Benzyl alcohols oxidation potentials

The goal of maximum energy generation by oxidation of carbonaceous species often thwarted detailed examination of occasional selective oxidations, such as ethylene oxidation to acetaldehyde on Pd or Au (28, 29, 370) or to ethylene oxide on Ag (330) or methanol and benzyl alcohol oxidation to formates and benzaldehyde, respectively (6-32, 54, 250, 333). Product yields were usually determined at one potential only or even galvanostatically (330), and the combined effects of potential, catalyst, reactant concentration, and cell design or mixing on reaction selectivity are unknown at present. Thus, reaction mechanisms on selective electrocatalysis are not well understood with few exceptions. For instance, ethylene oxidation on solid pal-... 

Benzyl alcohols Aryl alkyl carbinols (11) can be oxidized to ketones (12) by the direct electrochemical method (Eq. 4) since they possess their oxidation potentials at around 2.0 V versus SCE (saturated calomel electrode) however, cleavage products decrease the selectivity [14]. 

Ru" (0)(N40)]"+ oxidizes a variety of organic substrates such as alcohols, alkenes, THE, and saturated hydrocarbons. " In all cases [Ru (0)(N40)] " is reduced to [Ru (N40)(0H2)] ". The C— H deuterium isotope effects for the oxidation of cyclohexane, tetrahydrofuran, 2-propanol, and benzyl alcohol are 5.3, 6.0, 5.3, and 5.9 respectively, indicating the importance of C— H cleavage in the transitions state. For the oxidation of alcohols, a linear correlation is observed between log(rate constant) and the ionization potential of the alcohols. [Ru (0)(N40)] is also able to function as an electrocatalyst for the oxidation of alcohols. Using rotating disk voltammetry, the rate constant for the oxidation of benzyl alcohol by [Ru (0)(N40)] is found to be The Ru electrocatalyst remains active when immobilized inside Nafion films. 

Besides having smaller oxidation potential values than substituted benzyl alcohols (E° > 1.4 V/NHE), the DMAs have larger energy values (90-92 kcalmoD ) for the NC—H bond with respect to C—H bond energies around 75-85 kcalmoD of the benzyl alcohols (Scheme 12). Both factors disfavour the operation of the radical HAT route for PINO with the DMAs, and cause a mechanistic changeover to the ET route, as opposed to the reactions with the benzylic substrates listed in Table 4. 

Consistent with the results of this study is the outcome of the oxidation of 4-X-substituted phenols by use of PINO, generated from HPI with Pb(OAc)4 at 25 °C in MeCN containing 1% AcOH . The reactivity (fcn) of PINO towards phenolic O—H bonds (BDE 85-90 kcal moC ) was about one order of magnitude higher than that measured towards the C—H bond of benzyl alcohols (cf. Table 4). A p value of —3.1 was obtained from plotting log kn vs. for this reaction, where removal of H-atom from the phenolic O—H bond (which is weaker than the O—H bond of aliphatic or benzyl alcohols) induces an oxidative phenolic coupling with the PINO moiety. In view of the low redox potential of the substituted phenols (in the 0.8-1.1 V/NHE range), and of the substantial value of the kinetic isotope effect = 3.1-3.7 measured, ... 

Operation of the latter mechanism has also been invoked for the oxidation of X-substituted benzyl alcohols with TEMPO and the enzyme laccase becanse the redox potential of the enzyme (0.78 is adeqnate for the oxidation of TEMPO to oxoammonium ion (0.8 Strangely enough, no linear correlation of the log A x/ h ratios... 

The data in Table 2 show the potential of the Na2B407 based catalyst system tested over large number of representative alcohols. The primary alcohols were oxidized to the corresponding aldehydes at complete conversion of the alcohol and at 90-93% selectivity. The only by-products observed were the corresponding acid and minor amounts of the symmetrical ester (Entry 2, 3). Benzyl alcohol was quantitatively converted to benzaldehyde. The secondary alcohols, 4-methyl cyclohexanol and 4-methylpentanol were converted to the corresponding ketones at room temperature. 

Interestingly, the simple p-quinone (84a) is also able to oxidize certain unsaturated alcohols under harsh conditions.98 Because of its lower oxidation potential, p-quinone only oxidizes unsaturated alcohols devoid of steric hindrance and able to generate very stabilized carbocations. Thus, it is able to react with primary cinnamyl alcohols but not with secondary cinnamyl alcohols, simple allylic alcohols and benzylic alcohols. 

TABLE 12.4 Voltammetric Oxidation Potentials and Peak Currents (pA/mM) of Alkoxy-Substituted Benzenes, Phenols, and Benzyl Alcohols, and Their Derivatives in MeCN (0.1 M TEAP) at a Glassy-Carbon Electrode... 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

The [Ruv(N40)(0)]2+ complex is shown to oxidize a variety of organic substrates such as alcohols, alkenes, THF, and saturated hydrocarbons, which follows a second-order kinetics with rate = MRu(V)][substrate] (142). The oxidation reaction is accompanied by a concomitant reduction of [Ruv(N40)(0)]2+ to [RuIII(N40)(0H2)]2+. The mechanism of C—H bond oxidation by this Ru(V) complex has also been investigated. The C—H bond kinetic isotope effects for the oxidation of cyclohexane, tetrahydrofuran, propan-2-ol, and benzyl alcohol are 5.3 0.6, 6.0 0.7, 5.3 0.5, and 5.9 0.5, respectively. A mechanism involving a linear [Ru=0"H"-R] transition state has been suggested for the oxidation of C—H bonds. Since a linear free-energy relationship between log(rate constant) and the ionization potential of alcohols is observed, facilitation by charge transfer from the C—H bond to the Ru=0 moiety is suggested for the oxidation. 

Reactions of this type are relatively few, in comparison to those of DIB. Alkyl benzyl ethers afforded benzaldehyde and alkyl trifluoroacetates in a potentially useful reaction for the deprotection of benzylated alcohols under oxidative conditions, since trifluoroacetates are hydrolysed very easily [53] ... 

Redox potential is a catalytically relevant property of heme peroxidases, as in theory, sets the limit for the oxidative ability of the enzyme. An inverse correlation was found between the activity and the redox potential of methoxybenzenes and methoxy-substituted benzyl alcohols for lignin peroxidase (LiP) and horseradish peroxidase (HRP) [42, 43]. These enzymes were able to catalyze the oxidation of methoxybenzenes with redox potential as high as 1.45 V and 1.12 V, respectively [42]. In the case of methoxy-substituted benzyl alcohols, the maximum substrate redox potential was 1.39 V for both enzymes [43]. This type of correlation has allowed ranking enzymes from the more oxidant to the less oxidant. The inverse... 

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