The chemistry of organohalogenic molecules. Part 172. Kinetic studies of bromine transfer from N-bromosuccinimide to norbornene under protic conditions

Abstract

Kinetic studies of bromofunctionalisation of cycloalkenes with N-bromosuccinimide (NBS) in acetonitrile-water under various conditions were carried out. Reaction with bicyclo[2,2,1]heptene (norbornene) in acetonitrile-water solution gave three main products: 2-exo-hydroxy-7-syn-bromo norbornane, 2-exo- hydroxy-7-anti-bromo norbornane and bromonortricyclane in 5:1:1 ratio. The course of the reaction obeys a simple rate equation: v = d[NBS] / dt = k2 [NBS] [alkene] with k2 = 0.35 M -1s-1 at 10°C. In order to obtain further insight into the nature of the transition state during the bromine transfer from NBS, the effect of cycloalkene ring size was investigated and the following relative values were determined: 9.7 for the norbornene/cyclohexene pair, while a lower structural effect was found for the cyclopentene/cyclohexene pair, i.e. 1.2. For the bromination of norbornene the following activation parameters were determined: ΔH‡ = 6.1 kcal mol-1, ΔS‡ = -38 cal mol-1K-1. The solvent polarity variation (Grunwald - Winstein Y) was used to obtain information about the nature of the rate determining step in bromine transfer from NBS to norbornene and m = 0.8 was determined, indicating a large change in the polarity of the transition state as compared to the reactants. The observed kinetic data suggests that the rate determining transition state in bromofunctionalisation of cyclic alkenes has a three-centered cyclic structure, the nucleophile also playing an important role.