Aryl-benzyl bonds, cleavage

This prediction has now been confirmed by the observation (R. Alexander, C. Eaborn and T. G. Traylor, /. Organometal. Chem., 21 (1970) P65) that a solvent-isotope effect is obtained in base cleavage of benzyl- and aryl-tin bonds by NaOH in MeOH-MeOD. 

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine... 

The reduction of benzyl aryl ethers has been thoroughly investigated by voltammetric reduction, homogeneous redox catalysis,and currently, by convolution analysis. A family of ethers activated by proper substitution on the phenoxy side were chosen to provide a wide variation in the ET and bond cleavage properties of the molecule. ... 

Eaborn and co-workers28 have reported product isotope ratios for the cleavage of benzyl-silicon, benzyl-tin, and aryl-tin bonds using an equimolar mixture of MeOH and MeOD as the solvent. A free carbanion would not be expected to discriminate between MeOH and MeOD in the fast step (24), and hence28 the product isotope ratio kH/kD should be unity. The values of kJkD in Table 11 indicate that the carbanions are not entirely free, but that some degree of electrophilic attack by the solvent at the benzyl carbon atom takes place, as in (V). 

Castellan, A., Vanucci, C., Bous-Laurent, H., "Photochemical Degradation of Lignin Through a C-0 Bond Cleavage of Non-phenolic Benzyl Aryl Ether Units. A Study of the Photochemistry of a(2 ,4 -Trimethyl-Phenoxy)-3,4 Dimethoxy Toluene.", Hohforschmg, 1987, 41(4), 231. 

It has been shown that an electron-donating group on the aryl substituent promotes C-S bond cleavage because of better stabilization of the benzylic cation. In the case of R3 = R4 = Me, the aldehyde is obtained after hydrolysis of the imine (Scheme 26) <1998T8941>. 

Alkyl aryl ketones are known to be arylated at the a-position of the alkyl groups, via the corresponding enolates, by treatment with aryl halides in the presence of palladium catalysts [4, 9]. The ortho arylation of alkyl aryl ketones is also possible. For example, in the reaction of benzyl phenyl ketones with bromobenzenes, the arylation first occurs at the benzylic position the ortho positions are then arylated via C-H bond cleavage (Eq. 8) [15]. The ortho arylation is believed to occur after coordination of the enol oxygen to ArPd(II), which is followed by ortho palladation as in the reaction of 2-phenylphenols shown in Scheme 2. 

Finally, side-chain functionalization may be obtained through a PET process. Apart from the several arylations discussed in Sect. 2.1.3, oxygenation of benzyl radicals obtained from deprotonation or carbon-carbon bond cleavage is preparatively useful (the method has been recently reviewed [254]), and other functionalization are possible, notably fluorination of benzyl substrates by irradiation in the presence of titanium dioxide and fluoride ions [255], Furthermore, the modification of a substituent, as a typical example the reduction of a nitro group [18, 27, 256-259] or an azo group [266], is in some cases obtained through a PET process. 

Aryl and aliphatic amines are perhaps the best known D molecules primarily due to the availability of the nonbonded electrons on nitrogen for PET processes. The simplest bond cleavage reaction for amine cation-radicals is heterolytic C—H cleavage or deprotonation. It is well known that the acidity of protons on a species after loss of an electron, i.e., after one-electron oxidation, is greatly enhanced [7]. For example, the pKa of the benzylic protons in the toluene cation-radical have been estimated to be —10 [7] compared to pKa value of 35 before oxidation. The process of proton loss can be extremely fast, on the time scale of picoseconds [8]. 

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