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

## TL;DR

The study reveals that reactive sulfur species, not S8 molecules, drive a specific chemical reaction involving cyclopropenones and sulfur.

## Contribution

The paper identifies polysulfide anions as the reactive species in cyclopropenone cycloaddition, challenging the conventional S8-based mechanism.

## Key findings

- Reactive sulfur species like polysulfide anions, not S8, are responsible for the cycloaddition reaction.
- Oxidants like BQ or m-CPBA completely suppress the reaction.
- The reaction proceeds via a ketenethialdehyde and thiet-2-one sequence to form a single regioisomer.

## Abstract

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 (FS
n

–, HS
n

–, or S
n

2–) 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.