Photochemistry of six‐ and five‐membered‐ring α,β‐unsaturated lactones in cryogenic matrices

Abstract

The photochemistry of representative six and five-membered a,b-unsaturated lactones [a-pyrone and some of its derivatives, including coumarin and 3-acetamidocoumarin, 2(5H)-furanone] isolated in cryogenic inert matrices has been investigated by infrared spectroscopy and quantum chemical calculations. In these types of molecules, two main competitive photochemical reaction pathways could be identified: ring opening, leading to formation of the isomeric aldehyde-ketenes, and ring contraction to the corresponding Dewar isomers. For a-pyrone and 2(5H)-furanone, the ring-opening process dominates over the ring-contraction reaction, the same occurring for derivatives of these compounds bearing a voluminous substituent at position 3. In 2(5H)-furanone, the ring-opening reaction requires the simultaneous occurrence of a [1,2]-hydrogen atom migration. Nevertheless, it was found to be an easy process upon excitation at l > 235 nm. The ring-opening reaction was also found to occur much easily in a-pyrone than in coumarin, and factors explaining this observation were discussed. In turn, the Dewar forms of the studied compounds resulting from the ring-contraction photoreaction were found to undergo subsequent photo-elimination of CO 2 , with formation of the corresponding cycloalkenes. In the matrices, CO 2 and the simultaneously formed cycloalkenes were found to exist as associated forms, in which the CO 2 molecule is preferentially placed over the cycloalkene ring in a stacked-type geometry. For coumarin, a third photoreaction channel was observed, leading to the formation of benzofurane and CO. This additional reaction channel corresponds to the photoreaction previously observed for the compound in the gaseous phase.

Contrasting ring-opening propensities in UV-excited α-pyrone and coumarin

“…These authors provided several possible rationales for the different photochemistry, including the higher density of electronic and vibrational states for coumarin, and the increased relative stability of the ring-open isomers in a-pyrone. 24,25 Krauter and coworkers 26 used ultrafast transient electronic absorption (TEA) spectroscopy to probe the 330 nm induced photochemistry of coumarin dissolved in methanol, cyclohexane, and acetonitrile. The TEA data indicated that the initially excited S 1 state decayed on the ps time scale, and again provided no evidence of any photoproduct formation.…”

New Colours for Carotenoids – Synthesis of Pyran Polyenes

“…Pyranones and ketene aldehydes have been found to be interchangeable isomers. [35,36] The reaction steps outlined are mentioned only as a tentative mechanism as viable evidence is lacking. Especially intriguing is the observation that pyran formation only proceeds with C 10 aldehyde 1; C 20 aldehyde 6 (Scheme 5) did not react, nor did two molecules of C 20 -pyran dialdehyde 2 yield directly a C 40 -dipyran carotenoid.…”

Six‐Membered Rings with One Oxygen: Pyrylium Ion, Related Systems and Benzo‐Derivatives

“…Finally, other reactions such as photochemical reactions [111], hydrogenations [112], and palladium-catalyzed couplings [113] can also take place at the a-pyranone structure.…”