3
$\begingroup$

Predict major product:

cyclobutyl(cyclopropyl)methanol + H+

In this question first I've protonated the OH group and then water is removed to form a carbocation. Now I have to expand the ring, but I'm confused which ring to choose: 3-membered ring or 4-membered ring?

enter image description here

Note: I can make an alkene (which would be the final product of the reaction) by removing an alpha hydrogen if it is known which ring to expand.

Improve this question
$\endgroup$
2
  • $\begingroup$ Perhaps there is more than one rearrangement? Could both rings expand? $\endgroup$
    – Michael Lautman
    May 30, 2019 at 12:09
  • $\begingroup$ I think there will be two separate ring expansion products, but the major product will be the expansion of the cyclopropyl group. The reason ring expansions occur at all, is to relieve ring strain. The farther away you are from a hexagon, the more strain there will be. $\endgroup$
    – Argento
    May 30, 2019 at 13:23

4 Answers 4

Reset to default
2
$\begingroup$

It is clear to me that no matter which ring you open the first, you'd get the same product as A. Michael Lautman gave the correct product , first opening the cyclopropyl ring. Michael's mechanism is fair, but I'm reluctant to say it is the most preferable path because it gives you bicyclobutyl $2^\circ$ carbocation as an intermediate, which needed extra 1,2-hydride shift to get stabilized as $3^\circ$ carbocation (total of 5 steps). However, if you have opened the cyclobutyl ring first, you'd get the same final product A, through a relatively low energy cyclopropylcyclopentane carbocation (total of 5 steps) even though it is also needed a 1,2-hydride shift (in relatively more stable cyclopentyl $2^\circ$ carbocation when compared to $2^\circ$ carbocation in cyclobytyl ring in Michael's mechanism) to get stabilized as $3^\circ$ carbocation. Complete mechanism is depicted below:

cyclobutyl(cyclopropyl)methanol

This mechanism is supported by following reference:

  • G. K. Surya Prakash, V. Prakash Reddy, G. Rasul, J. Casanova, G. A. Olah, “The Search for Persistent Cyclobutylmethyl Cations in Superacidic Media and Observation of the Cyclobutyldicyclopropylmethyl Cation,” J. Am. Chem. Soc. 1998, 120(51), 13362–13365 (https://doi.org/10.1021/ja9828962).
Improve this answer
$\endgroup$
1
$\begingroup$

How about this:

  1. Ring expansion of the cyclopropyl group
  2. 1,2-hydride shift
  3. Expansion of the cyclobutyl group
  4. Expansion of the "new" cyclobutyl group
  5. Elimination

Carbocation Rearrangement

Improve this answer
$\endgroup$
3
  • 1
    $\begingroup$ The first two structures aren't really distinct. $\endgroup$
    – Zhe
    May 31, 2019 at 1:02
  • $\begingroup$ @Zhe can you elaborate please? The first structure is the initial carbocation formed in the reaction. The second has undergone a ring expansion. If you are going to downvote an answer, it would be nice if you provided some constructive feedback. $\endgroup$
    – Michael Lautman
    May 31, 2019 at 1:33
  • 1
    $\begingroup$ Cyclopropylcarbinyl, aka homoallyl, aka cyclobutyl cations are generally thought to be non-classical instead of distinct species. $\endgroup$
    – Zhe
    May 31, 2019 at 13:03
0
$\begingroup$

The $\ce{C4H7^+}$cation shown as the first entry in Scheme 1 is described as a bridged structure that is isomeric with cyclopropylmethyl and cyclobutyl ions.1

Scheme 1

enter image description here

The product following this pathway is shown in the post by Michael Lautman.

Second pathway is shown here in Scheme 2 with 2 in ring expansion to give2a .Next is elemination to give 3 and 4 .

Scheme 2

enter image description here

2a could undergo a hydride shift to 2c ,that on elemination gives third product 5.

enter image description here

Referance

1 Advanced Organic Chemistry FIFTH EDITION

Part A: Structure and Mechanisms

FRANCIS A. CAREY and RICHARD J. SUNDBERG

Improve this answer
$\endgroup$
5
  • $\begingroup$ Cyclopropyl carbinyl is generally thought to be a nonclassical cation and not two isomeric forms. $\endgroup$
    – Zhe
    May 31, 2019 at 1:01
  • $\begingroup$ @Zhe i thought so too ,but its a direct quote from advanced organic chemistry by francis carey and sundberg $\endgroup$
    – Chakravarthy Kalyan
    May 31, 2019 at 3:13
  • $\begingroup$ Historically, there are opposing view points on this subject, which Carey/Sundberg does not capture. Cf. the excellent work of George Olah. $\endgroup$
    – Zhe
    May 31, 2019 at 13:06
  • $\begingroup$ @Zhe, I understand that, but I would have appreciated some insight into what the concept of the non classical carbocation would change about the product. $\endgroup$
    – Michael Lautman
    May 31, 2019 at 16:45
  • $\begingroup$ @MichaelLautman For one thing, you don't ever have to consider ring expansion of the cyclopropyl carbinyl cation; the non-classical view would be that it exists in an intermediate form. On first principle, the unknown regarding this mechanism is which steps are under thermodynamic versus kinetic control? $\endgroup$
    – Zhe
    May 31, 2019 at 17:43
-2
$\begingroup$

The cycloproyl ring may expand as it is more angle strained. So after ring expansion a comparatively more stable carbocation will be formed.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.