Flash-photolysis of 2-(2′-hydroxyphenyl)-3-H-indole. Ground-state keto-enol tautomerization by mutual hydrogen exchange and by proton catalysis
- Stephan, Jens S.
- Rodríguez, Carmen Ríos
- Grellmann, Karl H.
- Zachariasse, Klaas A.
ISSN: 0301-0104
Ano de publicación: 1994
Volume: 186
Número: 2-3
Páxinas: 435-446
Tipo: Artigo
Outras publicacións en: Chemical Physics
Resumo
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.
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