Alcohol oxidation species reactivity

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

Another possible alternative oxidant that has recently been investigated is an Fe(VI) species, potassium ferrate, K2Fe04, supported on montmorillonite clay.14 This reagent gives clean, high-yielding oxidation of benzylic and allylic alcohols, but saturated alcohols are less reactive. 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

A Cr(VI)-catalyst complex has been proposed as the reactive oxidizing species in the oxidation of frans-stibene with chromic acid, catalysed separately by 1,10-phenanthroline (PHEN), oxalic acid, and picolinic acid (PA). The oxidation process is believed to involve a nucleophilic attack of the olefinic bond on the Cr(VI)-catalyst complex to generate a ternary complex.31 PA- and PHEN-catalysed chromic acid oxidation of primary alcohols also is proposed to proceed through a similar ternary complex. Methanol- reacted nearly six times slower than methanol, supporting a hydride transfer mechanism in this oxidation.32 Kinetics of chromic acid oxidation of dimethyl and diethyl malonates, in the presence and absence of oxalic acid, have been obtained and the activation parameters have been calculated.33 Reactivity in the chromic acid oxidation of three alicyclic ketoximes has been rationalized on the basis of I-strain. Kinetic and activation parameters have been determined and a mechanism... 

Br , Cr, F , and I ions can be useful to transform alcohols into ketones in very convenient and facile experimental conditions (see Table 6 and 7). Species suich as Cl, Br", or were propose to be formed in the anode reaction layer. Probably, polarized halogen dipoles in the high electric field present at the electrode interface could also explain the specific reactivity of anodically generated oxidizing species from halides. 

In studies of the reactivity of copper-phenoxyl radical species, particular emphasis has been placed on performing alcohol oxidations, either stoichiometrically or catalytically, and by using O2 as the final oxidant. One goal has been to obtain mechanistic insights into GAO function. In addition, the GAO reaction sequence has been used as inspiration for the development of new chemical catalysts for alcohol oxidations and H2O2 generation (a bioinorganic success story ). ... 

In the past decades, a significant number of fundamental investigations have been carried out in the field of low-temperature electrooxidation of small organic molecules [14-38]. Electrochemical studies have been carried out in combination with spectroscopy [21,27,28], mass spectroscopy [25,31], physio-chemical tools [20,30], as well as theoretical calculations (e.g., DFT) [38] in order to examine the adsorbed species and reactive intermediates on the electrode surface during the alcohol oxidation, and thus to elucidate the alcohol reaction pathways. 

The oxidation state of Cr in each of these species is (VI), and they are powerful oxidants. The precise reactivity, however, depends on the solvent and the form of Cr present, so that substantial selectivity can be achieved by choice of the particular reagent and conditions. The transformation most often effected with CrOs-based oxidants is the conversion of alcohols to the corresponding ketone or aldehyde. The mechanism that is believed to be operative in alcohol oxidations is outlined below ... 

Gupta et al. (2004) studied the cyclic voltammograms (Fig. 10) of ethanol electro-oxidation behavior on CuNi, CuNi/Pt and CuNi/PtRu alloys electro-catalysts in 0.5 M NaOH solution. Fig. 10 (a) shows a steady rise of the anodic peak current for the CuNi/Pt electro-catalyst. The peak current increases substantially from F to the 50 scan. Fig. 10 (b) shows the increase in reaction kinetics for ethanol electro-oxidation when Ru is added in the alloy. They have detected the presence of acetaldehyde and CO2 (as carbonate) with CuNi/PtRu electro-catalyst Authors found carbonate ions because of the cleavage of C—C bond of ethanol molecule. The temperature of ethanol electro-oxidation was not mentioned although the experimental work was done at room temperature. Tripkovic et al. (2001) studied the electro-oxidation of methanol, ethanol, -propanol and n-butanol (C[—C4 alcohol) in alkaline solution at the Pt (111) and vicinal stepped planes Pt (755) and Pt (332). The nature of the oxygen-containing species as well as their role in the alcohol oxidation is proposed. A dual path reaction mechanism as shown by eqs. (4) and (5) is proposed based on the assumptions that RCOa is a reactive intermediate of the main reaction path, while CO2 is a product of the poisoning species oxidation in a parallel reaction path. 

Mukaiyama s conditions have also been used in other aerobic oxidation reactions of substrates including thiols (Table 5.2, entries 1—4, 10 and 11), alkanes (entries 8, 12 and 14) and alcohols (entries 9 and 13), as well as reactions involving lactone formation via a Baeyer-ViUiger oxidation (entries 5-7) and oxidative decarboxylation (entry 16) [15-17]. While nickel, iron and cobalt aU selectively oxidize thiols to sulfoxides, Co(II) is the most active (entries 1—4) [15 b]. Of particular synthetic interest, the chemoselective and diastereoselective aerobic oxidation of the complex sulfide, exomethylenecepham (entries 10 and 11), was observed with no overoxidation to the suUbne or oxidation of the olefin [16 a]. The diverse substrate scope in entries 1-9 suggest iron and nickel species tend to have similar reactivity with substrates, but cobalt behaves differently. For example, both iron and nickel displayed similar reactivity in Baeyer-Villiger oxidations, with cobalt being much less active (entries 5-7), yet the opposite trend was observed for sulfide oxidation (entries 1—4) [15]. Lastly, illustrating the broad potential scope of Mukaiyama-type oxidations, alcohol oxidation (entries 9 and 13) and oxidative decarbonylation (entry 15) reactions, which are oxidase systems, have also been reported [16b, 17b]. 

Figure 29.6 Possible routes of ethanol to cancer. This schematic diagram shows some of the ways in which it is suspected that ethanol may promote carcinogenesis. Although not carcinogenic itself, ethanol can solubilise organic carcinogens that can intercalate between the bases of DNA and cause it to be misread. The metabolism of alcohol generates large amounts of free radicals and reactive oxygen species. These cause peroxidation of lipids and the products of lipid peroxidation can form adducts with DNA and its repair enzymes. The acetaldehyde produced by alcohol oxidation can also do this, but can also induce cell proliferation in some tissues as well as altering the levels of steroid hormones upon which some tumours depend.

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