ELECTROPHILIC BROMINATION OF 7-NORBORNYLIDENE-7'-NORBORNANE - THE OBSERVATION OF AN UNUSUALLY LARGE INVERSE DEUTERIUM KINETIC ISOTOPE EFFECT

The bromination of 7-norbornylidene-7'-norbornane (1) and its perdeuterated analogue (2) was studied in HOAc and MeOH by investigating the reaction kinetics and product distributions as a function of added [Br-]. In all cases there is a significant inverse deuterium kinetic isotope effect on the bromination rate constant (k(g)) which, in HOAc, is k(g)(2/1) 1.56 and 1.83 at [Br-] = 0 and 0.04 M, respectively. Added Br- causes a significant rate retardation which indicates the intervention of a reversibly formed bromonium ion intermediate. Product studies indicate the formation of four products, a normal dibromide and beta-bromo solvate and two that arise from capture of an alpha-bromo cation formed from a Wagner-Meerwein rearrangement of the first-formed bromonium ion. In HOAc, added [Br-] leads to an increase in the amount of the normal dibromide at the expense of the solvent-incorporated and rearranged products. In MeOH, dibromide is never an important product at any [Br-] investigated, the two major isolated products being the normal methoxy bromide and rearranged ketone. Quantitative analyses of the products formed from bromination of 1 and 2 in HOAc as a function of [Br-] indicate little difference in the product partitioning that is attributable to the presence of H or D. A unified mechanism that accommodates all the data is presented.