[52215-12-0] · C12H22Br3N3O3 · 2-Pyrrolidone Hydrotribromide · (MW 496.07)
(selective brominating reagent for ketones;1 used in bromination/dehydrobromination and bromination/oxidation sequences when high selectivity is required)
Alternate Name: PHT.
Physical Data: mp 88-90 °C.
Solubility: sol THF, dioxane, DMSO.
Form Supplied in: commercially available as red crystalline solid.
Analysis of Reagent Purity: active bromine content by KI/Na2S2O3 titration.
Handling, Storage, and Precautions: irritant, hygroscopic solid; complex is stable.
The reaction of molecular Bromine with 2-pyrrolidone in CHCl3 yields the crystalline 2-pyrrolidone hydrotribromide. PHT contains 31.6% available bromine and has been found to be a remarkably selective brominating agent.2 The relative reactivities towards bromination are: ketones >> alkenes >> enol acetates.1 Reactions are generally run in THF. As the reaction proceeds, the solution loses the red color of the PHT and (pyrrolidone)2.HBr is precipitated from solution as colorless needles. This not only facilitates the recovery of the pyrrolidone, but offers a convenient method of monitoring the reaction.
Acid sensitive molecules can be protected by adding excess pyrrolidone to the reaction to trap liberated HBr. The mild conditions have allowed many functional groups to survive. These include enones (eq 1),1 nitroxyl radicals (eq 2),3 ergot alkaloids (eq 3),4 benzothiophenes,5 and numerous polyfunctional intermediates in natural products synthesis (eqs 4 and 5).6,7
The solvent used can also play an important role in PHT brominations. Since the bromination is a radical reaction, trace hydroperoxides present in solvents such as THF and dioxane facilitate the bromination. If hydroperoxide-poor solvents are used, radical initiators may be used.4 When DMSO is used, either dehydrobromination8 or oxidation9 can occur, depending on the molecule (eqs 6 and 7).
The use of PHT selectivity followed by subsequent dehydrobromination10-12 or oxidation13 has contributed to the value of this reagent. The selectivity of PHT is considered superior to that of Phenyltrimethylammonium Perbromide (PHAT).1
Dean R. Lagerwall
Occidental Chemical Corp., Grand Island, NY, USA