Benzenes regioselective bromination

Fig. 5.13, Regioselective bromination of a benzene derivative that contains a strong acceptor and a weak donor substituent.

Unusual regioselectivity is reported in the bromination of methoxyindoles by pyridini-um bromide perbromide which brominates the C3 position while free bromine attacks the benzene moiety559. 

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)-... 

Explain the different regioselectivity in these two brominations of 1,2-dimethyl benzene. 

A number of methods of bromine production in situ have been reported. A regioselective and stereoselective methoxy-bromination of olefins has been accomplished using LiBr and (diacetoxyiodo)benzene as oxidants [3]. Zinc bromide... 

Electrophilic bromination of methylbenzene (toluene) is considerably faster than the bromi-nation of benzene itself. The reaction is also regioselective It results mainly in para (60%) and ortho (40%) substitutions, with virtually no meta product... 

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