Brominations with Pr4NBr9 as a Solid Reagent with High Reactivity and Selectivity­

Abstract

Tetrapropylammonium nonabromide (Pr 4 NBr 9 ) is introduced as a room-temperature solid reagent for rapid bromination reactions of various substrates. The reagent exhibits reactivity similar to that of elemental bromine, but shows higher selectivity and it is easier and safer to store and to handle.Brominated organics are valuable precursors for transition-metal-catalyzed cross-couplings, 1 and numerous other transformations for the synthesis of specialty chemicals and pharmaceutical products. 2 Therefore, reagents for the selective bromination of organic compounds are highly demanded tools for synthetic chemistry. Elemental bromine has been used in the past as a readily available electrophilic bromination reagent, but it is an undesirable chemical to work with because of its disadvantages: high vapor pressure, toxicity, and corrosiveness. Hence, other bromination reagents that are more user-friendly have been developed and used frequently. These include commercially available tribromide salts such as tetrabutylammonium tribromide (1), 3 or pyridinium tribromide (2), 4 and several variations as ionic liquids such as 3-7 [ Figure 1 (a)]. 5 Commercially available N-bromosuccinimide (NBS, 8) 6 and 1,3-dibromo-5-5-dimethylhydantoin (9), 7 hexabromocyclopentadiene (10), 8 and 4,4-dibromo-2,6-di-tert-butylcyclohexa-2,5-dienone (11) 9 are non-Br 2 -containing Br + sources, for example, that have been found useful for brominations of aromatic compounds and α-carbons of activated carbonyls [ Figure 1 (b)]. However, since these reagents transfer only one bromine equivalent to the target molecule, a large amount of waste is generated. Although some of these compounds showed a significant increase in chemo-and regioselectivity, elemental bromine remained the most efficient brominating agent. In addition, the reactivity was often much lower than that of Br 2 , which in return required longer reaction times or heating of the reaction mixture. A second approach has been the use of bromide salts or hydrogen bromide in combination with strong oxidants to generate the electrophilic bromination reagent in situ. However, this often led to undesired side reactions and in many cases produced a similar amount of waste. 10 We herein introduce tetrapropylammonium nonabromide (Pr 4 NBr 9 , 12), which we discussed in an earlier communication, 11 as a bromination reagent that overcomes most of the aforementioned limitations ( Figure 2). Quaternary ammonium nonabromides and other perbromides have been known in the literature for a long time, 12 and they were, in part, even applied in bromination reactions. 13 However, only recently has further structural evidence of higher polybromides been established, 11,14 and this report represents the first application and study of a solid nonabromide as a bromination agent for organic synthesis. The nonabromide 12 is a dark-red solid that can be handled under air, weighed on a balance, and only slowly releases bromine, which greatly reduces hazards to the user compared to Br 2 . It can be conveniently ...

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