Reversible Formation of Bromonium Ions. Preferential Reaction of Br(1-) on the Br(1+) of the Bromonium Ion Produced from the Solvolysis of the trans-2-Bromotriflate of Cyclohexane in MeOH or HOAc Containing Added Bromide

Article abstract of DOI:10.1021/jo00060a031

The trans-2-bromotriflate of cyclohexane (4) has been solvolyzed at ambient temperatures in HOAc and MeOH containing varying and in the presence of 0.5M of a scavenger olefin, cyclopentene.The kinetics of solvolysis of 4 have been determined by observing the rate of change in color of an acid/base indicator that instantly responds to the production of HOTf or HBr.The kinetics show that the rate of solvolysis of 4 at constant ionic strength is independent of Br(1-).The products of solvolysis of 4 under the various conditions have been determined by quantitative GLPC analysis.In the presence of Br(1-), the products consisted of the trans 1,2-dibromides and bromosolvates of cyclohexane and cyclopentane.The cyclopentyl products are shown to be formed from the electrophilic addition of Br2/Br3(1-) to cyclopentene while trans-1,2-dibromocyclohexane (2) was obtained from Br(1-) capture of the bromonium ion of cyclohexene on carbon.The Br2 arises from capture by Br(1-) of the bromonium ion on Br(1+).On the basis of the ratio of the cyclopentyl products/2, Br(1-) capture of the solvolytically produced bromonium ion by attack on Br(1+) is 4-5 times more prevalent than attack on carbon in HOAc and ca. 25 times more prevalent than attack on carbon in MeOH.The results have strong implications on the reversal of bromonium ions formed during the electrophilic bromination of olefins in hydroxylic solvents.