Carbon-hydrogen bonds benzylic

Benzylic carbon-hydrogen bonds in compounds such as methylpentafluoro-benzene, fluoromethylpentafluorobenzene, and difluoromethylpentafluoroben-zene are not capable of metalation by butyllithium Instead nucleophilic substitution of the para fluorines occurs m each example [55] (equation 13)... 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

Oxidation of Benzylic and Allylic sp3 Carbon-Hydrogen Bonds... 

The indirect anodic cleavage of carbon-hydrogen bonds in the benzyl position using triarylamine mediators was also used for mild and selective deblocking of hydroxy, carboxyl, and amino groups. The primarily formed cation radical of the protective group is readily deprotonated in the benzyl position by an added base (Eq. (107)). This benzylic radical is easily further oxidized to the benzyl cation which subsequently is cleaved by attack of a nucleophile, such as water (Eq. (108)). 

Benzyl esters, cyclization-hydrosilylation, 11, 386-387 Benzyl ethers, cyclization-hydrosilylation, 11, 386-387 Benzyl groups, C-H bond silylation, 10, 240 Benzylic alcohols, catalytic alkylation, 11, 146 Benzylic carbon-hydrogen bonds borylation, 9, 174... 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

The other type of carbamoyllithiums IIIc can also be prepared by reaction of CO with (V-lithioketimines, resulting from the addition of rert-butyllithium to aryl cyanides 10477,102. These intermediates 105 underwent selective cyclization to give 177-isoindole derivatives 10677 and six- (107)102 or seven-membered (108)102 cyclic products (Scheme 27). Compounds 107 result either by insertion of the carbene structure into the benzylic carbon-hydrogen bond, as in the case of carbamoyllithiums96, or by intramolecular protonation. 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

In the attack by the comparatively unreactive bromine atom, we have said (Sec. 2.23), the transit.bn state is reached late in the reaction process the carbon-hydrogen bond is largely broken, and the organic group has acquired a great deal of free-radical character. The factors that stabilize the benzyl free radical stabilize the incipient benzyl free radical in the transition state. 

In phenols with ortho or para unsaturated side chains, coupling reactions may occur outside the aryl ring, and may form new carbon-carbon or carbon-oxygen bonds. Benzylic radicals may also be generated, perhaps by hydrogen atom abstraction, and lead to coupling at this position. Finally the products arising from quinones, quinone ketals, or quinone methides may dominate. 

Wolczanski also investigated the chemistry of a tantalum imido system. In this system, elimination of hydrocarbon from the bis-amido imido complex occurs with difficulty at 183°C to give an amido bis-imido complex. The elimination is reversible, with the bis-imido species not being directly observed (Scheme 10). Under methane pressure, the phenyl complex loses benzene and adds methane. Neopentane, benzene, and toluene (benzylic activation) were also found to undergo activation, but not cyclohexane. The authors conclude from their equilibrium studies that the differences in metal-carbon bond strengths are approximately equal to the differences in carbon-hydrogen bond... 

Sammes, M.P. and Harlow, R.L. (1976) Intramolecular hydrogen bonds involving polar carbon hydrogen bonds infrared and H nuclear magnetic resonance spectra of some cyano-methyl and benzyl sulphones, J. Chem. Soc. Perkin Trans. H, 1130 1135. 

Since oxidation is a chemical process, the established carbon-hydrogen bond strengths can be used to estimate ease of hydrogen abstraction. Table 5.3 [75]. These suggest that PMA backbones should be most stable to oxidative attack OCP polymers would be expected to be less stable whilst the stability of styrene-diene polymers could vary depending on the extent of 1,2 vs. 1,4 structures present. The benzylic hydrogens from styrene are potential sites for attack, and it is obviously critical that hydrogenation of the olefinic unsaturation is as complete as possible. 

Polymers vary greatly in their susceptibility to autooxidation. Polystyrenes, because they have benzylic carbon-hydrogen bonds that afford highly stabilized free radicals on hydrogen atom abstraction, are among the most readily oxidized polymers under environmental conditions. As would be expected on these grounds, polymers that contain alkene groups, such as rubber (1), are readily autooxidized. The... 

Carbon-hydrogen bonds decrease in strength in R- H when R goes from primary to secondary to tertiary. Tertiary alkyl radicals are therefore the most stable and methyl radicals are the least stable. As C-H bonds next to conjugating groups such as allyl or benzyl are particularly weak, allyl and benzyl radicals are more stable. 

Initially, this field was pioneered by Li who reported in 2007 on the formation of new carbon-carbon bonds directly starting from diarylmethanes and 1,3-dicarbonyl compounds under mild reaction conditions (80-100 °C, 5-36 h) in the presence of catalytic quantities of an inexpensive iron precursor (iron(ii) chloride, 20 mol%) and stoichiometric quantities of an oxidant (DTBP). The overall reaction can be regarded as an iron(ii) chloride-catalysed oxidative activation of a benzylic carbon-hydrogen bond followed by a cross-coupling reaction to form a carbon-carbon bond (Scheme 13.7, bottom). The mechanistic proposal is based on a single-electron transfer pathway as shown in Scheme 13.7. Initial homolysis of DTBP in the presence of the iron(ii) salt forms both tert-butoxyl radical and an iron(iii) species. Then, proton abstraction from the diarenemethane generates the two key... 

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