The rate constants of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to an NAD(+) model, 9-phenylxanthylium ion (PhXn(+)), in acetonitrile (AN) and in water containing AN (80% H(2)O/20% AN) were determined over a temperature range from 36 to 67 degrees C. The reactions follow second-order rate laws. In the latter solution, formation of the water adduct of PhXn(+) was observed as a side-equilibrium (K). The observed inverse solvent kinetic isotope effect (k(H(2)O)(obs)/k(D(2)O)(obs) = 0.54), the larger than unity equilibrium isotope effect (K(H(2)O)/K(D(2)O) = 2.69), and the results of acid effect on the observed rate constants of the reactions are consistent with the "side-equilibrium mechanism". Kinetic isotope effects at all three H/D positions of i-PrOH for the net hydride-transfer process were determined in both solutions at 60 degrees C: KIE(alpha-D)(H) = 3.2(AN), 3.2(H(2)O); KIE(beta-D6)(H) = 1.05(AN), 1.16(H(2)O); KIE(OD)(H) = 1.08(AN), 1.04(H(2)O). These KIE values are consistent with the presence of the positively charged alcohol moiety in the transition state (TS) for cleavage of the alpha-C-H bond, the delocalization of the positive charge over the alpha-C-OH group, and the stepwise hydride and proton transfer processes. Comparison of the activation parameters for the reactions in the two solvent systems as well as those in the i-PrOH/AN (1:1 v/v) reported earlier suggests that the AN medium promotes the reaction by activating the ground-state alcohol reactant through weak interactions with the electron pairs on alcohol O, while water and parent alcohol media facilitate the reaction by H-bonding stabilization of the alcohol moiety of the TS. Results suggest that in the alcohol dehydrogenases without a Zn(II) cofactor in the active sites alcohols would be oxidized via hydride transfer to NAD(+) coenzyme followed by the rapid deprotonation to the nearby basic species in the active site of the enzymes.

Second‐order rate constants were determined for the oxidation of 27 alcohols (R1R2CHOH) by a carbocationic oxidizing agent, 9‐phenylxanthylium ion, in acetontrile at 60 °C. Alcohols include open‐chain alkyl, cycloalkyl, and unsaturated alcohols. Kinetic isotope effects for the reaction of 1‐phenylethanol were determined at three H/D positions of the alcohol (KIEα‐D = 3.9, KIEβ‐D3 = 1.03, KIEOD = 1.10). These KIE results are consistent with those we previously reported for the 2‐propanol reaction, suggesting that these reactions follow a hydride‐proton sequential transfer mechanism that involves a rate‐limiting formation of the α‐hydroxy carbocation intermediate. Structure–reactivity relationship for alcohol oxidations was deeply discussed on the basis of the observed structural effects on the formation of the carbocationic transition state (Cδ+OH). Efficiencies of alcohol oxidations are largely dependent upon the alcohol structures. Steric hindrance effect and ring strain relief effect win over the electronic effect in determining the rates of the oxidations of open‐chain alkyl and cycloalkyl alcohols. Unhindered secondary alkyl alcohols would be selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C7C11 cycloalkyl alcohols react faster than cyclohexyl alcohol, whereas the strained C5 and C12 alcohols react slower. Aromatic alcohols would be efficiently and selectively oxidized in the presence of aliphatic alcohols of comparable steric requirements. This structure–reactivity relationship for alcohol oxidations via hydride‐transfer mechanism is hoped to provide a useful guidance for the selective oxidation of certain alcohol functional groups in organic synthesis. Copyright © 2011 John Wiley & Sons, Ltd.

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Die Belichtung von Benzophenon (I) in Methanol in Gegenwart von konz. Schwefelsäure ergibt Benzoesäuremethylester (II), das Pinakol (III) und das Diphenyläthandiol (IV).

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