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

Abstract

a b s t r a c tIn this study, the infrared spectrum of N-(2-oxo-2H-chromen-3-yl)acetamide (3-acetamidocoumarin; 3AC) isolated in solid argon, at 10 K, was obtained and assigned. In consonance with the relative energies of the three conformers predicted theoretically, only the most stable form was observed experimentally. This conformer is stabilized by two intramolecular hydrogen bonds and is similar to the structural unit of 3AC found in crystalline phase. Upon in situ UV (k > 215 nm) irradiation of the matrix-isolated compound, the characteristic IR intense band due to the antisymmetric stretching vibration of the ketene (AC@C@O) group was observed, indicating occurrence of the ring-opening isomerization reaction to the open-ring ketene isomeric of 3AC. In consonance with the theoretical structural predictions for the most stable isomers of this photoproduct, the experimental data indicates that it is produced in the E arrangement of the (O@)CAC@CAC(@C@O) fragment. There were also experimental indications pointing to occurrence of a second photoreaction channel, corresponding to decarbonylation. On the other hand, contrarily to what is generally observed for a-pyrones derivatives, including unsubstituted coumarin, no photochemical production of Dewar isomer of 3AC was observed. This last result, follows the trend observed for 2-pyrone-3-carboxylate, and seems to be a quite general rule for matrix-isolated a-pyrones bearing relatively volumous substituents at the position 3, as a consequence of the unfavorable relaxation of the matrix around the guest molecule that would be required to accommodate the Dewar isomers of these compounds, whose structure deviates strongly from planarity, thus mismatching the primarily occupied matrix sites.

Infrared Spectrum and UV-Induced Photochemistry of Matrix-Isolated 5-Hydroxyquinoline

“…Besides, in general, the different possible isomeric ketene forms resulting from ringopening reactions analogous to that taking place in the present case are formed altogether. 22,47,48 This seems also to be the current situation, considering both the considerable broadness of the observed νCCO antisymmetric stretching band and the multiplet structure of the other bands that were possible to assign to the ketene species (in the 1550−1500 cm −1 spectral region, where the next more intense bands of the ketenes are predicted to occur; see Figure S4, Supporting Information).…”

“…In general, the spectroscopic identification of ketenes is easy because they strongly absorb in the infrared in the 2150−2100 cm −1 region (νCCO antisymmetric), 22,47,48 and such characteristic absorption is indeed observed in the spectra of the studied photolyzed 5HQ N 2 -matrixes ( Figure 4). However, almost all the remaining infrared bands of the ketenes that are possible to result from the photolysis of 5HQ are of low intensity (below 10 km mol −1 ), as shown in Figure S4 (Supporting Information).…”