A New Type of Excited-State Intramolecular Proton Transfer: Proton Transfer from Phenol OH to a Carbon Atom of an Aromatic Ring Observed for 2-Phenylphenol1

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Abstract
The photochemical deuterium incorporation at the 2‘- and 4‘-positions of 2-phenylphenol (4) and equivalent positions of related compounds has been studied in D2O (CH3OD)−CH3CN solutions with varying D2O (CH3OD) content. Predominant exchange was observed at the 2‘-position with an efficiency that is independent of D2O (MeOD) content. Exchange at the 2‘-position (but not at the 4‘-position) was also observed when crystalline samples of 4-OD were irradiated. Data are presented consistent with a mechanism of exchange that involves excited-state intramolecular proton transfer (ESIPT) from the phenol to the 2‘-carbon position of the benzene ring not containing the phenol, to generate the corresponding keto tautomer (an o-quinone methide). This is the first explicit example of a new class of ESIPT in which an acidic phenolic proton is transferred to an sp2-hybridized carbon of an aromatic ring. The complete lack of exchange observed for related substrates 6−9 and for planar 4-hydroxyfluorene (10) is consistent with a mechanism of ESIPT that requires an initial hydrogen bonding interaction between the phenol proton and the benzene π-system. Similar exchange was observed for 2,2‘-biphenol (5), suggesting that this new type of ESIPT is a general reaction for unconstrained 2‘-aryl-substituted phenols and other related hydroxyarenes.

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