Mechanistic Insights into the Regioselective (3 + 2) Cycloaddition of Unsymmetrical Cyclopropenones with Elemental Sulfur: Experimental and Computational Studies

This study investigates the (3 + 2) cycloaddition of substituted cyclopropenones with elemental sulfur. Combined experimental and computational results highlight the involvement of reactive sulfur species, such as those generated from KF or sulfide impurities. Oxygen exerts minimal influence, whereas oxidants such as BQ or m-CPBA completely suppress the reaction. DFT calculations challenge conventional S₈-based mechanisms, pointing to an energetically costly process from inactivated S₈. Our results support that the cyclic S₈ molecules of elemental sulfur are inactive toward cyclopropenone reagents. The true reactive species are polysulfide anions, which can be present in different forms (FSn⁻, HSn⁻, or Sn²⁻) depending on the additive type and purity of the sulfur source. These species undergo conjugate addition to the least substituted carbon of the cyclopropenone C═C bond, ultimately affording a five-membered heterocycle─1,2-dithiol-3-one─as a single regioisomer, through a ketenethialdehyde and thiet-2-one sequence.