Manipulating the
equilibrium between a ketone and an enol by exposure to light opens up ample
opportunities in material chemistry and photopharmacology since it allows one
to reversibly control the content of the enol tautomer, which acts as a
hydrogen atom donor, with high spatio-temporal and energy resolution. Although
tautomerization of β-ketoesters or their analogs was studied in numerous
papers, their light-induced reversible tautomerization to give thermally stable
enols (photoenolization) is an unexplored area. To shed light on this “blind
spot”, we report an unprecedented property of diarylethene <b>2A</b> assembled from fragments of photoactive dithienylethene and a
β-ketoester as part of the cyclohexenone bridge. In a pristine state, the
tautomeric equilibrium of <b>2</b> is
almost completely shifted towards the ketone. Photocyclization of the
hexatriene system results in a new equilibrium system containing a significant
fraction of the enol tautomer, both in polar and non-polar solvents. Due to the
considerable spectral separation (35 nm), the keto-enol tautomerization process
could be observed visually. The tendency of <b>2A </b>to undergo light-induced enolization was proved by isolating a
related byproduct of photochemical 1,2-dyotropic rearrangement stabilized in the
enolic form. Our results provide a novel tool for controlling the keto-enol
tautomerism that might find use in the development of novel photocontrollable
processes.
Reversible Shifting of a Chemical Equilibrium by Light: The Case of Keto-Enol Tautomerism of a β-Ketoester