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The Photochemistry of 4‐Chlorophenol in Water Revisited: The Effect of Cyclodextrins on Cation and Carbene Reactions
Author(s) -
Manet Ilse,
Monti Sandra,
Bortolus Pietro,
Fagi Maurizio,
Albini Angelo
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200401227
Subject(s) - chemistry , nucleophile , carbene , photochemistry , medicinal chemistry , stoichiometry , yield (engineering) , triplet state , hydrogen atom abstraction , ground state , hydrogen , molecule , organic chemistry , catalysis , materials science , physics , quantum mechanics , metallurgy
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 CCl 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 (≈1 μs). Specific trapping of both intermediates has been achieved. The cation adds to 2‐propenol, k add ∼1.3×10 8 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 3 3 is trapped by oxygen to give benzoquinone and is reduced by D ‐glucose ( k q = 8.5×10 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 = 9.4×10 8 m −1 s −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 ≥5×10 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.