Ethylbenzene Ziegler bromination

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). 

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)-... 

Competition experiments (ethylbenzene versus toluene) confirm the role of Br as chain carrier in the Ziegler bromination in SCCO2 [57]. The role of NBS in this reaction is to maintain a low, steady-state concentration of Br2 (Scheme 4.4-9). 

A competition experiment pitting ethylbenzene vs toluene in SC-CO2 (40 C, 342 bar) yielded a relative reactivity, r(2 /lO) = 29 2. This result is identical to that observed with direct bromination with molecular bromine (Table I) and suggests that Br is the chain carrier for the Ziegler bromination in SC-CO2. Consequently, there does not appear to be a change of mechanism with the conversion from CCI4 to SC-CO2 as solvent for the Ziegler bromination. 

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