Geometric and solvent effects on intramolecular phenolic hydrogen abstraction by carbonyl n,π* and π,π* triplets

The photochemistry of a series of alkoxyacetophenone, -benzophenone, and -indanone derivatives, which contain a remote phenolic group linked to the ketone by a para,para'- or meta,meta'-oxyethyl spacer, has been studied in acetonitrile and dichloromethane solutions using laser flash photolysis techniques. The corresponding methoxy-substituted compounds and, in the case of the alkoxyindanones, derivatives bearing just a remote phenyl substituent, have also been examined. The triplet lifetimes of the phenolic compounds are determined by the rates of intramolecular abstraction of the remote phenolic hydrogen, and depend on the solvent, the geometry of attachment and the configuration of the lowest triplet state. In contrast to the large (>500-fold) difference in lifetime of the para,para'- and meta,meta'-alkoxyacetophenone derivatives, both of which have lowest π,π* triplet states, smaller differences are observed for the alkoxyindanone (lowest charge transfer triplet, ~twofold difference) and alkoxybenzophenone (lowest n,π* triplet, ~18-fold difference) derivatives in acetonitrile solution. The triplet lifetimes of the acetophenone and benzophenone are significantly shorter in dichloromethane than in acetonitrile, consistent with the intermediacy of a hydrogen-bonded triplet exciplex in the reaction. This is not the case with the para,para'-indanone derivative, sugesting that hydrogen abstraction in this compound is dominated by a mechanism involving initial charge transfer rather than hydrogen bonding. This is most likely due to orientational constraints that prevent the remote phenolic -O-H group from adopting a coplanar arrangement with the n-orbitals of the carbonyl group.Key words: photochemistry, aromatic ketone, phenol, triplet, intramolecular, quenching, hydrogen abstraction, phenoxyl radical, kinetics, kinetic isotope effect, laser flash photolysis.