Methanol Oxidation hydride transfer

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... 

Two possible mechanisms for methanol oxidation by MDH enzymes have been proposed in the literature, the Addition-Elimination (A-E) and the Hydride Transfer (H-T) mechan-isms. " The A-E is a three-step mechanism (Fig. 2a) that involves a proton transfer from methanol to an active site base, which is proposed to be ASP303. It is believed that the presence of this catalytic base at the MDH active site initiates the oxidation reactions by subtracting a proton (H16) from methanol (Fig. 2a). This proton addition to ASP303 leads to the formation of a covalent hemiketal intermediate, since the resulting oxyanion (016 ) in the methanol molecule is then attracted to the C5 of PQQ. The second step consists of the proton (H16) elimination from ASP303 and transfer to 05 of PQQ, and the final step is characterized by a... 

Figure 2. (a) Addition-Elimination (A-E) and (b) Hydride-Transfer (H-T) methanol electro-oxidation mechanisms by Methanol Dehydrogenase Enzyme proposed in the literature. 

According to step 1 of the H-T methanol oxidation mechanism (Fig. 2b), there should be a direct hydride transfer (H17) from methanol to C5 of PQQ in concert with proton abstraction (HI6) by 014 of ASP, thus resulting in the formation of by-... 

Figure 7. Geometry optimized struetures involved in step 1 of the methanol Hydride-Transfer oxidation meehanism by MDH aetive site Model B.

At the BLYP/DNP theory level it is found, however the possibility of oxidizing methanol through an alternative mechanism, which we call the two-step hydride transfer (2 step H-T) mechanism as shown in Fig. 9. 

The latter results from an unusual intramolecular Cannizzaro-tjrpe reaction involving a transannular hydride transfer from C-20 to C-7. Oxidation of LXXVI followed by reduction gave the epimeric hydroxyl lactam LXXVII with the hydroxyl (p ax 1682 cm i) in the original configuration of ajaconine. The hydroxyl in LXXVI is thus trans to the i r-bridge and in LXXVII is cis thereto. Further evidence on this point is provided by the behavior of the methiodide (LXXVIII) of the azo-methine alcohol (LXVII). The product liberated from this salt by hot, methanolic alkali is a hydroxyl-free base (LXXIX, 10) and hence... 

Possible additional support for a photonucleophilic hydride transfer mechanism comes from further observations on the photolysis of PCP. When this compound was irradiated in methanol rather than in water, no oxidation or displacement of chlorine by hydroxyl was detected instead, photoreduction produced primarily 2,3,5,6-tetrachlorophenol (XVI) together with a small proportion of what seemed to be the 2,3,4,5-tetra-chloro isomer (Figure 6) (52). Hydrogen abstraction from solvent by polychlorophenyl radicals is expected to produce some of each of the three isomers representing replacement of an o-, m-, and p-chlorine, with the first two predominating because of doubled probability. 

All the reactions of (NH3)4(H20)Rh0, with the possible exception of C2H5CHO, appear to take place by hydrogen atom transfer as judged by a deuterium C-H kie of 5.0 for methanol and 3.2 for 2-propanol. The lack of a chain reaction in the oxidation of alcohols argues against hydride transfer for these substrates (72). 

The oxidative aromatization of tetrahydro-5(l/f)-quinolinones and tetrahydropyrido [2,3-fif]pyrimidin-4(//)-one withpara-benzaldehydes as oxidants in NaOEt/EtOH results in the formation of the corresponding quinolone and aryl methanol because of the hydride transfer from tetrahydroquinoline to arylaldehydes during the oxidation process. The yield of the products basically depends on the substituents with -l-M effect attached to the para position of benzene rings connected to the 2- and 4-positions of the hydro-quinolinone moiety and substituents with -I effect attached to the aryl aldehydes. 

The layer of K2C03 separated quantitatively, 0.2 ml of 2% periodate solution was added and the substrate was oxidized at 50°C for 30 min. The reaction was stopped by the addition of 0.2 ml of 10% sodium metabisulphite, the reaction mixture was cooled with water and vanillin was reduced to vanillyl alcohol by reaction with 100 ml of potassium boro-hydride at room temperature for 10 min. The pH was then adjusted to 7.0 with 5 N acetic acid and 0.6 ml of phosphate buffer (pH 7.2) was added. After mixing, the mixture was extracted with two 10-ml portions of ethyl acetate and the extracts were combined and evaporated to dryness at decreased pressure. In order to remove borate, methanol was added and the mixture again evaporated to dryness. The residue was transferred into a small vial with 2 ml of ethyl acetate and was treated with 0.5 ml of trifluoroacetic-anhydride at room temperature for 1 h. The contents of the vial were evaporated to dryness in an evacuated desiccator, the residue dissolved in 1 ml of ethyl acetate and 1 jul analysed on 3% OV-17 at 150°C. 

---