On the pyrolysis mechanism of 2-pyranones and 2-pyranthiones: thermally induced ground electronic state chemistry of pyran-2-thione.

[reaction: see text] This work reports studies of thermochemistry of pyran-2-thione (PT), a sulfur derivative of alpha-pyrone (AP). Moderate heating of PT results in scrambling of sulfur and oxygen atoms in the molecule and formation of isomeric thiapyran-2-one (TP). The products of pyrolysis of PT were studied experimentally by a combined use low temperature matrix isolation and Fourier transform infrared spectroscopy. The infrared spectrum of the TP monomer isolated in solid argon at 10 K was completely assigned based on comparison with theoretical calculations undertaken at the DFT(B3LYP)/6-311++G(d,p) level. The upper limit of thermal stability of PT was investigated using the differential scanning calorimetry technique. It was found that pyrolysis of PT is already initiated at temperatures below 130 degrees C. The mechanism of the observed pyrolytical conversion has been studied theoretically at the MP2/6-311++G(d,p) level, in the ground electronic state. The primary step of the pyrolytical reaction in PT is the alpha-cleavage of the C-O single bond. It proceeds via an open-ring thioketene-aldehyde structure, TK1. According to the calculations, the ring-opening reaction from PT to TK1 requires an activation energy less than 80 kJ mol(-1), at 130 degrees C, being the rate-determining step. Further steps of the pyrolytical reaction involve internal rotations around single bonds and [1,5] sigmatropic shift of the aldehydic hydrogen. Pyrolytical ring-opening reactions were studied theoretically also for AP and TP and compared to the pyrolysis of PT. It is suggested that the relative ease of the pyrolytical transformation in PT can be explained in terms of existence of the additional minimum TK1 in the reaction path. No counterparts for this structure could be theoretically located for AP and TP.