FLASH-PHOTOLYSIS OF 2-(2'-HYDROXYPHENYL)-3-H-INDOLE - GROUND-STATE KETO-ENOL TAUTOMERIZATION BY MUTUAL HYDROGEN-EXCHANGE AND BY PROTON CATALYSIS

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 1K(tr) whose transient ab sorption 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 h as been synthesized as a bridged, rotationally blocked counterpart of HBC. In hydrocarbon solvents, two 1K(tr) 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 propo rtional to T/eta over a wide range of the solvent viscosity eta. In th e presence of alcohols (ROH) a first-order, pseudo-monomolecular proto n-catalysed re-enolization reaction competes with the second-order rea ction. 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-equil ibrium between a nonreactive 1K(tr) and a reactive 1K(tr):::(ROH)2 com plex is the reason for this unusual temperature dependence of the firs t-order 1K(tr) decay rate.