Bromoiodinanes--conveniently and directly prepared from iodobenzenecarbinols and N-bromosuccinimide, and characterised for the first time crystallographically--act as electrophilic bromine donors.

A mild and versatile procedure for the bromination of olefins and activated arenes by in situ generation of 'Br + ' using (diacetoxyiodo)benzene and lithium bromide is presented. The reactions were carried out in open vessels at room temperature and were typically complete in 30 minutes. The brominated products were isolated by column chromatography, which also allowed for the isolation of the iodobenzene by-product for recycle.Aryl bromides and alkyl bromides are fundamental to organic chemistry, finding use as key intermediates for subsequent reactions such as metal-catalysed carbon-carbon bond formation (aryl bromides) 1 and nucleophilic substitution reactions (alkyl bromides). 2 Typically aryl bromides are generated by electrophilic substitution of an aromatic with molecular bromine. 3 Alkyl halides, are often generated from the alcohol by activation of the hydroxyl group and subsequent displacement with bromide anion (e.g., the well-known CBr 4 -PPh 3 system), but for the generation of 1,2-dibromides, the electrophilic addition of molecular bromine to an alkene via a bromonium ion 4 remains the method of choice. However, despite the widespread use of molecular bromine as an electrophilic reagent it is a toxic, difficult to handle, low-boiling lachrymatory liquid, causing severe burns on contact with skin. 5 Moreover, since molecular bromine is a strong oxidizing agent, attempted bromination of complex organic substrates can be hampered by undesired competing oxidation processes. 6 In the electrophilic substitution of arenes with Br 2 , the stoichiometric by-product (HBr) is toxic and leads to strongly acidic aqueous waste streams. These concerns over selectivity, handling and toxicity issues associated both with Br 2 and HBr have fuelled research into new strategies for the electrophilic bromination of organic substrates. 7 Hypervalent iodine compounds have recently gained increasing popularity for a number of synthetically valuable transformations, exemplified by oxidation chemistry with the Dess-Martin periodinane, 8 and the so-called IBX reagent. 9 We were drawn to investigate whether an iodine(III) compound -specifically, (diacetoxyiodo) benzene, PhI(OAc) 2 1 (DIB) -could be used as a synthetically useful co-reagent with anionic bromide for electrophilic bromination of arenes and alkenes. From a practical perspective, DIB is a readily available compound, 10 which also benefits from being an air-stable, crystalline solid, with good reactivity at ambient temperature. Mechanistically, it was envisaged that electrophilic 'Br + ' could be generated in situ by the reaction of the bromide anion with DIB and displacement of acetate to give putative intermediate 2 (Scheme 1). The resulting Br-I(III) bond of 2 should render the bromine atom electrophilic. Scheme 1The use of DIB has previously been reported to facilitate electrophilic bromination of selected substrates in combination with trimethylsilyl bromide or quaternary ammonium bromide salts. 11-15 Rho has reported the use of DIB and TMSBr 11a or Bu 4 NBr 11...

Suitably ortho-substituted iodobenzenes act as organocatalysts for the transfer of electrophilic bromine from N-bromosuccinimide to alkenes via the intermediacy of bromoiodinanes.

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