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While reading a post , i came across the following mechanism .(source : Which ring expansion in cyclobutyl(cyclopropyl)methanol is favourable?) enter image description here

The rearrangement is a series of shifts.

  • The first ring expansion is 1,2 alkyl shift.
  • The second ring expansion is 1,3 aalkyl shift.
  • Third step is again 1,2 Hydride shift.

I am aware of 1,2 shift but not not 1,3 shift.So I researched from various sources which are tabulated below .

  1. ORGANIC CHEMISTRY ( SECOND EDITION ) by Jonathan Clayden ,Nick Greeves, Stuart Warren .Quoting from page no. 942 "The methyl group can slide smoothly from one orbital to another—there are bonding interactions all the way. The next step, migration of H, is just the same—except that the HOMO is now a C–H σ bond.

enter image description here

enter image description here

enter image description here

in all the above , 1,2 shifts are seen.

  1. Advanced Organic Chemistry , FIFTH EDITION ,Part A: Structure and Mechanisms by Francis A. Carey and Richard J. Sundberg.

    Quoting from page no.446 it says

    "The preferred alignment of orbitals for a 1,2-hydride or alkyl shift involves coplanarity of the p-orbital at the carbocation ion center and the
    orbital of the migrating group "

enter image description here

1,2 shift pathway is seen.

  1. https://www.masterorganicchemistry.com/2012/08/15/rearrangement-reactions-1-hydride-shifts/

Quoting from the above link.

"Here are some examples of “allowed” rearrangement reactions. Notice how we’re always going from a less substituted carbocation to a more substituted carbocation. One exception is at the very bottom; the rearrangement is favorable because the new carbocation is resonance stabilized."

enter image description here

A rearrangement can occur to give the more stable tertiary carbocation.(involves 1,2 shift)

enter image description here

  1. https://www.masterorganicchemistry.com/2012/08/22/rearrangement-reactions-2-alkyl-shifts/

Quoting from the above link.The Mechanism Of Alkyl Migration Reaction is a 1,2 shift in two examples below.

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Ring-Expansion Reactions

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Therefor ,the above sources have all talked about 1, 2 shifts only .

So my question is , is 1,3 alkyl shift as given in the post (Which ring expansion in cyclobutyl(cyclopropyl)methanol is favourable?)

enter image description here allowed ?

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    $\begingroup$ This may be a stupid question, but have you checked the literature? And if you can't, can we get a volunteer? $\endgroup$
    – Zhe
    May 31, 2019 at 15:03
  • $\begingroup$ @ Zhe i donot have axcess to litrature as i have encountred a paywall.I needed to clarify regarding 1,3 shifts as shown.Then we may have to urge authors to rewrite books so that such information is accessable (imagine asking clayden to change).Please SHARE THAT INFORMATION.That should help every one $\endgroup$
    – Chakravarthy Kalyan
    May 31, 2019 at 15:22
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    $\begingroup$ That urging authors to rewrite part might be taking it too far. Just because a paper says something for limited scope, does not mean it is true everywhere. Hell, it doesn't even mean it's true at all. Also, this would be absolute hell for beginning students. $\endgroup$
    – Zhe
    May 31, 2019 at 15:35

1 Answer 1

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There may be 1,3-alkyl shifts but they are not needed here. Cation 1 should expand the more strained cyclopropane ring first to form cyclobutyl cation 2. After a 1,2-hydride shift to form a tertiary cation 3, the cyclobutane ring in 3 expands to form spirocyclopentyl cation 4. Another cyclobutane ring expansion of 4 leads to cation 5 and eventually to alkene 6.

enter image description here

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  • $\begingroup$ then the mechnism given in chemistry.stackexchange.com/questions/116143/… is wrong . $\endgroup$
    – Chakravarthy Kalyan
    May 31, 2019 at 14:02
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    $\begingroup$ I see my answer is the same as Lautman's, Zhe's comment notwithstanding. Didn't scroll down far enough. The point is why release strain from the cyclobutane ring first? $\endgroup$
    – user55119
    May 31, 2019 at 14:15
  • $\begingroup$ In the answer above, as well as my answer to the linked question, the strain is released from the cyclopropane first. $\endgroup$
    – Michael Lautman
    May 31, 2019 at 18:35
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    $\begingroup$ @Michael Lautman: I belatedly saw the duplication after I posted it. If you are concerned about the duplication, I can delete mine or link to your comment. $\endgroup$
    – user55119
    May 31, 2019 at 20:32

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