Bromodimethylsulfonium bromide (BDMS): A versatile reagent in organic synthesis

(A) Preparation of alkyl bromides from corresponding alcohols: Furukawa et. al first demonstrated the application of this reagent for the conversion of alcohols to bromides in high yield. The reaction proceeds via an inversion, i.e. an optically active alcohol gives the corresponding bromide with inversion of configuration. (B) Conversion of epoxides and enamines to a-haloketones: Epoxides upon treatment with halodimethylsulfonium halides (chloride/bromide) in the presence of triethyl amine give α-halo ketones in high yields. The reaction proceeds well with alkene oxides and cycloalkene oxides of small ring sizes. Epoxides with medium and large ring sizes undergo transannular rearrangements, giving mixtures of products. Similarly, enamines react with BDMS to give α-bromoketones. (C) Electrophilic addition to olefins: Chow et. al showed that bromdimethylsulfonium bromide reacts with various alkenes and provides corresponding addition product sulfonium bromides in good yields. The resultant sulfonium bromide on treatment with aqueous potassium carbonate gives dehydrobrominated products. (D) Efficient synthesis of a-bromo enonesfrom enones: Bromodimethylsulfonium bromide reacts with conjugated enones at ≤0° and give α-bromo-β-sulfonium carbonyl compounds, which on subsequent treatment with aqueous K2CO3 give α-bromo enones in excellent yields. (E) Regioselective para-halo genation of activated arenes: Bromodimethylsulfonium bromide as well as its chloro analogue is an efficient regioselective halogenating agent for activated aromatics such as phenols, anisole, diphenyl ether and N-alkyl anilines.15 The observed high para selectivity is a consequence of the transfer of halogens going through a 'late' arenium ion like transition state and of the bulky nature of the halogenating agents. For para-substituted aromatics it gives no halogenations. (F) Deprotection of thioacetals and protection of carbonyl compounds as acetals, oxathioacetals and thioacetals: Bromodimethylsulfonium bromide (a soft electrophilic brominating agent) in stoichiometric amount combines with the soft sulfur atoms of thioacetals and gives the bissulfonium ion (A), which subsequently can be hydrolyzed to the corresponding ketones. 1 On the other hand, a catalytic amount of BDMS is effective for thioacetalization, acetalization, transthioacetalization,9 as well as oxathioacetalization6 of carbonyl compounds. (G) Electrophilic aromatic bromination by in situ generated BDMS: Majetich et al. showed that BDMS, generated in situ by treating DMSO with aqueous HBr, is a milder and more selective reagent for electrophilic aromatic bromination than elemental bromine.2 Indole alkaloids can also be brominated using the same reagent.16 © Georg Thieme Verlag Stuttgart.