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Phenyl has greater migratory power. However, why did methyl migrate in step 2 of the reaction?

  • $\begingroup$ What makes you think the reaction depicted is right? $\endgroup$
    – Zhe
    May 2 '18 at 15:25
  • 2
    $\begingroup$ ^ In other words, what is the source for this mechanism? Note that your thought may possibly be correct as well. $\endgroup$ May 2 '18 at 16:02
  • $\begingroup$ One might compare the stabilities of the cations formed from phenyl vs. methyl migration. $\endgroup$
    – user55119
    May 3 '18 at 16:37

In general, migratory aptitude of different groups follow following order: $\ce{-Ph} > \ce{-C(CH3)3} > \ce{-CH(CH3)2} > \ce{-CH2CH3} > \ce{-CH3} > \ce{-H}$

For example, 2,3-diphenylbutane-2,3-diol (1) was allowed to react under homogeneous (25 mol% of p-toluenesulfonic acid) or heterogenious (20 mol% of graphene oxide) catalytic conditions using toluene as a solvent at $\pu{100 ^\circ C}$. Under both conditions, only pinacol rearrangement with phenyl migration was observed affording 3,3-diphenylbutan-2-one (2) in a 90 and 95% conversion. Methyl migration has not been observed [Reference].



Pinacol Rearrangement and Direct Nucleophilic Substitution of Allylic Alcohols Promoted by Graphene Oxide and Graphene Oxide $\ce{CO2H}$: M. Gomez-Martinez, A. Baeza, D. A. Alonso, ChemCatChem, 2017, 9, 1032–1039 (https://doi.org/10.1002/cctc.201601362)


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