Photochemical α‐Cleavage in Pyran‐2‐thione: Generation of Aldehyde–Thioketene and Thioaldehyde–Ketene Photoproducts

Abstract

The photochemical ring-opening reaction leading to cleavage of the CÀO a-bond in a-pyrones, and other similar photoreactions observed in closely related unstrained cyclic carbonyl compounds such as 2,4-cyclohexadienones [1] and pyrimidinones, [2] are well documented. In the case of a-pyrones (Scheme 1), because of the high reactivity of the ketene moiety, the open-ring conjugated ketene photoproducts can be stabilized only in solid, low-temperature inert matrices. [3,4] The intense, characteristic infrared band due to the antisymmetric stretching vibration of the ÀC=C=O group, appearing in the 2140-2130 cm À1 spectral region, facilitates the identification of ketene photoproducts. Open-ring conjugated ketenes are usually presented as structures shown in Scheme 1. However, it is improbable that such isomers of conjugated ketenes could be stabilized under any conditions. It was recently demonstrated [3] that these forms revert barrierlessly to stable, closed-ring structures. The detailed structural analysis carried out for the conjugated aldehyde-ketene generated upon UV irradiation of a-pyrone strongly suggests [3] that the isomers, which are stabilized in a matrix, must adopt a conformation with at least one of the groups rotated around a single or double bond of the openchain structure.Contrarily to the carbonyl compounds, the photochemical cleavage of an a-bond is believed not to occur for thiocarbonyl compounds, [5] though Pirkle et al. reported [6] pyranthiones that underwent thermal rearrangements (similar to that depicted on Scheme 1) during pyrolysis at elevated temperatures. The known exceptional cases of photochemical a-cleavage concern only the species with C=S groups directly attached to small (three-or four-membered), strained rings. [7] Herein, we report the experimental observation of the photochemical ring opening, by cleavage of the CÀO a-bond in pyran-2-thione (Scheme 2). In this compound the thiocarbonyl group is attached to a relaxed, six-membered ring.The unimolecular photochemistry of pyran-2-thione was studied by a combined usage of the matrix isolation technique and FTIR spectroscopy. The infrared spectral signatures of the primary photogenerated aldehyde-thioketene (TK) isomeric forms, as well as those of the secondary products, thioaldehyde-ketene (TA) isomers, were assigned on the basis of theoretical calculations carried out at the B3 LYP/6-311 + + G(d,p) level.In the matrix isolation experiments, a sample of pyran-2-thione [8] (PT) was placed into a glass tube protected against light and connected to the chamber of the cryostat with a needle valve. The vapor of the compound was introduced into the cryostat chamber through the needle valve together with large excess of the host matrix gas (argon N60, Air Liquide) coming from a separate line. A cold (10 K) CsI window mounted on the tip of an APD Cryogenics DE-202 A closed-cycle helium refrigerator was used as optical substrate. Care was taken to keep the guest-to-host ratio in the matrixes low enough to avoid association. The matrices we...

UV-induced unimolecular photochemistry of 2(5H)-furanone and 2(5H)-thiophenone isolated in low temperature inert matrices

“…As for the previously studied six-membered heterocyclic a-carbonyl compounds [19][20][21][22]30], a dependence on the irradiation wavelength was found. The absorptions due to thiophenone started to decrease and new absorptions appeared in the spectrum of the irradiated sample when the cutoff wavelength was decreased to 285 nm.…”

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix

“…There are multiple experimental reports of similar photoequilibria demonstrated for α-pyrones and the respective open-ring ketenes. When one of the atoms in α-pyrones is isotopically labeled ( 2 H, 13 C, or 18 O) ,− or one of the oxygen heteroatoms is substituted with sulfur , or when the ring substituents at positions 3 and 5 are not equivalent, , the asymmetry in the structure permits to observe a series of isomerizations that evidence the existence of photoequilibrium between the closed-ring and open-ring isomers (see Scheme ). Another evidence in favor of such ring-open/ring-closure photoequilibria in the present case of carvacrol is the intensity of the band near 2109 cm –1 .…”

Hydrogen atom transfer reactions in thiophenol: photogeneration of two new thione isomers

“…28 However, several years ago, the first experimental observation of such a photoprocess, occurring in matrix-isolated 2H-pyran-2thione, was reported. 29,30 For that compound, the photoinduced cleavage of the C-O a-bond in the heterocyclic ring led to the generation of an open-ring aldehyde-thioketene product. The present study provides the first example of an a-bond-cleavage photoreaction occurring in a thiocarbonyl compound (24 thione tautomer of thiophenol) with a six-membered carbon ring that does not contain any heteroatoms.…”