Cocatalysis by an internal carboxyl group and by α-cyclodextrin. The debromination of 4-bromo-4-methyl-2,5-cyclohexadienone-2-carboxylic acid

Debromination of the title dienone ( 1 ), formed during the bromination of 5-methylsalicylic acid ( 2 ), is strongly catalyzed by α-cyclodextrin (CD). The bromination of 2 is also catalyzed by CD. The extent of the catalysis of the debromination of 1 is independent of pH in the range pH 3-5 and it tends to saturate at high [Br − ] and high [CD]. Analysis of the [Br − ] dependence data in the presence of 1 mM CD yields a value of 0.68 M for the dissociation constant of the CD*Br − complex. Also, from the dependence of the rate of debromination on [CD], the anion of the dienone 1 (i.e., 1 − ) forms a complex with CD with a K d of 15 mM, which appears to react with Br − at a rate 170 times faster than does free 1 − . However, the catalysis is more likely due to the reaction of free dienone, reacting as 1 , with the CD*Br − complex, which is 3400 times more reactive than uncomplexed Br − . The combined effect of the internal catalysis by the carboxyl group of 1 and the external catalysis by CD is an overall rate enhancement of 12 million. Keywords: catalysis, cocatalysis, cyclodextrin, debromination, intramolecular.