I'm trying to understand how the sulfur was removed in the following reaction, taken from a paper by Ben Feringa:1
After some research I found out that this is a Barton-Kellogg reaction. Based on this I went through the mechanism as follows:
I'm stuck on the final step of how to remove the sulfur. I looked over multiple papers and all of them used $\ce{PPh3}$ to open the episulfide but the reaction from the paper I'm considering does not have $\ce{PPh3}$. I looked at all the sources and supporting information the paper had linked but couldn't find anything to explain how the double bond forms. My only guess is that something happens when they purify it using $\ce{SiO2}$ flash column chromatography. Here is the excerpt from the SI (L2 is the alkene product):
Freshly prepared diazafluorene 1 ($\pu{350 mg}$, $\pu{1.80 mmol}$) and thioketone (±)-2 ($\pu{383 mg}$, $\pu{1.80 mmol}$) in THF ($\pu{10 mL}$) were heated at reflux for 14 h under a $\ce{N2}$ atmosphere. The solution was cooled to rt, concentrated and purified by FC ($\ce{SiO2; EtOAc}$) to afford (±)-L2 ($\pu{327 mg}$, $52\%$) as yellow solid [...]
I would like any suggestions on what people think may be happening.
Reference
- Wezenberg, S. J.; Chen, K.-Y.; Feringa, B. L. Visible-Light-Driven Photoisomerization and Increased Rotation Speed of a Molecular Motor Acting as a Ligand in a Ruthenium(II) Complex. Angew. Chem. Int. Ed. 2015, 54 (39), 11457–11461. DOI: 10.1002/anie.201505781.