Pyrrolidone hydrotribromide: a brominating agent with selectivity for ketones

The relative reactivities of a ketone, an olefin, and an enol acetate towards bromination in tetrahydrofuran by "pyrrolidone–bromine complex", (pyrrolidone)3•HBr3 (PHT), are [Formula: see text] acetate. The conversion of benzalacetone to C6H5CH = CHCOCH2Br is given as an example. The selectivity of PHT is enhanced by dilution and is diminished upon addition of pyrrolidone to the reaction mixtures. The selectivity of phenyltrimethylammonium tribromide (PTAT) is not as great as that of PHT and is subject to acid catalysis.At a sufficiently low concentration in a nonpolar solvent, the enol of cyclohexanone reacts at least 1 000 000 times faster with molecular bromine than does cyclohexene. This observation suggests that the selectivity of perbromides may be due to their ability to furnish a low equilibrium concentration of bromine when dissolved in solvents of low polarity.