H. Slebockatilk et al., ELECTROPHILIC BROMINATION OF 7-NORBORNYLIDENE-7'-NORBORNANE - THE OBSERVATION OF AN UNUSUALLY LARGE INVERSE DEUTERIUM KINETIC ISOTOPE EFFECT, Journal of the American Chemical Society, 117(34), 1995, pp. 8769-8776
The bromination of 7-norbornylidene-7'-norbornane (1) and its perdeute
rated analogue (2) was studied in HOAc and MeOH by investigating the r
eaction kinetics and product distributions as a function of added [Br-
]. In all cases there is a significant inverse deuterium kinetic isoto
pe 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 interventi
on of a reversibly formed bromonium ion intermediate. Product studies
indicate the formation of four products, a normal dibromide and beta-b
romo solvate and two that arise from capture of an alpha-bromo cation
formed from a Wagner-Meerwein rearrangement of the first-formed bromon
ium ion. In HOAc, added [Br-] leads to an increase in the amount of th
e normal dibromide at the expense of the solvent-incorporated and rear
ranged products. In MeOH, dibromide is never an important product at a
ny [Br-] investigated, the two major isolated products being the norma
l 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 at
tributable to the presence of H or D. A unified mechanism that accommo
dates all the data is presented.