Isoselenazolones as Catalysts for the Activation of Bromine: Bromolactonization of Alkenoic Acids and Oxidation of Alcohols

Isoselenazolones were synthesized by a copper-catalyzed
Se–N
bond forming reaction between 2-halobenzamides and selenium powder.
The catalytic activity of the various isoselenazolones was studied
in the bromolactonization of pent-4-enoic acid. Isoselenazolone 9 was studied as a catalyst in several reactions: the bromolactonization
of a series of alkenoic acids with bromine or N-bromosuccinimide
(NBS) in the presence of potassium carbonate as base, the bromoesterification
of a series of alkenes using NBS and a variety of carboxylic acids,
and the oxidation of secondary alcohols to ketones using bromine as
an oxidizing reagent. Mechanistic details of the isoselenazolone-catalyzed
bromination reaction were revealed by 77Se NMR spectroscopic
and ES-MS studies. The oxidative addition of bromine to the isoselenazolone
gives the isoselenazolone­(IV) dibromide, which could be responsible
for the activation of bromine under the reaction conditions. Steric
effects from an N-phenylethyl group on the amide
of the isoselenazolone and electron-withdrawing fluoro substituents
on the benzo fused-ring of the isoselenazolone appear to enhance the
stability of the isoselenazolone as a catalyst for the bromination
reaction.