UV-induced photochemistry of methyl coumalate (methyl 2-pyrone-5-carboxylate) isolated in low-temperature inert matrices

Abstract

Photochemical transformations of methyl coumalate have been studied by matrix-isolation technique. Two photoreactions were induced by UV (k > 295 nm) light: isomerisation to the Dewar form and a-bond cleavage leading to the open-ring aldehyde-ketene. The first photoprocess was found to be strongly dominating. Experimental evidence of photoreversibility of both photoisomerisation processes has been provided. Upon k > 200 nm irradiation, the photochemically formed monomeric Dewar species underwent decarboxylation, with production of methoxycarbonyl-substituted cyclobutadiene. This antiaromatic photoproduct was experimentally observed for the first time. All the photoproduced species were identified by comparison of their IR spectra with the spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level.

In situ direct photoproduction of ketenes from substituted coumarins isolated in solid argon: The case of N-(2-oxo-2H-chromen-3-yl)acetamide

“…This has been recently observed for matrix-isolated 3-methyl coumalate, where only the ring-opening reaction was observed [13,14]. On the other hand, the 5-substituted analogue (5-methyl coumalate) shows the expected dominance of the Dewar isomerization process over the ring-opening reaction, as found for the other substituted a-pyrones already studied.…”

“…As mentioned in Section 1, the a-pyrone moiety has been shown to exhibit an interesting photochemistry [9][10][11][12][13][14], with both the ring-opening (to isomeric ketene) and ring-contracting (to Dewar isomer) reactions being generally observed and their relative importance being influenced by the substituents present in the heterocyclic ring. An interesting point was the observation that the presence of a volumous group at the position 3 of the a-pyrone ring strongly reduces the importance of the ring-contraction reaction for the matrix-isolated compounds, in view of the energetically demanding reorganization of the matrix host atoms that would be required to accommodate the newly formed strongly .…”

“…The calculated frequencies were scaled by 0.987; the spectrum was then simulated by Gaussian functions (with 4 cm À1 halfband width) centred at the calculated scaled frequencies. non-planar Dewar species [13,14]. Since 3AC is a molecule bearing a relatively volumous substituent at the position 3 of the a-pyrone ring, the photochemical ring-contraction reaction could be expected not to take place easily for the matrix-isolated compound.…”

“…In general, a-pyrones undergo two main competitive photochemical reactions: ring-opening, leading to formation of the isomeric open-ring ketenes, and ring-contraction to the corresponding Dewar isomers [9][10][11][12][13][14]. However, the precise photochemistry and relative importance of these two photochannels is strongly dependent on the substituents present in the heterocyclic ring.…”

“…The properties of coumarins are in large extent determined by their constituting a-pyrone unit, which has been shown to present a very rich photochemistry [9][10][11][12][13][14]. In general, a-pyrones undergo two main competitive photochemical reactions: ring-opening, leading to formation of the isomeric open-ring ketenes, and ring-contraction to the corresponding Dewar isomers [9][10][11][12][13][14].…”