Alcohol oxidation kinetics

One of the reasons to use AEM in PEM fuel cells is to reduce alcohol crossover due to the elimination of electrosmotic drag by protons moving from anode to cathode. In AD AFC the current through the AEM is conducted by HO ions, which are transported in the opposite direction minimizing the electrosmotic flux and, in addition, alcohol oxidation kinetics can be facilitated in alkaline media. 

Bruhsett et al. [34] examined the performance of air-breathing membraneless LFFC operated with ethanol and methanol imder acidic (H2SO4) and alkaline (KOH) conditions. Methanol and ethanol showed improved open-circuit potential and maximum power density in alkaline media (1.2 and 0.7 V, 17.2 and 12.1mWcm ) compared with acidic conditions (0.93 and 0.41V, 11.8 and 1.9 mW cm ). The improved performance in alkaline media was the result of the enhanced alcohol oxidation kinetics and oxygen reduction kinetics compared with acidic media. 

HMF has been selectively converted into FDCA (99 mol% yield) in water, under mild conditions (65-130°C, 10 bar air) using gold nanoparticles on nanoparticulated ceria. ° As an alternative to FDCA, 2,5-dimethylfuroate (FDMC) has also been synthesized using the same catalyst in the absence of a base in methanol. The oxidation of HMF into FDCA or FDMC comprises two steps aldehyde oxidation and alcohol oxidation. Kinetic studies show that the rate-limiting step of the reaction is alcohol oxidation to aldehyde. Once the aldehyde is formed, the corresponding hemiacetal is obtained, which is rapidly oxidized into the acid or the ester (Fig. 13.5). 

SCHEME 34.35. Structure of (—(-aurantioclavine 133 and details of the secondary alcohol oxidation kinetic resolution step. 

SCHEME 34.37. Structure of (-l-)-amurensinine 141 and details of the asymmetric secondary alcohol oxidative kinetic resolution step. 

SCHEME 34.41. Asymmetric secondary alcohol oxidation kinetic resolution step in the synthesis of human neurokinin receptor antagonist h-NKl 156. 

Okamoto et al. found that A-oxidation activates 4-halogeno-quinolines in the reaction with piperidine in aqueous alcohol by kinetic factors of 9 to 25, at 100°. This rate-enhancing effect is accompanied by a fairly large decrease in the enthalpy of activation (up to 10 kcal/mole in the chloro compounds), the effect of which is partly offset by a decrease in the entropy of activation. 

Since the transition state for alcohol oxidation and ketone reduction must be identical, the product distribution (under kinetic control) for reducing 2-butanone and 2-pentanone is also predictable. Thus, one would expect to isolate (R)-2-butanol if the temperature of the reaction was above 26 °C. On the contrary, if the temperature is less than 26 °C, (S)-2-butanol should result in fact, the reduction of... 

Scheme 9.4 Kinetic resolution by alcohol oxidation toward chiral products.

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

In an effort to develop more active catalyst systems for the oxidative kinetic resolution of non-activated alcohols, Stoltz et al. discovered a modified set of conditions that accomplishes similar resolutions in a fraction of the time [43]. 

Sigman et al. have optimized their system too [45]. A study of different solvents showed that the best solvent was f-BuOH instead of 1,2-dichloroethane, which increased the conversion and the ee. To ensure that the best conditions were selected, several other reaction variables were evaluated. Reducing the catalyst loading to 2.5 mol % led to a slower conversion, and varying temperature from 50 °C to 70 °C had little effect on the selectivity factor s. Overall, the optimal conditions for this oxidative kinetic resolution were 5 mol % of Pd[(-)-sparteine]Cl2, 20 mol % of (-)-sparteine, 0.25 M alcohol in f-BuOH, molecular sieves (3 A) at 65 °C under a balloon pressure of O2. 

In 2003, Sigman et al. reported the use of a chiral carbene ligand in conjunction with the chiral base (-)-sparteine in the palladium(II) catalyzed oxidative kinetic resolution of secondary alcohols [26]. The dimeric palladium complexes 51a-b used in this reaction were obtained in two steps from N,N -diaryl chiral imidazolinium salts derived from (S, S) or (R,R) diphenylethane diamine (Scheme 28). The carbenes were generated by deprotonation of the salts with t-BuOK in THF and reacted in situ with dimeric palladium al-lyl chloride. The intermediate NHC - Pd(allyl)Cl complexes 52 are air-stable and were isolated in 92-95% yield after silica gel chromatography. Two diaster corners in a ratio of approximately 2 1 are present in solution (CDCI3). 

Interestingly, the scope of the reaction using this catalyst can be extended to oxidative kinetic resolution of secondary alcohols by using (-)-sparteine as a base (Table 10.2) [25]. The best enantiomeric excess of the alcohol was obtained when a chiral enantiopure base and an achiral catalyst were used. The use of chiral enantiopure catalyst bearing ligand 17 led to low enantioselectivity. 

Scheme 10.6 Mechanism of aerobic oxidation catalysed by complex 13 [23] Table 10.2 Oxidative kinetic resolution of alcohols using (-)-sparteine [25]...

Axially chiral Pd-NHC complexes reported by Shi and co-workers [26-28] have shown high selectivity in the oxidative kinetic resolution of alcohols without the need of addition of a chiral base. Enantiomeric excesses of up to 99% were obtained (Scheme 10.7). 

Scheme 10.7 Oxidative kinetic resolution of alcohols using chiral bis-NHC hgands [26-28] 10.3.1.2 Anaerobic Oxidation...

It was seen when analyzing the kinetic data for alcohol oxidation reactions that the catalytic action of nickel oxide is due to a mediator mechanism. Higher oxide forms interact with the adsorbed organic species and oxidize them. In the following step the higher oxide forms are regenerated by electrochemical oxidation of lower oxide forms. 

This reaction can be used as a kinetic test to determine which radicals predominate in an alcohol oxidized under given conditions. If hydroxyperoxide radicals predominate, the... 

In the oxidation of isopropyl alcohol, the kinetics of formation of hydrogen peroxide for [H202] = 0.7 mol L 1 is described by the equation [58]... 

Figure 5.8 Oxidation kinetics in the aerobic conversion of benzyl alcohol to benzal-dehyde in toluene mediated by 10 mol% TPAP either encapsulated in the sol-gel hydrophobic matrix A-Me3 or unsupported. (Reproduced from ref. 17, with permission.)...

Complex (1) is a catalyst for selective oxidation of benzylic, allylic alcohols to aldehydes, and secondary alcohols to ketones using r-butyl hydroperoxide. Primary aliphatic alcohol oxidation failed. The use of cumyl hydroperoxide as radical probe discounted the involvement of i-BuO /t-BuOO. Hammett studies p = -0.47) and kinetic isotope effects kn/ku = 4.8) have been interpreted as suggesting an Ru—OO—Bu-i intermediate oxidant. 

The oxidations of secondary alcohols and sulfides by halamine polymers produce ketones and sulfoxides, respectively, with some sulfones and chlorosulfoxides produced in the latter case. A mechanism is proposed based on the oxidation kinetics. 

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