Towards the pyrolytic preparation of carbonothioic O,O-acid (monothiocarbonic acid)

- We are reporting on our attempts to generate, for the first time, thiocarbonic acid, a hitherto elusive molecule that plays an important role in the earth's sulfur cycle. While we have, after a lot of experimentation and computation, not been successful in preparing the target molecule, we outline why the pyrolysis is different from carbonic acid recently prepared in our labs. Hence, our study combines experiment and theory to elucidate the pyrolysis conditions from appropriately chosen thiocarbonate starting materials. We are pleased to report that we prepared the monothioester, which is an intermediate en route to thiocarbonic acid.

Carbonothioic O,O-acid (monothiocarbonic acid), the sulfur analogue of carbonic acid, is thus far an experimentally unreported molecule of atmospheric and biochemical relevance. Computations at the CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ level of theory reveal that its carbonothioic O,S-acid tautomer is energetically favored over the carbonothioic O,O-acid tautomer and that the decomposition of the former to H2S and CO2 is prevented by a barrier in excess of 30 kcal mol−1. Therefore, we attempted to prepare carbonothioic-O,O-acid under matrix isolation conditions via high vacuum flash pyrolysis (HVFP) of O,S-di-tert-butyl carbonothioate in analogy to the previously reported successful preparation of carbonic acid. Pyrolysis at 900 °C only produced isobutene, CO2, and tert-buthylthiol, while S-tert-butyl O-hydrogen carbonothioate is the most likely additional product identifiable at 690 °C pyrolysis temperature. Density functional theory computations at the M06-2X/cc-pVTZ level were used to understand the pyrolysis pathways and help determine the leaving group ability in the ester pyrolysis step, and our inability to observe the carbonothioic acids via our chosen pyrolysis path.