ELECTROPHILIC ADDITION OF BR2 TO OLEFINS IN THE PRESENCE OF NUCLEOPHILIC TRAPPING ANIONS - IMPLICATIONS FOR THE LIFETIMES OF BROMONIUM ION INTERMEDIATES PRODUCED FROM ELECTROPHILIC BROMINATION OF OLEFINS IN METHANOL

The product ratios for Br2 or NBS additions to cyclopentene, cyclohexene, tetramethylethylene, and styrene in MeOH containing varying concentrations of added N3- or Br- have been determined with an aim of determining the lifetimes of the bromonium ion intermediates. On the basis of the ratio of trans bromo azide to methoxy bromide products, the partitioning rate constant ratios (k(N3)/k(CH3OH)) for the four olefins are 5.9, 4.9, 9.3, and 2.7 M-1, respectively. That the far better nucleophile (N3-) does not lead to a marked increase in product formation relative to solvent suggests that both species capture a highly reactive intermediate in a non-activation-limited process. Assuming that the N3- reacts with the intermediate with a diffusion-limited rate constant of 10(10) M-1 s-1, the respective lifetimes of the ions produced from bromination of the four olefins are 5.9 x 10(-10), 5.0 x 10(-10), 9.3 x 10(-10), and 2.7 x 10(-10) s, respectively. On the basis of existing comparisons, these values indicate the following: the cyclic olefins produce ions that live about 100 times longer than a secondary carbocation; tetramethylethylene gives a bromonium ion that lives approximately 10 times longer than a tertiary cation; and styrene gives an ion (bromonium or beta-bromo cation) that is approximately 40-fold longer lived than the 1-phenylethyl cation. In the case of Br2 or NBS addition to cyclohexene in the presence of varying [Br-], the ratio of the trans dibromide to methoxy bromide product tends to zero as [Br-] --> 0. This indicates that the trans dibromide cannot be formed by ion pair collapse. The solvolysis of trans-2-bromocyclohexyl trifluoromethanesulfonate in MeOH containing N3 or Br- produces significantly less azide or bromide capture product than does electrophilic addition of Br2 or NBS to cyclohexene under the same conditions, suggesting that the ions produced in the two cases are not identical.