New chemical species isolated

Like a previously unknown species emerging into the glare of camera flashlights from the tropical undergrowth or crawling out from under a deep ocean rock, a seemingly simple small molecule, an organosulfur compound called oxathiirane, has been synthesised at long last and reveals itself under infrared and ultraviolet spectroscopic inspection.

Peter R. Schreiner and Hans Peter Reisenauer of the Institute of Organic Chemistry, at the Justus-Liebig University, in Giessen, Germany, and Jaroslaw Romanski and Grzegorz Mloston of the Department of Organic & Applied Chemistry at the University of Lodz, Poland, point out that oxathiirane and its derivatives have been suggested to be the key intermediates in photolytic sulfur transfer reactions of thiocarbonyl S-oxides (sulfines). These are common enough reactions but the intermediates have never been observed by direct experiment. Moreover, they're also involved in various biochemical pathways. For instance, as the component in cut onions that make one's eyes water, the lachrymatory factor.

Theoretical studies suggest that the oxathiiranes should be observable while previous experimental research had hinted at the existence of a diphenyl derivative that produced a transient UV absorption at 390 nm in the photochemical decomposition of thiobenzophenone S-oxide. But, the only firm spectroscopic evidence for the formation of an oxathiirane derivative (6,7-dimethyl-1,5-dioxa-2-thiaspiro[2.4]heptan-4-one) was to be found in the photolytic rearrangement of its corresponding sulfine precursor.

Schreiner and colleagues hoped to synthesise the parent compound and observe it in isolation. The challenge facing them was in the "unexpectedly rich" photochemistry of H₂C=S=O, which one might imagine cyclising into the triangular oxathiirane structure. This compound is apparently wont to undergo unimolecular rearrangement with little effort. The team anticipated that irradiation at an appropriate wavelength might lead to oxathiirane.

They have now successfully prepared the compound, isolated it and confirmed its existence by spectroscopic and high-level computational characterization.

"Oxathiirane was characterized by the extraordinarily good agreement of experimentally measured and (unscaled) CCSD(T)/cc-pVTZ computed vibrational frequencies for both the perhydrogenated and perdeuterated species," the researchers say. They add that evidence of the existence of other related small organosulfur molecules, might open a route to a novel carbene HO-C-SH.

From the perspective of fundamental chemical understanding, the study also has mechanistic implications, carbenes aside. Indeed, the team's concluding remark brings the oxathiirane tale full circle: "The stability ordering and the relative energy differences of carbonyl versus thiocarbonyl groups underline the likely role oxathiiranes play in sulfur transfer reactions," the team concludes.