Benzyl alcohols electronic effects

Kinetic studies of competitive alcohol oxidation over Au/TiOj have postulated a unique ensemble in which a carbocation can be stabilized adjacent to a neighboring gold-oxo center [163]. This work proposed that benzylic alcohols oxidize via C-H cleavage at the benzylic position. Electronic effects in PVP-stabilized gold nanoparticles for p-hydroxy benzyl alcohol selox have also been reported [164] XANES, XPS, and FT-IR suggest that anionic 1.5 nm Au clusters are the most catalytically active, with electron donation from the polymer stabilizer creating Au-superoxo species (Figure 2.11). 

The bare nitrogen analog of dihydroazaborine, where the R substituent on N is effectively replaced by an electron pair, is the azaboratabenzene anion H4C4BRN. The synthesis of this ligand and a ruthenium sandwich complex Cp Ru(H4C4BPhN) that is catalytically active in the acylation of benzyl alcohol have been reported.151... 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

The reduction of benzoic acid at a lead cathode in aqueous sulphuric/citric acids yields the two-electron products benzaldehyde and the four-electron product benzyl alcohol rather than one-electron hydrodimer. In all cases studied by the authors they found that ultrasound favoured the process involving the smaller number of electrons per molecule. This is the opposite of the sonoelectrochemical effect seen in carboxylate electrooxidation [57,59,60] where the process involving the greater number of electrons was favoured by ultrasound. 

Verevkin, S.P. Strain effects in phenyl-substituted methanes, geminal interaction between phenyl and the electron-releasing substituent in benzylamines and benzyl alcohols, J. Chem. Eng. Data, 44(6) 1245-1251, 1999. 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

From the temperature dependence of the substantial kinetic isotope effect (KIE) observed in the oxidation of diols to hydroxycarbonyl compounds by 2,2/-bipyridinium chlorochromate (BPCC), it is proposed that hydride transfer occurs in a chromate ester intermediate, involving a six-electron Hiickel-type transition state.9 A similar conclusion is drawn for the oxidation of substituted benzyl alcohols by quinolinium chlorochromate.10... 

The Pd-catalysed intermolecular reaction of aryl bromides containing electron-withdrawing substituents with a wide variety of alcohols including MeOH, 2-propanol, benzyl alcohol and i-butyl alcohol gives the aryl ethers 416 under milder conditions than uncatalysed reactions. Bidentate ligands such as BINAP and DPPF (XLIX) are effective [206,207]. The aryl Pd alkoxide 417 was isolated as an intermediate, and the formation of the aryl ethers 418 by reductive elimination of 417 was confirmed. 

Several catalysts have been found for the ring opening of epoxides. For instance, cyclohexene oxide gave an excellent yield of the trans-fi-amino alcohol when treated with either aromatic or aliphatic amines in the presence of a scandium triflate catalyst.21 Aromatic epoxides react in a regiospeciflc reaction at the benzylic carbon when treated with aromatic amines and scandium triflate but at the least substituted carbon of the epoxide ring when aliphatic amines react. Electronic effects are more important in the reactions of the aromatic epoxides whereas steric factors control the reaction with aliphatic epoxides. 

The influence of the alcohol on the reaction was evaluated (Scheme 26). The results of a competition experiment between the alcohols are shown in Table 7. Both alcohols were treated with mono-alkoxysilane le using 10 % Pd/C as the catalyst. The silyl ketals of both alcohols were isolated as a mixture and the area under the methine protons, from the (+)-ethyl lactate moiety of both silyl ketals, was compared by NMR analysis. The difference in reactivity of primary, versus secondary, versus tertiary alcohol was small. The differences in reactivity range from 1.5 1 for 1° vs 2°, to 3 1 for 1° vs 3°. The reactivity of a benzyl alcohol is slower than the aliphatic alcohol as shown in entries 4 to 6. Entries 4 and 5 show an increase in the ratio of 1° 2° alcohol and a decrease in ratio for the 2° 3° for the secondary benzyl alcohol. Entries 6 and 7 confirm that benzyl alcohols are less reactive than aliphatic alcohols. The inductive electron withdrawing effect of the aryl group in the benzyl alcohol renders it less nucleophillic and this may affect the rate of reaction with the silane. Although the difference in reactivity is small, this trend may be informative. The influence of the alcohol s nucleophilicity on the reaction mechanism will be addressed in a later section. 

Another similarity between the two catalysts was the small electronic effect observed for 2-phenylethanol versus benzyl alcohol. The same 1.1 1 ratio in favor of 2-phenylethanol was observed when a competition experiment was performed between the two alcohols. 

We have briefly looked at the electronic effects on this mechanism by the competition experiment of two primary alcohols (2-phenethyl ethanol and benzyl alcohol). The information we obtained only explained the importance of the nucleophilicity of the alcohol. In order to determine the electronic effects on the dissociating alcohol, an experiment could be performed that treats the silyl ether of a primary alcohol with a second primary alcohol, both alcohols having different electronic environments (Scheme 40). The ratio of exchange product (B) to expected product (A) would be an indication of the effect of the electronic environment on the dissociating alcohol in intermediate IV of Figure 11. 

Photoheterolytic cleavage of benzyl alcohol in neutral media occurs only when it gives rise to highly stabilized cations, such as triphenylmethyl cations (compare Sec. 15.1 the reaction may be at least in part adiabatic in this case and yield the excited cation) [79-81], xanthyl [82] or fluorenyl cations [83]. However, acid catalysis is effective and methyl ethers are by far the main products from benzyl alcohols in acidified aqueous methanol [84-88]. Electron donating substituents in the ortho and, to a lesser extent, in the meta position enhance the quantum yield (acid catalysis may be... 

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