Benzyl alcohol anodic oxidation

Benzyl alcohol anodic oxidation in water over TEMPO DE (V=1.4Vvs. Ag/AgCI)... 

Because the reduction potential of ether is usually more negative than that of halides, examples that belong to this category are rather rare. Generally, cathodic reduction of ethers is similar to that of alcohols, and nonactivated ethers are not reducible under the conditions of electroreduction. Activated ethers such as benzylic and allylic ethers are elec-trochemically reduced to a limited extent (Scheme 7) [1, 15, 16]. Reduction of epoxides is usually difficult however, electroreductive cleavage of activated epoxides to the corresponding alcohols is reported [17, 18]. The cleavage of the C—O bond of ethers is more easily accomplished in anodic oxidation than in cathodic reduction, which is stated in Chapter 6. 

One of the first notions of EGA-catalyzed reactions was the rationalization [8, 14] of the unexpected outcome of anodic oxidation of methyl arenes, (1), in MeGN containing various amounts of water. Preferentially A-benzyl acetamides, (3), rather than the benzyl alcohols, (2), were formed [15, 16] (with increasing amounts of water, increasing amounts of aldehyde was formed as a side product [16]). Since water is a more powerful nucleophile than MeCN, it is reasonable to believe that the carbocation formed by overall two-electron oxidation and deprotonation is initially trapped by water. However, the process is reversible in the presence of a strong EGA (protons liberated from the oxidized substrate), and the carbocation is eventually trapped by the excess MeCN, Scheme 1. 

Typical examples are listed in Table 2.1. A few oxidations are effected by RuO but in general it is too powerful an oxidant for this purpose. The system RuCyaq. NaCl-CCy Pt anode oxidised benzyl alcohol to benzaldehyde and benzoic acid and p-anisaldehyde to p-anisic acid [24], and a wide range of primary alcohols and aldehydes were converted to carboxylic acids, secondary alcohols to ketones, l, -diols to lactones and keto acids from RuOj/aq. NaCl pH 4/Na(H3PO )/Pt electrodes (Tables 2.1-2.4). The system [RuO ] "/aq. K3(S303)/Adogen /CH3Cl3 oxidised benzyhc alcohols to aldehydes [30]. The oxidation catalyst TPAP (( Pr N)[RuO ]) (cf. 1.3.4) is extremely useful as an oxidant of primary alcohols to aldehydes and secondary alcohols to ketones without... 

The behavior of BnMgBr (5d) is similar to that observed for compounds with higher alkyl groups, i.e. only the coupling product was detected and the earlier report on the additional formation of benzyl alcohol was not confirmed. On the other hand, reactions of Ar" radicals formed in the anodic oxidation of aryl Grignard reagents are different from those established for Aik, as is evident from the percent distribution of parent radicals in major products given in Table 7. 

Fuchigami and coworkers and Yoshida and coworkers independently found that anodic oxidation of benzylsilanes in the presence of nucleophiles such as alcohols and carboxylic acids resulted in a selective cleavage of the C—Si bond and the oxygen nucleophiles were introduced exclusively into the benzylic position (equation 6)11-13. In the absence of nucleophiles, the benzylsilane itself plays a role of a nucleophile and benzyl(trimethylsilyl-methyl)benzene is formed (equation 7)11,12. 

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. 

Anodic oxidation of A -benzyl-A/ -methylethanolamine leads to the formation of a mixture of 3-methyl-2-phenyl- and 3-benzyloxazolidine by intramolecular attack of the hydroxy group at the intermediate iminium ion, mainly in the benzylic position [181]. The typical fragmentation of jS-amino alcohols occurs as a side reaction, in this case giving foiTnaldehyde and A/ -benzyl-A/ -methoxymethyl-A -methyl amine. The direct anodic oxida-... 

Anodic oxidation of cephalosporins in methanol-tetrahydrofuran mixtures containing Et4NOTs provides a useful synthesis of the corresponding 2-methoxy derivatives [105]. When ethanol, 2-propanol, or benzyl alcohol are substituted for methanol, the 2-alkoxy derivatives are formed in acceptable yields ... 

Benzyl alcohols [95] and ethers [96] may be oxidized anodically. This has been employed in an anodic removal of benzylic protecting groups. Anisyl ethers of the protected alcohols were preferred because of the relatively low oxidation potential of these ethers [1.65 V (SCE)] [97] ... 

Allyl and benzyl silanes may be anodically oxidized in the presence of an alcohol, a carboxylic acid, or water to the corresponding ether, ester, or alcohol [208]. The initially formed radical cation is believed to lose the silicon moiety by attack by the nucleophile on silicon ... 

One of the side reactions gives the corresponding benzyl alcohol (and its oxidation product, the aldehyde). Related to these reactions is the anodic hydroxylation of 1-carbo-methoxy-l,2,3,4-tetrahydrocarbazoles to the corresponding 1-hydroxy-1-carbomethoxy-1,2,3,4-tetrahydrocarbazoles in MeCN-H20 mixtures [24]. 

In some cases, the hydrocarbon oxidation in the presence of a reductant does not need a transition metal complex as catalyst. Thus, a cathode of a carbon whisker has been found to be active for the oxidation of toluene into benzaldehyde and benzyl alcohol during the H2-O2 fuel cell reaction [58a]. During the electrolysis of water at room temperature, the epoxidation of hex-1-ene and hydroxylation of benzene to phenol and hydroquinone occurs simultaneously on the anode and the cathode, respectively [58b]. Finally, selective oxidation of terminal isopropyl groups to the corresponding tertiary alcohols have been carried out by an electrochemical method [58c], Using the... 

If the solubility of poorly soluble substances in electrochemical reactions can be increased by using hydrotropes, remarkable enhancements of the rate can be achieved. Most of the hydrotropes withstand cathodic reduction much better than anodic attack, and permanent loss of hydrotrope may occur due to anodic attack. Two early examples are the reduction of nitro compounds such as nitro-phenol to aminophenol in quantitative yield and the oxidation of benzyl alcohol or benzaldehyde to benzoic acid. 

The general reactivity of oxidized Ni anodes in various f-butanol/H20 mixtures was followed by cyclic voltammetry. " The coulombic and organic product yields of aldehyde and acid were determined for various primary alcohol derivatives. Substituent effect on the anodic oxidation rates of a series of benzyl alcohols were evaluated. Attempts were made to relate the oxidation rates to the Hammett cr parameter for substituent properties. 

The reactivities of alkyl halides are in the sequence RI > RBr > RCl and MeX > EtX > PrX. Benzyl hahde reactions with tin do not require catalysts (equation 2). For less reactive halides, the catalysts and promoters employed include metals (sodium, magnesium, zinc, or copper), Lewis bases (amines, triorganophosphines and -stibines, alcohols, or ethers), iodides, and onium salts (R4MX). The use of tin-sodimn alloys can result in tri- or tetraorganotin products. Electrochemical synthesis has also been reported, e.g. the formation of R2SnX2 from the oxidation of anodic tin by RX in benzene solution and the formation of ILtSn from RI (R = Me or NCCH2CH2) and cathodic tin. 

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