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This is a rxn in the synthesis of Tramadol. I found in in IChO prep problem 2001, Q19...

Edit: I have seen in clayden a rxn with HSbF6 whereby the benzene ring gets protonated so I followed that path of thinking and got stuck. Thanks for the answers and enlightenment!

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Fluoroantimonic acid - $\ce{HSbF6}$ is made by combining $\ce{HF}$ and $\ce{SbF5}$. In this system, $\ce{HF}$ releases its proton ($\ce{H+}$) concomitant with the binding of $\ce{F-}$ by antimony pentafluoride, which is a Lewis acid. The resulting anion ($\ce{SbF6-}$) is both a weak nucleophile and an extraordinarily weak base.

Two things to note are:

  1. $\ce{HSbF6}$ is a very strong acid.
  2. Its conjugate base is a very poor nucleophile.

The hydroxyl group of phenol attacks $\ce{H+}$ and makes phenylhydronium ion which would behave like a nitro group. Therefore, the substitution would occur preferably on the 3 position.

Since $\ce{HSbF6}$ is such a strong acid, you can safely assume that virtually all of the phenol is protonated all the time.

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