaHSO₃ reagent for the chemoselective oxidation of vicinal diols was limited to simple systems and failed with more complex monosaccharide compounds probably due to acid catalyzed dehydrogenation reactions. Despite the simple reaction set-up and good selectivity towards the α-hydroxy ketone product, the actual oxidation reaction mechanism is highly complex and postulated to involve at least six different equilibria with a plethora of bromine containing species. A possible oxidation reaction mechanism is discussed.     

Metal-free oxidation of secondary benzylic alcohols using aqueous TBHP

We report a simple, yet efficient metal-free oxidation of secondary benzylic alcohols in the presence of t-butyl hydroperoxide (70% TBHP) with high yields of up to 98%. This type of reaction can be carried out using a wide variety of substrates, requires no other organic solvent, and proves to be tolerant toward a variety of different functional groups.     

IBX/n-Bu4NBr/CH2Cl2-H2O: a new mild system for selective oxidation of secondary alcohols

A new alternative system for the chemoselective oxidation of secondary hydroxyl group to ketone with IBX/n-Bu₄NBr in CH₂Cl₂-H₂O has been developed. Under the reaction conditions, the secondary hydroxyl group was highly chemoselectively oxidized to the corresponding ketone, in moderate to good yields at rt, in the presence of primary hydroxyl group within the same molecule.     

Bromination of phenols in bromoperoxidase-catalyzed oxidations

Phenol and ortho-substituted derivatives furnish products of selective para-bromination, if treated with sodium bromide, hydrogen peroxide, and the vanadate(V)-dependent bromoperoxidase I from the brown alga Ascophyllum nodosum. Relative rates of bromination in morpholine-4-ethane sulfonic acid (MES)-buffered aqueous tert-butanol (pH 6.2) increase by a factor 32, as the ortho-substituent in a phenol changes from F via Cl, OCH₃, C(CH₃)₃, and H to CH₃. The polar effect in phenol bromination by the enzymatic method, according to a Hammett-correlation (ρ=−3), compares to reactivity of molecular bromine under identical conditions (ρ=−2). Hypobromous acid is not able to electrophilically substitute bromine for hydrogen at pH 6.2 in aqueous tert-butanol. The tribromide anion behaves in MES-buffered aqueous tert-butanol as electrophile (ρ∼−3), showing a similar polar effect in phenol bromination as molecular bromine.     

CuCl catalyzed selective oxidation of primary alcohols to carboxylic acids with tert-butyl hydroperoxide at room temperature

Direct oxidation of primary alcohols to the corresponding carboxylic acids is performed highly efficiently at room temperature with anhydrous tert-butyl hydroperoxide in the presence of a catalytic amount of easily available CuCl under ligand free conditions in acetonitrile. Benzylic alcohols are more reactive than aliphatic alcohols, and these benzylic alcohols are selectively oxidized to the corresponding acids in the presence of aliphatic alcohols such as 1-octanol and 1-decanol.     

Salen-Mn（Ⅲ）-Complex-Catalyzed Oxidations of Secondary Alcohols

Secondary alcohols were catalytically oxidized with diace-toxyiodobenzene as oxidant in the presence of salen-Mn(Ⅲ)complex to aiTord the eorrespoltding ketones, in up to 99% yield, using CH2Cl2 or water as reaction media.     

Standard partial molar volumes of alcohols in aqueous dodecyltrimethylammonium bromide solutions

