Eiectrophilic additions of bromonium nitrate to unsaturated substrates

Abstract

Bromonium nitrate (generated in situ by the reaction of bromine with silver nitrate in chloroform–pyridine) reacts trans-stereospecifically and regiospecifically in eiectrophilic additions to a series of Z–E pairs of alkenes to give bromoalkyl nitrate esters and bromoalkyl pyridinium nitrates. Addition to the less hindered (Z)-[β-2H]-styrene is also stereospecific, eliminating the possibility of restricted rotation during addition. Reaction with conjugated dienes gives rise to bipyridinium salts from 1,4-conjugate addition and displacement. The accompanying alkyl bromonitrate esters rearrange readily to the more stable 1,4-bromonitrates in the liquid phase. In contrast to the behavior of iodonium nitrate, bromonium nitrate reacts with 3,3,3-triphenylpropene to give an allylic pyridinium nitrate, which results from phenyl migration, the structure of which was proven by synthesis. Bromonium nitrate reacts with pent-4-en-1-ol to give 2-bromomethyltetrahydrofuran. Similar reaction with cyclohex-2-en-1-ol provides products corresponding to the addition of the bromonium ion both cis and trans to the hydroxyl group, unlike iodonium nitrate where specific interaction between the hydroxyl and the pyridine group leads exclusively to cis addition.