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It has been repeatedly emphasized that proton transfers "involving" three or four membered ring transition states are not favorable and should not be drawn as the primary mechanism for any reaction.

However, with regard to a recent discussion involving $\ce{H3CSH}$ it was noted that the rate of proton tunneling in the anionic form $\ce{H3CS-}$ was likely extremely high.

Carbon-Sulfur Bond Lengths; Resonance Effects (Or Lack Thereof)

So:

a) Is this a valid rule of thumb: avoid intramolecular proton transfers unless the transition state has 5 or 6 members in its ring?

b) What's more significant with regard to proton transfer - proton tunneling or physical proton transfer? Or is the answer an "it depends"?

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  • $\begingroup$ 1) Was the discussion on Chem SE? If so, could you reference it? 2) I don't follow, in what sense is the proton in the anion tunneling, what is the proton doing? $\endgroup$ – ron Dec 6 '14 at 18:34
  • $\begingroup$ @ron chemistry.stackexchange.com/questions/18724/… $\endgroup$ – Dissenter Dec 6 '14 at 18:35
  • $\begingroup$ @ron I'd even guess there's a significant H tunneling rate in the syn form (or an inversion from anti to syn followed by H shift). So I agree it would at least be tough. Unfortunately, my microwave expert retired a few years ago, so I can't ask. – Geoff Hutchison $\endgroup$ – Dissenter Dec 6 '14 at 18:36
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a) Is this a valid rule of thumb: avoid intramolecular proton transfers unless the transition state has 5 or 6 members in its ring?

Yes

b) What's more significant with regard to proton transfer - proton tunneling or physical proton transfer? Or is the answer an "it depends"?

It depends, at temperatures around absolute zero, tunneling can become significant. At ambient temperatures or above, physical proton transfer is much more common.

As to the methyl mercaptan anionic example you cite, to the extent that the various carbon-sulfur double bond resonance structures contribute, I would expect a lowering of the barrier around carbon anion planarization - and consequently a lowering of the barrier to inversion at the carbon atom in the anion. Therefore, it is not clear to me that tunneling of the sulfur bound proton would be preferred to carbon inversion in order to bring about the syn-anti interconversion, especially since any presumed experiments to generate the anion would not be performed near absolute zero.

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