Density measurements of water-dodecyltrimethylammonium bromide (DTAB)-alcohol ternary systems as a function of alcohol and surfactant concentrations were carried out at 25°C. The alcohols were propanol (PrOH), 2-propanol (2-PrOH) and hexanol (HexOH). The apparent molar volume V_,R of alcohols have been calculated and the standard (infinite dilution) partial molar volumes of alcohols V _R at each surfactant concentration were obtained by means of a least squares fit of V_,R vs. the alcohol concentration. The V _R vs. surfactant concentration curves have been rationalized in terms of the partial molar volume of alcohol in the aqueous V _f and the micellar V _b phases and the distribution constant of alcohol between the aqueous and the micellar phases K. The V _b values for PrOH and HexOH together with those of butanol and pentanol previously reported satisfy the additivity rule giving a methylene group contribution of 16.7 cm³-mol^–1 which is identical to that reported in the literature from the study of pure liquid alcohols. No difference between V _b for PrOH and 2-PrOH has been found. From density data of water-alcohol and water-surfactant binary systems and of water-surfactant-alcohol ternary system, the apparent molar volume of the surfactant in the water-alcohol mixed solvent V_,S have been calculated as a function of the surfactant concentration and of the mixed solvent composition. The effect of the alkyl chain length of the alcohols and the effect of isomerization of the alcohols on the V_,S vs. surfactant concentration trends have been analyzed.     

Aerobic oxidation of primary alcohols in the presence of activated secondary alcohols

Chemoselective aerobic oxidation of primary alcohols in the presence of activated secondary alcohols was effected under irradiation of visible light by using (nitrosyl)Ru(salen) complex 6 that possesses bulky 1-ethyl-1-methylpropyl groups at C3, C3′, C5 and C5′, as catalyst. For example, oxidation of n-decanol was >50 times faster than oxidation of 1-phenylethanol at 10 °C.     

Covalent organic framework stabilized CdS nanoparticles as efficient visible-light-driven photocatalysts for selective oxidation of aromatic alcohols

Noble-metal-free photocatalysts with high and stable performance provide an environmentally-friendly and cost-efficient route for green organic synthesis.In this work,CdS nanoparticles with small particle size and different amount were successfully deposited on the surface of covalent organic frameworks(COFs).The deposition of suitable content of CdS on COFs could not only modify the light adsorption ability and the intrinsic electronic properties,but also enhance the photocatalytic activity and cycling performance of CdS for the selective oxidation of aromatic alcohols under visible light.Especially,COF/CdS-3 exhibited the highest yield(97.1%) of benzalde hyde which is approximately 2.5 and 15.9 times as that of parental CdS and COF,respectively.The results show that the combination of CdS and COF can improve the utilization of visible light and the separation of photo-generated charge carriers,and COF with the π-conjugated system as supports for CdS nanoparticles could provide efficient electron transport channels and improve the photocatalytic performance.Therefore,this kind of COF-supported photocatalysts with accelerated photo-induced electrons and charge-carrier separation between semiconductors possesses great potentials in future green organic synthesis.     

Oxidative amidation of benzyl alcohols with amino acid esters mediated by N-hydroxysuccinimide/phenyliodine diacetate

A simple protocol involving metal-free oxidative amidation of benzyl alcohols with amino acid esters has been presented. The amidation proceeds in a radical pathway unlike in conventional metal-mediated extrusion of dihydrogen. The method is advantageous in terms of metal-free conditions, nonexpensive commercial starting substrates. Also various substituents in the starting materials are tolerated and sterically hindered amino acid side chains could provide good yields of amide products.     

Peroxidase-catalyzed polymerization and depolymerization of coal in organic solvents

Peroxidases from horseradish roots (HRP) and soybean hulls (SBP) catalyze the efficient polymerization of a 4-kDa dimethylformamide (DMF)-soluble fraction of Mequininza (Spanish) lignite in 50% (v/v) DMF with an aqueous component consisting of acetate buffer, pH 5.0. Under these conditions, HRP and SBP catalyze the oxidation of free phenolic moieties in the coal matrix, thereby leading to oxidative polymerization of the low-molecular-weight coal polymers. The high fraction of nonphenolic aromatic moieties in coal inspired us to examine conditions whereby such coal components could also become oxidized. Oxidation of nonphenolic aromatic compounds was attempted using veratryl alcohol as a model substrate. SBP catalyzed the facile oxidation of veratryl alcohol at pH <3.HRP, however, was unable to elicit veratryl alcohol oxidation. The potential for SBP to catalyze interunit bond cleavage on complex polymeric substrates was examined using l-(3,4-dimethoxyphenyl)-2-(phenoxy)propan-1,3-diol (1) as a substrate. SBP catalyzed the Cα-Cβ and β-ether bond cleavage of this compound, suggesting that similar reactions on coal, itself, could lead to depolymerization. Depolymerization of a >50 Da coal fraction was achieved using SBP in 50% (v/v) DMF with an aqueous component adjusted to pH 2.2. Approximately 15% of the initial high-molecular-weight lignite fraction was depolymerized to polymers 4 Da in size. Hence, SBP is capable of catalyzing the depolymerization of coal in organic solvents, and this may have important ramifications in the generation of liquid fuels from coals.     

