In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, but only if a nonclassical ion is formed. So, does that happen?

The substrate is 8a-methyl-1,3,4,8a-tetrahydronaphthalen-4a(2⁠H)-ol.

  • $\begingroup$ If classical carbocation is already heavily stabilised, it would be surprising if additional "nonclassical" stabilisation would be added. In this case it's a difference between sigma and pi complex between substituted benzene ring and methyl group - it would be rather a pi complex then nonclassical carbocation, even if it was an actual intermediate here. $\endgroup$ – Mithoron May 29 '19 at 21:20

I don't think it is necessary to consider a mechanism with a nonclassical carbocation. It would undergo normal rearrangement with a 1,2-methide shift within the same ring. See the mechanism I posted in previous question you were directing to:

Rearrangement to Aromatic

Answer to this question would support for my mechanism.

  • 1
    $\begingroup$ So the answer seems to be "No". Fair enough. $\endgroup$ – Oscar Lanzi May 28 '19 at 21:32

While there are examples of 1,2-methide shifts, this question has an uncanny resemblance to the dienone-phenol rearrangement whose mechanism was first elucidated by Woodward and Singh in 1950. Dienone 1 under acidic conditions undergoes rearrangement to phenol 6 and not, based on earlier speculation, to phenol 4. The direct 1,2-methide shift (2b --> 3) does not occur but rather the reaction proceeds through the spiro carbocation 5. continued

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Applying this approach to the carbocation 7 generated from the alcohol in this question, spiro carbocation 8 forms the predicted tetrahydronaphthalene 10. One way to distinguish between the two mechanisms is via a labeling experiment. All of the carbon label in 7 will retain its location as the red star in 10 if the 1,2-methyl shift mechanism applies. The spiro mechanism will partition the label ~50:50 between the two ring benzylic carbons. For related studies, see reference 2.

1) R. B. Woodward and T. Singh, J. Am. Chem. Soc., 1950, 72, 494.
2) A. J. Waring, J. H. Zaidi and J. W. Pilkington, J. Chem. Soc, Perkin Transactions I, 1981, 1454.


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