Group Shifts

Two examples of 1,2-phenyl shifts are shown in the following diagram. In each case the driving force for the rearrangement is the conversion of a less stable anion into a more stable one. The reversible addition of hydroxide ion to one of the benzil carbonyl groups produces an intermediate which undergoes a pinacol-like rearrangement. In contrast to the carbocation "pull" that initiates the pinacol rearrangement, this benzilic acid rearrangement complements a weak electrophilic pull by the adjacent carbonyl carbon with the "push" of the alkoxide anion. A rapid proton transfer then forms the relatively stable carboxylate anion. In the second example, a very reactive 1º-carbanion (pK_a ≈ 45) is converted to a diphenyl resonance stabilized carbanion (pK_a ≈ 34).

Similar 1,2-shifts of hydrogen or alkyl groups may also be favored by loss of a stable anion, as illustrated by the following example. Once again, an alkoxide anion provides a "push", and loss of the stable tosylate leaving group terminates the rearrangement. The LiAlH₄ reagent not only generates the oxy-anion, but also reduces the resulting carbonyl products to alcohols. This rearrangement contrasts with the Wagner-Meerwein rearrangement in which a stable anion leaving group initiates the process by generating a carbocation species.