The effect of α-cyclodextrin upon the kinetics of aqueous bromination of anisole

Abstract

Th~s paper is dedicated to Professor Gerald E. Durzn on the occasiorz of his 65th birthday OSWALD S. TEE and JANICE M BENNETT. Can. J . Chem. 62, 1585 (1984). I a-Cyclodextrin (cyclohexaamylose) (CD) reduces the rate of bromination of anisole and p-methylanisole in aqueous KBr solutions (0.05-0.1 M). This retardation is largely due to the formation of a CD -tribromide ion complex and, to a lesser extent, to the encapsulation of bromine and of the anisole substrate. Forp-methylanisole no other effects seem to be important. However, for anisole a pathway involving free (or complexed) anisole and complexed (or free) bromine seems to be operative. The intermediacy of a ternary complex (CD-anisole-Br,) cannot be ruled out. Both equilibrium and kinetic measurements indicate that the dissociation constant of the CD-Br3-complex is -2 X lo-" M.

Organic Reactions Mediated by Cyclodextrins

“…Furthermore, since the formation of complexes with the CD lowers the concentrations of the free reactants in solution, one might reasonably expect that bromination in aqueous solution would be strongly retarded or even inhibited by CDs. The bromination of anisol 57 and phenols 58 were fairly retarded in the presence of R-CD. Why?…”

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives

“…The bromination of anisole and some phenols shows at most a slight acceleration by R-CD and is often retarded because tribromide ion binds into the cyclodextrin cavity. 145,146 However, R-CD catalyzes the debromi- 6) 112 or as a dimer (7). 113 Figure 5.…”

“…Also, there are many examples in which the point of reaction of an external reagent is influenced when a substrate binds into a cyclodextrin cavity − ,− but so far with no evidence that the cyclodextrin catalyzes the process as it does in the HOCl chlorinations. The bromination of anisole and some phenols shows at most a slight acceleration by α-CD and is often retarded because tribromide ion binds into the cyclodextrin cavity. , However, α-CD catalyzes the debromination of some bromocyclohexadienones (the proposed mechanism is shown in Figure ), the reverse of one of the steps in the bromination of aromatic rings. , α-CD also modestly catalyzes the reaction of formic acid with bromine 7 Cyclodextrin binding promotes the alkylation of substrate 8 140 and the debromination of a bromocyclohexadienone …”

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

“…The apparent second-order rate constants (k;PP) increase at low [CD] but then begin to decrease at high [CD], similar to the behaviour observed previously with simple phenols (6). The increase in k; PP at low [CD] indicates that there is a significant contribution from a CD-catalyzed pathway for bromination while the decrease at high [CD] is due to the combined effects of the complexation of the substrate, Br2, Br3-, and Br-by CD (5,6). If the bromination of 2 were not catalyzed by CD, the rate constants k; PP would decrease substantially with increasing [CD] (see Fig.…”