Benzene derivatives, halogen bromo

Bromo- and -iodoindoles, and the similarly reactive 2- and 3-triflates, " undergo palladium-catalysed couplings as normal aryl halides. Since 2- and 3-haloindoles are unstable it is expedient to employ their A -acyl derivatives. Halogen and triflate on the benzene ring of indoles take part unexceptionally in coupling reactions. 

The benzene ring of fused dihydropyrans is more easily attacked by electrophiles than that of the corresponding pyran. However, the activating effect of the carbonyl group on the 3-position of chromanones and flavanones ensures that halogenation normally occurs at that position preferentially. Thus, 3-mono- and 3,3-di-bromination is readily achieved with appropriate molar proportions of bromine but bromination of 2,3-dihydronaphtho[2,l-Z>]pyran-l-one (675) in acetic acid gave the 7-bromo derivative (676) (48JA599). 

Schlosser and co-workers have reported the shift of lithium in lithiated l-bromo-3-(tri-fluoromethyl)benzene 2,fa Quenching at — 100 C gives exclusively the product derived from 2, whereas after 2 hours at — 75 C, arene 2 is completely converted into less basic 3. A lithium-iodine exchange takes place in lithiated 2-chloro-3-iodo-6-(trifluoromethyl)pyridine 4, which at —85 C is totally converted into the less basic isomer 5.7 These rearrangements have been discussed in terms of a base-catalyzed halogen dance or halogen-shuffling mechanism. 

Both halogens in di-bromo- or di-iodobenzene derivatives can be replaced by Li in halogen-Li exchanges. If the two halogens are ortho, aryne formation can occur from the monoexchange product, l-Li-2-X-benzene, although such compounds can be trapped at low T, e.g. ... 

Common hypervalent iodine(III) reagents, such as PhI(OAc)2 and PhI(OH)OTs, in the presence of bromide anions can be used for oxidative bromination of organic substrates. The oxidative halogenation of 1,4-dimethoxynaphthalene derivatives with (diacetoxyiodo)benzene and trimethylsilyl bromide or chio-ride affords the corresponding halogenated or haloacetylated products [91, 92]. For example, the treatment of 1,4-dimethoxynaphthalene (87) with PhI(OAc)2 and trimethylsilyl bromide affords 3-bromo-l,4-dimethoxynaphthalene (88) in quantitative yield (Scheme 3.33) [92]. 

So far we have steered clear of the reactions of halogenated derivatives of benzene. Before we explain their reactions, have a look at the table, which shows the rates of nitration of fluoro, chloro, bromo, and iodobenzene relative to benzene itself, and also gives an indication of the products formed in each case. ----... 

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