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Cyclodextrin (CD) has little or no effect on the rates of enolization of transient 4-bromo-2,5-cyclohexadienones (2), formed during the aqueous bromination of alkylphenols. In contrast, saturation kinetics and large catalytic effects are observed for the debromination of the title dienones (4), formed by ipso bromine attack on 4-alkylphenols (alkyl = Me, Et, z'-Pr, n-Pr, Z-Bu, 3,4-diMe). With the exception of the -propyl case, the extent of the catalysis (kz/ku = 23-78) and the dissociation constants of the CD-dienone complexes (ATd = 2.32-4.83 mM) show surprisingly little variation for the different alkyl groups. The simplest interpretation of the results is that the CD-catalyzed debromination reaction involves attack by free bromide ion on the CD-dienone complex. However, the kinetically equivalent pathway, reaction between the free dienone 4 and the CD complex of bromide ion, is much more consistent with the low sensitivity of the catalysis to the length and size of the different alkyl grpups. For this mechanism the rate enhancements are much larger (2400-4600) and almost constant. They imply that Br" in its CD complex is a stronger nucleophile than bromide ion that is completely solvated by water. The preferred mechanism is the microscopic reverse of that postulated for the CD-catalyzed bromination of phenols. The common transition state for the ipso bromination of 5 (R = Me) and the debromination of 4 (R = Me) is strongly bound by CD (Afd ~4.5 X 10"5 M).

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.

The title compounds ("ipso-dienones") 5 have been observed during the reaction of bromine with six p-alkylphenols 4 (R = Me, Et, n-Pr, -Pr, t-Bu, 3,4-Me2) in aqueous solutions of pH 0-3. Their formation by ipso bromine attack on 4 accounts for about 10% of the initial consumption of bromine. The decomposition of 5, which is catalyzed by H+ and by Br~, is attributed to debromination. The rates of this reaction and of the attack of bromine on 4 are not very sensitive to the nature of the alkyl substituents. Studies of the behavior of 5 (R = Me) in buffers give curved buffer plots which provide additional support for the debromination mechanism and also demonstrate general acid catalysis. Decomposition of 5 (R = Me) in the presence of a trap for liberated bromine give straight buffer plots from which a Bronsted a 0.27 is deduced. The ipso-dienone 8, derived from 5-methylsalicyclic acid, shows intramolecular catalysis by the carboxyl group (EM = 58 M) and no catalysis by buffer acids.