Flash-photolysis of 2-(2′-hydroxyphenyl)-3-H-indole. Ground-state keto-enol tautomerization by mutual hydrogen exchange and by proton catalysis

Abstract

Efficient excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethyl-3H-indole (HBC) leads to the formation of an unstable trans-keto tautomer 1Ktr whose transient absorption can be observed in flash experiments. The assignment of this transient absorption to the trans-keto tautomer is corroborated by the observation that a similar transient is not formed from BHBC, which has been synthesized as a bridged, rotationally blocked counterpart of HBC. In hydrocarbon solvents, two 1Ktr molecules revert by a mutual hydrogen-exchange reaction to the stable enol form of HBC. This second-order reaction is diffusion-controlled and its rate constant is proportional to T/η over a wide range of the solvent viscosity η. In the presence of alcohols (ROH) a first-order, pseudo-monomolecular proton-catalysed re-enolization reaction competes with the second-order reaction. With methanol and ethanol as proton donors it is shown that two donor molecules are involved in this process. The rate of the proton-catalysed reaction increases with decreasing temperatures. A pre-equilibrium between a nonreactive 1Ktr and a reactive 1Ktr::: (ROH)2 complex is the reason for this unusual temperature dependence of the first-order 1Ktr decay rate.