Matrix isolation FTIR and theoretical study of α-pyrone photochemistry

Abstract

A combined matrix isolation FTIR and molecular orbital study on the photochemistry of alpha-pyrone (AP) was undertaken. The most efficient of the observed photoprocesses was the Norrish type I, ring opening reaction leading to conjugated ketene. Upon irradiation of matrix isolated AP with UV (λ > 285 nm) light, rapid formation of Z isomers of the aldehyde–ketene was observed. After a few minutes of irradiation the IR bands assigned to these photoproducts stopped growing. Further irradiation generated, by excited state internal rotations around the central CC bond in the Z forms, new aldehyde–ketene isomers, which were assigned to the E forms of the conjugated ketene. These forms were identified in the present work for the first time. Upon subsequent UV (λ > 337 nm) irradiation, the Z aldehyde–ketene isomers reverted back to the closed-ring, initial form of AP, while the E forms did not react further. The ring opening reaction occurring upon UV (λ > 285 nm) irradiation was accompanied by very slow valence isomerization of AP to the Dewar form (2-oxa-3-oxobicyclo[2.2.0]hex-5-ene; OOBH). Observation of cyclobutadiene, produced by shorter wavelength UV irradiation (λ > 235 nm) of OOBH, was also achieved in the present study. The structure of the cage confined complex of cyclobutadiene with CO₂ was theoretically investigated. The possibility of formation of 4-formyl-2-cyclobutene-1-one, by photochemical ring closure of aldehyde–ketene isomers with the central CC bond and the ketene group in s–cis position with respect to each other is also discussed.