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I'm trying to understand how the sulfur was removed in the following reaction, taken from a paper by Ben Feringa:1

Diazo and Thioketone

After some research I found out that this is a Barton-Kellogg reaction. Based on this I went through the mechanism as follows:enter image description here

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

  1. 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.
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    $\begingroup$ In almost every example I've seen, the episulfide has been isolated and then subjected to some kind of phosphine reagent to cause the elimination. I'd be inclined to think, given the nature of the paper (not really discussing this reaction apart from the SI), that its simply an oversight in the preparation. Are you looking to do this reaction (in which case there are other more detailed/explained preparations)? Or is it simply as mechanistic challenge? $\endgroup$
    – NotEvans.
    Commented May 26, 2017 at 20:00
  • $\begingroup$ There were other papers by Feringa that does the same reaction and none of them used PPH3 which makes me think that this is not simply an oversight. I'm not planing to do the reaction, I'm only trying to understand the mechanism. $\endgroup$
    – ChemDude
    Commented May 26, 2017 at 20:47
  • $\begingroup$ @MathHelpNeeded The paper's site states both Mr Nobel Laureate Professor Feringa, as well as Dr Wezenberg as corresponding author. Why not putting a foot forward, and writing an e-mail in tune of "I read your publication 2015ACIE11457, and by comparision with other's work on the Barton-Kellog reaction I noticed the lack of $\ce{PPh3}$ in your protocol. The comparision with earlier papers A, B, and C by your group suggests to me this is not a mere oversight. May you assist me in the rationale how the episulfide forms the alkene?" And, for the benefit of ChemSE, to share the rationale here? $\endgroup$
    – Buttonwood
    Commented May 26, 2017 at 22:10
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    $\begingroup$ This is a very heavily substituted episulfide. This suggests the possibility of removing the sulfur just under reflux. I've never seen this before, but that's how I would rationalize the mechanism. $\endgroup$
    – Zhe
    Commented May 26, 2017 at 22:32
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    $\begingroup$ @Buttonwood I sent an email; I'll post whatever answer if he replies. $\endgroup$
    – ChemDude
    Commented May 27, 2017 at 4:51

1 Answer 1

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After contacting the authors of the paper, one author, Prof. Wezenberg said that a conclusive mechanism is not yet available for this case but he believes that the unstable episulfide undergoes desulfurization by itself under reflux.

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