Solvents Wohl-Ziegler reaction

Solvent-free benzylic bromination (Wohl-Ziegler reaction) of diquinolines 147 in a ball mill with NBS has been reported by Rahman (Scheme 4.41) [27]. For completion of this reaction, 6h of milling with Af-bromosuccinimide (3.5eqniv.) in Retsch MM200 mill was required. Mechanochemical procedure is advantageous over... 

The Wohl-Ziegler reaction is the reaction of an allylic or benzylic substrate with 7V-bromosuccinimide (NBS) under radical initiating conditions to provide the eor-responding allylie or benzylie bromide. Conditions used to promote the radieal reaction are typieally radical initiators, light and/or heat earbon tetrachloride (CCI4) is typieally utilized as the solvent. 

A further useful application of SC-CO2 as a reaction medium is the free-radical side-chain bromination of alkylaromatics, replacing conventional solvents such as tetra-chloromethane or chlorofluorohydrocarbons having no abstractable hydrogen atoms [920]. For example, bromination of ethylbenzene in SC-CO2 at 40 °C and 22.9 MPa yields 95 cmol/mol (1-bromoethyl)benzene, with practically the same regioselectivity as obtained in conventional tetrachloromethane as the solvent. Even the classical Wohl-Ziegler bromination of benzylic or allylic substrates using A-bromosuccinimide (NBS) can be conducted in SC-CO2 [920]. Irradiation of a solution of toluene, NBS, and AIBN (as initiator) in SC-CO2 at 40 °C and 17.0 MPa for 4 hours gave (bromomethyl)-... 

When this reagent is used, the reaction is known as Wohl-Ziegler bromination. A nonpolar solvent is used, most often CCI4, but the reaction has been done in an ionic liquid. A variation in the reaction used NBS with 5% Yb(OTf>3 and 5%... 

Togo, H., Hirai, T. Environmentally-friendly Wohl-Ziegler bromination Ionic-liquid reaction and solvent-free reaction. Synlett 200Z, 702-704. Greenwood, J. R., Vaccarella, G., Capper, H. R., Mewett, K. N., Allan, R. D., Johnston, G. A. R. Theoretical studies on the free-radical bromination of methylpyridazines in the synthesis of novel heterocyclic analogs of neutro-transmitters. THEOCHEM1996, 368, 235-243. Gainsforth, J. L., Klobukowski, M., Tanner, D. D. Structure and Reactions of the Succinimidyl Radical A Density Functional Study. J. Am. Chem. Soc. 1997,119, 3339-3346. 

The bromination of aromatic compounds using N-bromosuednimide (NBS) occurs in high yield in ionic liquids such as dibutylimidazohum tetrafluoroborate [110]. The yields of the monobrominated product were all in the 80% to 98% range. NBS was also used in the Wohl-Ziegler Q -bromination of aryl methyl groups [111]. The reaction in [BMIM][PF6] typically gave a 5 to 10% improvement in the yield, compared with a solvent-free reaction. 

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