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Throughout my journey in organic chemistry I’ve only seen ring expansions by a compound in order to attain more stability, but yesterday I thought about the following reaction.

enter image description here

I feel that ring contraction resulted in a more stable carbocation.

Can someone tell whether the idea behind this reaction is correct or not?

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  • $\begingroup$ "I feel that ring contraction resulted in a more stable carbocation" Why do you think so, btw? I can't directly infer your reasoning from the diagram you've supplied. Mind explaining? Thanks $\endgroup$ – Gaurang Tandon Apr 7 '18 at 15:19
  • $\begingroup$ chemistry.stackexchange.com/questions/19090/… $\endgroup$ – Ritwik Das Apr 7 '18 at 15:43
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Ring contraction can certainly occur in many cases where the stereochemistry compels the molecule to undergo that to form a more stable carbocation.

In your described case, theoretically, it looks fine, because of more stability due to non-classical resonance (sometimes called dancing resonance). But, if you want to perform that experimentally, the percentage yield may not be guaranteed. It is possible to form the desired carbocation, as it is thermodynamically more stable. But the formation step is expected to have a large rate constant. So, at normal temperature, it may be difficult to form. However, at higher temperature its yield may be increased.

An excellent example of ring contraction is observed to happen during the pinacol-pinacolone rearrangement of (1R,2R)-1,2-dimethylcyclohexane-1,2-diol.

In pinacol-pinacolone, to form a stable carbocation, generally, a methyl or a hydride shift occurs. But for this shift, the migratory group has to be in anti to the leaving group. In the taken compound, the methyl shift can't occur, as it is not in the anti position w.r.t the leaving group $\ce{H2O}$. But a more stable carbocation is formed adjacent to the $\ce{-OH}$ group (like this, $\ce{-C+H(Me) -OH}$), if the bond migration occurs, and due to this bond migration, a ring contraction occurs, and the product which is expected to form theoretically is also verified to be produced experimentally.

A possible mechanism for this beautiful reaction is shown below.

enter image description here

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  • $\begingroup$ Shouldn't the bond of the ring also be anti for migration to be possible? $\endgroup$ – FreakyLearner Apr 7 '18 at 8:03
  • $\begingroup$ How would you define a bond to be in anti w.r.t a group ?? $\endgroup$ – Soumik Das Apr 7 '18 at 8:56
  • $\begingroup$ It's not the bond that is migrating, it's the carbon atom of the cyclic ring. I was just saying that the carbon atom should also be anti to the -OH group for migration to be possible. I think in your particular example, an epoxide will form instead of the product you made. $\endgroup$ – FreakyLearner Apr 7 '18 at 10:32
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Your idea of ring contraction to give more stable carbocation is correct and similar contraction has observed before (see example 4 below):

Courtesy of Department of Chemistry, Michigan State University, East Lancing, MI: link to webpage

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