Insights into Pinacol Rearrangement: Oxidative versus Acid‐Catalyzed Mechanism

Abstract

Reactions of both diastereomers of 1,2‐bis(4‐methoxyphenyl)cyclohexane‐1,2‐diol (1 d) with tris(2,4‐dibromo‐phenyl)aminium hexachloroantimonate as the oxidant in dichloromethane have been investigated to get evidence on the possibility that a pinacol rearrangement may be oxidatively activated rather than acid‐catalyzed. Relevant to this, we found that in the presence of an excess (4 eq) of oxidant, cis‐1 d afforded the rearrangement product 2,2‐bis(4‐methoxyphenyl)cyclohexanone in large amounts, while trans‐1 d gave exclusively the oxidative cleavage product 1,6‐bis(4‐methoxyphenyl)hexane‐1,6‐dione. Stereospecific implications explaining the reactivity difference between the two diastereomeric radical cation intermediates, cis‐ and trans‐1 d⋅+, are discussed.