An ef®cient bromination of alkenes using ceriumIV) ammonium nitrate CAN) and potassium bromide Vijay Nair, a,p Sreeletha B. Panicker, a Anu Augustine, a Tesmol G. George, a Siji Thomas a and M. Vairamani b a Organic Chemistry Division, Regional Research Laboratory CSIR), Trivandrum 695 019, India b Indian Institute of Chemical Technology, Hyderabad 500 007, India Dedicated with best wishes to Professor James P. Kutney on the occasion of his 70th birthday. Received 5 December 2000; revised 12 June 2001; accepted 5 July 2001 Abstract ÐBromination of alkenes using a combination of potassium bromide and ceriumiv) ammonium nitrate CAN) in a two phase system consisting of water and dichloromethane affords the corresponding dibromides in excellent yield. The reaction most likely involves the addition of the bromine radical generated from bromide ion by CAN and subsequent formation of the dibromide. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Recently, C±C bond forming reactions mediated by ceriumIV)ammoniumnitrateCAN)havebeenthesubject of considerable interest. 1 Less attention however has been paid to the use of CAN in C-heteroatom bond formation despite the report of CAN mediated addition of azide to alkenes by Trahanovsky as early as 1971 2 and subsequent exploitation of the reaction in the synthesis of aminosugars by Lemieux et al. 3 Very recently we have observed the ef®cient thiocyanation of styrenes and indoles using NH 4 SCN and CAN. 4±5 We have also reported the conver- sion of cinnamates to a-azidocinnamtes, cinnamic acids to b-azido styrenes 6 and styrenes to phenacyl azides and phenacyl thiocyanates. 7 It appeared that a similar strategy would be applicable for the bromination of alkenes without thedirectuseofbromine.Thebrominationofalkenesisan important organic transformation and it is worthy of note that protection and deprotection of double bonds via bromination±debromination strategy is ®nding increasing application in organic synthesis. 8 A number of protocols are available to achieve the bromination of alkenes. 9±13 Very recently, Fraser-Reid has reported the bromination of alkenyl glycosides using a combination of LiBr and CuBr 2 . 14 The bromination of alkenes using hydrobromic acid and tert-butyl hydroperoxide has also been reported. 15 In spite of the variety of reagents available for bromination, lack of selectivity and unwanted side reactions continue to be problems. In this context a search for mild, selective and easy-to-handle reagents assume importance. 2. Results and discussion Against the background presented above, we have carried out a detailed investigation aimed at the bromination of various alkenes and our results are presented in this paper. Our studies were initiated by the treatment of styrene with potassiumbromideandCANinatwo-phasesystemconsist- ing of water and dichloromethane. A facile reaction occurred and the dibromide 2 was isolated in 91% yield Scheme 1). Mechanistically, by comparing the redox potentials of Br 2 /Br and Ce IV)/Ce III) 16,17 it is reasonable to assume thatbromideionisundergoingoxidationtobromineradical byCANandsubsequentadditionofthelattertothedouble bondproducingabenzylicradicalfollowedbytrappingwith another bromine radical vide infra for a detailed dis- cussion). A number of styrenes were subjected to similar reactionconditionsandinallthecases,thedibromideswere Tetrahedron 57 2001) 7417±7422 Pergamon TETRAHEDRON 0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-402001)00712-8 Scheme 1. i) CAN, KBr, H 2 O,CH 2 Cl 2, rt,30min. Keywords: cerium compounds; bromination; dibromide. p Corresponding author. Tel.: 191-471-490-406; fax: 191-471-491-712; e-mail: gvn@csrrltrd.ren.nic.in