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 the mechanism of isoborneol to camphene

After the formation of carbocation, I was confused why would the reaction not go in favor of forming bornylene. I am still not sure why but I think that $\mathrm{sp^2}$ hybridized carbon will cause a lot of strain in the ring. Another thing confuses me, why does methyl not shift in place of the ring itself breaking? Is this anywhere similar to Bredt's rule? Please help me with these two doubts.

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  • $\begingroup$ There is no alkyl shift here; the carbocation is inherently non-classical in nature. Note how the “shift” is drawn using resonance arrows <->, not a reaction arrow ->. Google ‘norbornyl cation’ for more info. $\endgroup$
    – orthocresol
    Jan 18 '20 at 20:42
  • $\begingroup$ In addition, the bond that does migrate has good overlap with the initially formed carbocation while the methyl C-C sigma bond is essentially orthogonal to the carbocation p-orbital. $\endgroup$
    – user55119
    Jan 18 '20 at 22:59
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This is a class of carbocation 1,2-rearrangement reactions, and historically named Wagner–Meerwein rearrangement after two chemists who discovered the reaction (see following diagram):

Classical & Nonclassical Carbocations

Although it can be explained considering as an rearrangement of a classical carbocation, there are a tremendous amount of work has been done considering 2-norbornene cation as non-classical carbocation (see comment of orthocresol elsewhere). However, my interned task is to explain why it is not possible to have 1,2-methide shift instead of alkyl shift (Wagner–Meerwein rearrangement) considering classical carbocation.

First, why the reaction not go in favor of forming bornylene? Since the reaction is in acid medium, fast equlibrium of protonation (to give initial 2-norbonyl carbocation derivative) and deprotonation (to give bornylene derivative) could be expected until most thermodynamically stable product is formed. This can be expected from a $2^\circ$-carbocation, if there is a possibility of rearrangement to a more stable $3^\circ$-carbocation.

Now, the initial 2-norbonyl carbocation derivative is $2^\circ$-carbocation with a possibility of rearranging to a more stable $3^\circ$-carbocation by either 1,2-methide shift or alkyl shift (which is the one eventually happens). If you look at initial carbocation without the bridge-head, the bottom cyclohexyl part is in boat confirmation, which is locked in by the rigid bridge-head carbon. Now that restricts the 1,2-methide migration, which would made energetically unfavored triangularpyramidal $3^\circ$-carbocation. Thus, the readily available alkyl migration is energetically favored and fast. The rearanged crbocation would readily dehyrogenate to give camphene (2,2-dimethyl-3-methylidenebicyclo[2.2.1]heptane).

Also keep in mind that not only it is not favored by the steric resons, but also 1,2-hydride shift would generate a $3^\circ$-carbocation, which would have violate Bredt's rule.

Note: Nonetheless, a recent publication has discussed this scenario and 1,3-hydride migration to give 7,7-dimethyl-2-methylidenebicyclo[2.2.1]heptane as elimination product (Ref.1).


References:

Huidong Zheng, Yi Lin, Mengchan Wang, Jie Liu, Dan Wu, Jingjing Chen, Guanghua Yin, S. Ted Oyama, Suying Zhao, “The influence of solvent polarity on the dehydrogenation of isoborneol over a $\ce{Cu/ZnO/Al2O3}$ catalyst,” Catalysis Today 2019, 323, 44-53 (https://doi.org/10.1016/j.cattod.2018.10.021).

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