The photochemistry of 4-chlorophenol in water revisited: effect of cyclodextrins on cation and carbene reactions

The photochemistry of 4-chlorophenol (1) in water and in the presence of cyclodextrins has been studied by means of steady-state and time-resolved experiments. These have shown that I undergoes photoheterolysis of the C-Cl bond in the triplet state to yield the 4-hydroxyphenyl cation (3)2 in equilibrium with 4-oxocyclohexa-2,5-dienylidene, (3)3. These triplet intermediates scarcely react with a n nucleophile, such as water, nor abstract hydrogen from this solvent, thus they are long-lived (approximate to 1, mu s). Specific trapping of both intermediates has been achieved. The cation adds to 2-propenol, k(add) similar to 1.3x 10(8) m(-1) s(-1), to form the long-lived phenonium ion 11 (with lambda(max) = 290 nm), which then converts to 3-(4-hydroxyphenyl)propane-1,2-diol (10). Carbene 3 3 is trapped by oxygen to give benzoquinone and is reduced by D-glucose (k(q) = 8.5 x 10(6) M-1 s(-1)) to give the phenoxyl radical (8) and phenol (9). Cyclodextrins have been found to trap the intermediates much more efficiently (k(q) = 9.4 x 10(8) m(-1) s(-1) with beta-CD), which indicates that inclusion is involved. Ground state 1 forms inclusion complexes with 1:1 stoichiometry and association constants of 140 and 300 m(-1) with alpha- and beta-CD, respectively. Complexation does not change the efficiency or the mode of photofragmentation of 1; however, it does influence the course of the reaction because the major portion of the intermediates are reduced to phenol within the cavity (k'(red) >= 5 x 10(7) s(-1)) either via a radical 8 or via a radical cation 9(+). Under these conditions, neither 2-propenol nor oxygen trap the intermediates to a significant extent.