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I have studied that when an epoxy ring opening takes place in acidic medium, the ring opens in a way that the most stable carbocation forms. This is due to the positive charge on the oxygen, which gives partial positive character to the carbon atoms.

A very similar intermediate is formed in NGP (neighboring group participation) reactions with sulphur as the neighboring atom (source). How is the attack carried out then? Is it decided sterically as a regular SN2, or does the logic with oxygen extend to sulphur?

NGP Reaction Mechanism

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Inductive reasoning in organic chemistry can be somewhat hard at times, given all of the exceptions that can pop up, for example the SN2-like mechanism of opening an epoxide in basic conditions versus the acidic conditions.

For the epoxide opening in base; as you know sterics wins, and in acid the stability of the reactive transition state is our reasoning for the product observed. I believe for the NGP reactivity that stabilization of transition states is an acceptable platform to reason the important question as to exactly how this reaction happens in the first place.

lets look at the NGP example you've pulled from Wikipedia
(Awesome btw! reading organic chem on wiki for fun?!)

I have a hard time fully following your question but here's the best answer I could come up with:

I see a symmetrical transition state where the reactive-intermediate charged cyclopropyl-S-heterocycle can probably just be attacked from either side in a SN2 like fashion. If you prefer charge polarization to be our main reasoning in the S-hetero cycle, given that it is a symmetrical substrate I believe the same product will form either way due to symmetry.

Did you read the part of that wiki article below that depicts NGP reactivity by an Alkene? The driving factor there appears to be pi-stabilization of the carbocation in the transition state.

This is a physical organic chemistry question that is not trivial. I applaud your curiosity.

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  • $\begingroup$ Thanks for the answer. The picture I used was just to show the similarity between epoxide and the intermediate formed here. I'm mainly interested to know which way the attack happens predominantly (to the least hindered site or the site which stabalizes the positive charge the best) because the products will vary in case of an unsymmetrical substrate. I'm not able to find much regarding this $\endgroup$
    – himanshu
    Apr 26 '19 at 0:38
  • $\begingroup$ Thank you for clarification! I wasn't sure if that exact substrate was what was throwing you off or what.. So I am not a published peer reviewed expert on NGP reactions - but I would cautiously advise that I believe that the charge stabilized product would be logically supported by inductive reasoning by comparing to epoxides. Sulfur and Oxygen both are chalcogens so they are similar electronic environments - I would imagine that their reactivity wouldn't be too far off either. What makes S so different than just being "fat oxygen"? Hope it helps - this is a great question. $\endgroup$
    – jbussing
    Apr 26 '19 at 0:49
  • $\begingroup$ @himanshu if the question is adequate, I would encourage you to accept it (click the check mark) $\endgroup$
    – A.K.
    Apr 26 '19 at 2:14

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