enter image description here

My attempt: I went ahead with an $\mathrm{S_N1}$ type reaction followed by the attack of $\ce{OH-}$ on the 3 degree carbocation formed.

This does not match with the answer given (see image below), in which they have first formed a cumulated diene with the $\ce{OH}$ on the benzylic position. This enol tautomerizes to give an α,β-unsaturated ketone.

enter image description here

The aryl-vinyl carbocation should be more unstable than a 3 degree carbocation with 6 α-hydrogens, so I don't understand why the major product is not formed by the attack of $\ce{OH-}$ on the 3 degree carbocation. What's the proper mechanism here?

  • $\begingroup$ Consider an SN2 process with OH attack at the benzylic position. Is the benzylic position less sterically crowded than the tertiary centre? $\endgroup$
    – Waylander
    Oct 27 at 15:15
  • 1
    $\begingroup$ But why can't we consider SN1 mechanism? Br- is a moderate leaving group and we are getting a 3° carbocation. Also we are getting a unstable cummulated Diene in the SN2 one which is unstable though it does get converted keto form but isn't the first reasoning better? Am I ignoring something important? $\endgroup$ Oct 27 at 17:47
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    $\begingroup$ I don't think it is possible to assign one mechanism over the other. Both are possible pathways and both probably occur. What you cannot dispute is the identity of the product. Remember, all the mechanisms we draw for these reactions are only models. $\endgroup$
    – Waylander
    Oct 27 at 18:17
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    $\begingroup$ Related: en.wikipedia.org/wiki/Meyer%E2%80%93Schuster_rearrangement $\endgroup$ Oct 29 at 3:36
  • $\begingroup$ Thanks! That was helpful! $\endgroup$ Oct 30 at 13:44

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