Benzyl bromide radical

To conclude this section, radical bromination is a very useful reaction for preparing tertiary, allylic, and benzylic bromides. Radical chlorination is not very useful because all hydrogen atoms in a molecule react to give a mixture of products. Only in cases where only one type of reactive hydrogen atom is available does radical chlorination become a useful process. 

This ether is formed from the alcohol and the benzyl bromide (NaH, DMF, 94% yield). It was used primarily because of its stability toward NBS during the radical-promoted conversion of a 4,6-benzylidenepyranoside to the 6-bromo-4-benzoate. Cleavage occurs quantitatively upon hydrogenolyis (Pd/C, H2). ... 

Benzyl radical, C6H5CH2, and its deuterated analogues (C6H5CD2, CfiDsCHz) were obtained by vacuum thermolysis of dibenzyl derivatives or by pyrolysis of the corresponding benzyl bromides (17) (Baskir, 1989). 

Benzyl radicals have a reduction potential only slightly more negative than the reduction potential of the corresponding benzyl bromide. At a mercury cathode in... 

The synthesis of anastrozole (Scheme 3.3) began with an 8 2 displacement of commercially available 3,5-fc (bromomethyl)toluene (19) using potassium nitrile and a phase-transfer catalyst, tetrabutylammonium bromide (Edwards and Large, 1990). The resulting fcw-nitrile 20 in DMF was then deprotonated with sodium hydride in the presence of excess methyl iodide to give the fc -dimethylated product 21. Subsequently, a Wohl-Ziegler reaction on 21 was carried out using A-bromosuccinamide (NBS), and a catalytic amount of benzoyl peroxide (BPO) as the radical initiator. Finally, an Sn2 displacement of benzyl bromide 22 with sodium triazole in DMF afforded anastrozole (2) as a white solid. 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

With Fe, the products are o-, p-, and some m-BrCftH4CH,. In light the product is benzyl bromide, PhCHjBr. Like allylic halogenation (Section 6.5), the latter reaction is a free-radical substitution ... 

Ellis K. Fields We have found o-methylbenzyl bromide among the oxidation products of o-xylene. Even though this bromide persists during the oxidation period and appears to solvolyze somewhat more slowly than other benzyl bromides, it releases its bromine eventually to initiate and propagate free radical chains. 

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

Free-radical bromination of toluene is selective for the benzylic position. Benzyl bromide cannot undergo elimination, and so nucleophilic substitution of bromide by hydroxide will work well. 

Benzyl cyanide is the product of nucleophilic substitution by cyanide ion on benzyl bromide or benzyl chloride. The benzyl halides are prepared by free-radical halogenation of the toluene side chain. 

Nitrobenzyl halides are reduced in a 1 e-process to radical anions, which rapidly lose halide ion to form the neutral nitrobenzyl radical. The rates for this dissociation were calculated from cyclic voltammetry data to be k = 2,5 for m-nitrobenzyl chloride and k = 2 109 secfor o-nitrobenzyl bromide. The nitrobenzyl radicals predominantly dimerize (90%), whereas a small amount yields nitrotoluenes (< 10%) by hydrogen abstraction 4541. From a series of substituted benzyl bromides those with the more positive reduction potential form bibenzyl in 25—74% yield, whereas from the less easily reducible ones dibenzyl-mercury derivatives are obtained (50—60%) 485). Reduction of benzyl chloride at the plateau of the first wave yields dibenzylmercury 4 By reduction of diphenyliodonium hydroxide at -1,6 V 51% diphenylmercury is obtained 488 ... 

The formation of oxygen- and carbon-centered radicals by the thermolysis of peroxides or azo compounds is well known. Today, these compounds have been also used as radical initiators. For example, treatment of a CC14 solution of toluene and Af-bromosuccinimide (NBS) in the presence of a catalytic amount of benzoyl peroxide in refluxing conditions gives benzyl bromide in good yield as shown in Scheme 1.5. This is called the Wohl-Ziegler reaction. 

In general, the excited radicals shown have been found to possess enhanced donor/acceptor properties, especially in interactions with such quenchers as oxygen, dienes, amines, and halides. To date, the most well-characterized intermolecular processes are charge (electron) transfer reactions between excited diphenyl-methyl radicals and electron donors and acceptors. Thus, excited 132 reacts with methyl benzoates and benzyl bromides (Schemes 23 and 24) with rates that increase with the increasing electron withdrawing ability of substituents on the... 

Recently, a nonchain radical mechanism has been proposed402 for the oxidative addition of methyl iodide, ethyl iodide, and benzyl bromide to tris(tri-pheny lpho sphine)platinum(O) ... 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

The nucleophilic nature of these radicals allows addition to carbon-nitrogen multiple bonds, and in selected cases this has been demonstrated. Suitable substrates for such additions include protonated pyridines, thiazoles and imidazoles (equation 183), and nitroalkyl anions359. Yamamoto and coworkers have also described cobalt-mediated multiple additions of carbon tetrahalides, benzyl bromides or a-bromo ketones to isocyanides, which are postulated to occur through the corresponding alkylcobalt(III) complexes360. 

Zhou, D.-L., Carrero, H. and Rusling, J.F. (1996) Radical vs anionic pathway in mediated electrochemical reduction of benzyl bromide in a bicontinuous microemulsion. Langmuir 12, 3067-3074. 

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