Matrix Photochemistry, Photoelectron Spectroscopy, Solid-Phase Structure, and Ring Strain Energy of β-Propiothiolactone

Abstract

The four-membered heterocyclic beta-propiothiolactone compound was isolated in a low-temperature inert Ar matrix, and the UV-visible (200 < or = lambda < or = 800 nm) induced photochemistry was studied. On the basis of the IR spectra, the formation of methylketene (CH(3)CHCO) was identified as the main channel of photodecomposition. The formation of ethene and thiirane, with the concomitant elimination of OCS and CO, respectively, was also observed as minor decomposition channels. The valence electronic structure was investigated by HeI photoelectron spectroscopy assisted by quantum chemical calculations at the OVGF/6-311++G(d,p) level of theory. The first three bands at 9.73, 9.87, and 12.06 eV are ascribed to the n''(S), n'(O), and pi''(CO) orbitals, respectively, denoting the importance of the -SC(O)- group in the outermost electronic properties. Additionally, the structure of a single crystal, grown in situ, was determined by X-ray diffraction analysis at low temperature. The crystalline solid [monoclinic system, P21/c, a = 8.1062(1) A, b = 10.3069(2) A, c = 10.2734(1) A, beta = 107.628(1) degrees, and Z = 8] consists of planar molecules arranged in layers. The skeletal parameters, especially the valence angles [angleC2-C1-S = 94.55(7) degrees, angleOC-C = 134.20(11) degrees, angleC-S-C = 77.27(5) degrees], differ from those typically found in acyclic thioester compounds, suggesting the presence of strong strain effects. The conventional ring strain energy was determined to be 16.4 kcal/mol at the G2MP2 level of calculation within the hyperhomodesmotic model.

Native Chemical Ligation and Extended Methods: Mechanisms, Catalysis, Scope, and Limitations

“…Regarding cyclic thioesters, i.e., thiolactones, small and medium-sized thiolactones (4 < n < 7), such as β-propiolactone (n = 4), 153 γ-butyrothiolactone (n = 5), 154 and δvalerothiolactone (n = 6) 155 are constrained in the less favorable anti conformation (Figure 9). Computational analysis suggests that this is also the case for ε-caprothiolactone (n = 7).…”

Deciphering Stability of Five-Membered Heterocyclic Radicals: Balancing Act Between Delocalization and Ring Strain

“…In this regard, we have raised the following questions: (a) What is/are the major factor(s) for stabilizing and/or destabilizing the radical? Since five-membered ring systems will have more strain, − to what extent it can control the stability? (b) How different are the roles of oxygen and sulfur (divalent) from the other heteroatoms such as boron, nitrogen, and phosphorus (trivalent) in stabilizing the heterocyclic radicals?…”

Conformational Transferability of the Sulfenyl Carbonyl Group −SC(O)– in Cyclic Thioesters

“…Molecular and crystal structures of derivatives of higher molecular weight have also been studied by X-ray diffractometry. , In this sense, a broader knowledge of the molecular structure of simple thiolactones is of major interest. Quite recently, the structural and electronic properties for the nonsubstituted four-, five-, and six-membered species have been studied in our group by using a combined experimental and quantum chemical approach. In all these molecules, the central −SC(O)– unit is forced to adopt an antiperiplanar (anti) conformation by the constraints of the ring system.…”