Benzyl-transfer reactions

Analogous side-chain oxidations occur in various biosynthetic pathways. The neurotransmitter norepinephrine, for instance, is biosynthesized from dopamine by a benzylic hydroxylation reaction. The process is catalyzed by the copper-containing enzyme dopamine /3-monooxygenase and occurs by a radical mechanism. A copper-oxygen species in the enzyme first abstracts the pro-R benzylic hydrogen to give a radical, and a hydroxyl is then transferred from copper to carbon. 

This benzyl-methyl transfer reaction appears to be general in these systems. This process could involve the dehydrogenation of methanol on the surface of palladium, which produces formaldehyde. The formaldehyde could than add to the nitrogen atom to produce a quaternary carbinolamine (Scheme 4.92). 

Emphasis in this study was placed upon two reactions carried out at 450°C with sample times between 0 and 180 min. The reference run is that of HgPh, neat, and the second run is the hydrogen-transfer reaction of HgPh withdibenzyl, in which the benzyl radical is formed at conditions typical of coal liquefaction. 

It appears that the formation of benzyl Tetralin only occurs during the hydrogen transfer reactions at low... 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

To what extent is the partitioning of simple aliphatic and benzylic a-CH-substituted carbocations in nucleophilic solvents controlled by the relative thermodynamic driving force for proton transfer and nucleophile addition reactions It is known that the partitioning of simple aliphatic carbocations favors the formation of nucleophile adducts (ksjkp > 1, Scheme 2) and there is good evidence that this reflects, at least in part, the larger thermodynamic driving force for the nucleophilic addition compared with the proton transfer reaction of solvent (A dd U Scheme 6).12 21,22,24... 

To what extent are the variations in the rate constant ratio /cs//cpobserved for changing structure of aliphatic and benzylic carbocations the result of changes in the Marcus intrinsic barriers Ap and As for the deprotonation and solvent addition reactions It is not generally known whether there are significant differences in the intrinsic barriers for the nucleophile addition and proton transfer reactions of carbocations. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The deuterium KIE values are generally in the range expected for linear three-center hydrogen transfer reactions,44107 and they track nicely with the rate constants for the reactions with the faster, more exothermic reactions displaying smaller KIEs. The large KIE value for reaction of the benzyl radical is noteworthy in that it exceeds the theoretical maximum for the classical model in a manner apparently similar to that seen with tin hydride (see below). 

Pines and Kolobielski (18) have shown that phenylcyclohexene, although it is not a cyclic diolefin, will also undergo reactions similar to those that cyclic diolefins undergo when treated with base catalysts. When heated to 200-220 with a sodium-benzyl-sodium catalyst, it underwent a hydrogen transfer reaction resulting in the formation of biphenyl and of phenyl-cyclohexane molecular hydrogen was not produced. The mechanism of this reaction may be pictured as an elimination of sodium hydride from one molecule with the hydride ion being accepted by another molecule (A"-E"). 

A hydrogen-transfer reaction involving the olefin and the addition product of the benzylic carbanion and olefin may accompany the side-chain alkylation reaction. The result is that alkanes and arylalkenes are produced 19). This hydride-tiansfer reaction may take place by elimination of a hydride ion from the carbanion adduct followed by addition of the hydride ion to the olefin [Reaction (6)]. The amount of this side reaction probably depends largely on the severity of reaction conditions used. 

The oxidative behaviour of the acridinium carbocations 61 was also explored by the group of Lacour in the photoinduced electron transfer reaction [160]. In the amount of 2 mol%, the achiral hindered acridinium salt 61 catalyzed the aerobic photooxidation of the primary benzylic amine to benzylimine in the yield of 74% (Scheme 63). 

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

It is now generally admitted that this reaction involves both one-electron and two-electron transfer reactions. Carbonyl compounds are directly produced from the two-electron oxidation of alcohols by both Crvl- and Crv-oxo species, respectively transformed into CrIV and Crm species. Chromium(IV) species generate radicals by one-electron oxidation of alcohols and are responsible for the formation of cleavage by-products, e.g. benzyl alcohol and benzaldehyde from the oxidation of 1,2-diphenyl ethanol.294,295 The key step for carbonyl compound formation is the decomposition of the chromate ester resulting from the reaction of the alcohol with the Crvl-oxo reagent (equation 97).296... 

The process of chain transfer has received very little quantitative study insofar as the anionic systems are concerned. The first study of an anionic chain transfer process was that of Robertson and Marion 274) on the polymerization of 1,3-butadiene by sodium in toluene. The reaction of toluene with the sodium active center led to the formation of benzyl sodium. This work was the first to demonstrate the important role of solvent in transfer reactions involving anionic active centers ... 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

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