Kinetics and mechanism of quinolinium dichromate mediated oxidation of sugar alcohols in Bronsted acid media

Bronsted acid catalyzed oxidation of certain sugar alcohols (polyols) has been studied by quinolinium dichromate (QDC) using aqueous sulfuric, perchloric, and hydrochloric acids at different temperatures. At constant acidity, reaction kinetics revealed the second-order kinetics with a first order in [Alcohol] and [QDC]. Zucker-Hammett, Bunnett, and Bunnett-Olsen criteria were used to analyze acid-dependent rate accelerations. Bunnett-Olsen plots of (log k + H_ν) versus (H_ν + log [H⁺]), and (log k) versus (H_ν + log [H⁺]) afforded slope values (ϕ and ϕ^*, respectively) > 0.47, suggesting that a water molecule acts as a prton transfer agent in the slow step of the mechanism in the oxidation of alcohols by QDC in the presence of aqueous sulfuric, perchloric, and hydrochloric acids.     

Thermochemistry of copper(II) ion solvation in aqueous organic solvents

The integral heat effects of CuCl₂ dissolution in aqueous DMSO, aqueous ethanol and aqueous acetone solutions at 298. 15 K in the electrolyte concentration range 0.001–0.01M were measured by means of calorimetry. ΔH _sol ⁰ values were obtained by extrapolation to zero electrolyte concentration. Literature data were used to determine the thermodynamic characteristics of Cu²⁺ transfer from water to aqueous organic solvents.     

Construction of hydrogen-bonded ternary organic crystals derived from L-tartaric acid and their application to enantioseparation of secondary alcohols

Ternary organic crystals consisting of an L-tartaric acid-derived dicarboxylic acid, a commercially available achiral diamine, and a chiral secondary alcohol have been developed and characterized by X-ray crystallography. 1D, 2D, and 3D hydrogen-bonded supramolecular networks were constructed, depending on the structure of the diamine used. Benzylic and aliphatic secondary alcohols were enantioselectively incorporated into the crystal and were successfully enantioseparated with up to 86 and 79% enantiomeric excess (ee), respectively. Selective incorporation of one enantiomer of 2-butanol, which is a small chiral aliphatic alcohol, was achieved by the cooperative effects of hydrogen bonds, CH···π interactions, and van der Waals interactions between the guest and host molecules, with the aid of two water molecules. The high host potential of the binary supramolecular system is mainly attributed to the skewed conformation of two rigid aromatic groups of tartaric acid derivatives, which prevents dense packing of the molecules and enhances the formation of multicomponent inclusion crystals.     

Absorption and emission behaviour of trans-p-coumaric acid in aqueous solutions and some organic solvents

