Imidazoles 2-bromination, mechanism

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... 

A modification of GSR has been reported by Classon and co-workers.190 The idea remains the same create a covalently bound phosphorus cation and displace with a nucleophile—in this case, a halogen. Both bromine and iodine have been used.190 Three different systems were evaluated (1) chlorodiphenylphophine, iodine-bromine, and imidazole (2) p-(dimethylamino)phenyldiphenylphosphine, iodine-bromine, and imidazole or (3) polymer-bound triphenylphosphine, iodine-bromine, and imidazole. The last two were found to be very similar to just triphenylphosphine itself, and displayed reactivity inferior to the first system. The polymer-bound reagent does allow for easier removal of triphenylphosphine oxide produced in the course of the reaction. As with the original procedure, and consistent with a Sn2 mechanism, inversion of configuration occurred. Again, as with the original method vicinal diols were readily converted into alkenes.191 This... 

In both imidazoles and their /V-substituted derivatives, halogenation occurs preferentially in the 4(5)-positions there is a slight preference for 5-substitution in 1-substituted substrates. Although the 2-position is much less reactive, it is difficult to prevent substitution at that site. Indeed, polyhalogenation is so facile that it is seldom feasible to make monohalogenated imidazoles directly. Both sodium hypochlorite and NCS convert imidazole into its 4,5-dichloro derivative contaminated by the 2,4,5-trichloro product. Even very mild conditions are unlikely to promote monochlorination, and bromination and iodination arc similar. Mechanisms can vary, however, from substrate to substrate. It is likely that C-2 halogenations are the result of addition-elimination [1]. 

Cyclization of l-aroyl-3-arylthioureas 1355 with a variety of carbonyl compounds bearing an a-H, in the presence of bromine and triethylamine, provides 1-aroyl-3-aryW-substituted imidazole-2-thiones 1356 <20030L1657>. The underlying mechanism of this condensation is illustrated in Scheme 348. 

Interest in cyanogen bromide as a brominating agent for imidazoles has been rekindled. It seems that bromo products can be made in this way, for example, 1-benzylimidazole reacts with cyanogen bromide in the presence of 4-dimethylaminopyridine to give l-benzyl-2-bromoimidazole, perhaps via an ylide mechanism. Cyanogen chloride and the pre-formed adduct of cyanogen bromide and... 

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