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

Article abstract of DOI:10.1002/chem.200401227

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 1 undergoes photoheterolysis of the C-Cl bond in the triplet state to yield the 4-hydroxyphenyl cation ³2 in equilibrium with 4-oxocyclohexa-2,5-dienylidene, ³3. These triplet intermediates scarcely react with a n nucleophile, such as water, nor abstract hydrogen from this solvent, thus they are long-lived (≈1 μs). Specific trapping of both intermediates has been achieved. The cation adds to 2-propenol, k_add～1.3× 10⁸ M^-1 s^-1, to form the long-lived phenonium ion 11 (with λ_max = 290 nm), which then converts to 3-(4-hydroxyphenyl)propane-1,2-diol (10). Carbene ³3 is trapped by oxygen to give benzoquinone and is reduced by D-glucose (k_q = 8.5 × 10^-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 = 9.4×10 ⁸M^-1s^-1 with β-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 α- and β-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 × 10⁷ 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.