The absorption and fluorescence behaviour of trans-p-coumaric acid (trans-4-hydroxycinnamic acid) is investigated in buffered aqueous solution over a wide range from pH 1 to pH 12, in un-buffered water, and in some organic solvents. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and degrees of fluorescence polarisation are measured. p-Coumaric acid exists in different ionic forms in aqueous solution depending on the pH. There is an equilibrium between the neutral form (p-CAH₂) and the single anionic form (p-CAH⁻) at low pH (pK_na ≈ 4.9), and between the single anionic and the double anionic form (p-CA²⁻) at high pH (pK_aa ≈ 9.35). In the organic solvents studied trans-p-coumaric acid is dissolved in its neutral form. The fluorescence quantum yield of trans-p-coumaric acid in aqueous solution is ?_F ≈ 1.4 × 10⁻⁴ for the neutral and the single anionic form, while it is ?_F ≈ 1.3 × 10⁻³ for the double anionic form. For trans-p-coumaric acid in organic solvents fluorescence quantum yields in the range from 4.8 × 10⁻⁵ (acetonitrile) to 1.5 × 10⁻⁴ (glycerol) were measured. The fluorescence spectra are 7700–10,000 cm⁻¹ Stokes shifted in aqueous solution, and 5400–8200 cm⁻¹ Stokes shifted in the studied organic solvents. Decay paths responsible for the low fluorescence quantum yields are discussed (photo-isomerisation and internal conversion for p-CA²⁻, solvent-assisted intra-molecular charge-transfer or ππ^∗ to nπ^∗ transfer and internal conversion for p-CAH₂ and p-CAH⁻). The solvent dependence of the first ππ^∗ electronic transition frequency and of the fluorescence Stokes shift of p-CAH₂ is discussed in terms of polar solute–solvent interaction effects. Thereby the ground-state and excite-state molecular dipole moments are extracted.     

Kinetics of the oxidation of 1-phenyl ethanols with phase-transferred monochromate in organic solvents

Kinetic studies on the oxidation of 1-phenyl ethanol and its para-methyl, para-methoxy and para chloro derivatives have been carried out using phase transferred chromate as oxidant in pure organic solvents. The inorganic oxidant was transferred to the non polar media using the phase transfer catalyst, tetrabutyl phosphonium bromide (TB PB). The electronic effects of the substituents, effect of varying the temperature on the rate, as well as the effect of varying the dielectric constant of the medium on the rate have been investigated. The rate data fitted well with the Hammett equation.     

Solvent polarity and organic reactivity in mixed solvents: evidence using a reactive molecular probe to assess the role of preferential solvation in aqueous alcohols

Product selectivities [S = ([ester product]/[acid product]) x ([water]/[alcohol solvent])] are reported for solvolyses of p-methoxybenzoyl chloride (2) in aqueous methanol, ethanol, 2,2,2-trifluoroethanol, n-propyl alcohol, isopropyl alcohol, and tert-butyl alcohol at 25, 35, and 45 degrees C. S values are small and depend significantly on the alcohol cosolvent, varying from 1.3 in methanol to 0.1 in tert-butyl alcohol, but S depends only slightly on the solvent composition, and on the temperature. As S adjusts the product ratios for changes in bulk solvent compositions, it is suggested that preferential solvation by either alcohol or water at the reaction site is not a major factor influencing rates or products. Logarithms of rates of solvolyses of 2 correlate well with Kosower Z values (based on solvatochromism). In contrast, another solvatochromic polarity index, E(T)(30), shows "dispersion" in correlations with the solvent ionizing power parameter, Y(OTs), probably due to aromatic ring and other solvation effects.     

Selective oxidation and chlorination of trifluoromethylsulfide using trichloroisocyanuric acid in ionic liquid

A route to chemoselective oxidation and chlorination of aryltrifluoromethylsulfide using trichloroisocyanuric acid (TCCA) in ionic liquid, an efficiently O-methylation reaction and a reduction of nitro- to amido- in excellent yields have been developed.     

Temperature-dependence of the solubility of some acetanilide derivatives in several organic and aqueous solvents

The thermodynamic functions free energy, enthalpy, and entropy of solution, were evaluated from the solubility data of acetanilide, acetaminophen, and phenacetin, determined at several temperatures in water, octanol, isopropyl myristate, and chloroform. These three organic solvents mutually saturated with water, and finally, in cyclohexane. In the aqueous media, the solubility was determined at pH 7.4 and ionic strength 0.15?mol?L^?1. The excess free energy and the activity coefficients of the solutes were also determined. The solubility for acetanilide and phenacetin was higher in organic media such as octanol and chloroform than is those obtained in the aqueous media and cyclohexane, while for acetaminophen the solubility was higher in octanol than those obtained in the other solvents.