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.

مواضيع ذات صلة

تفاعلات البروبان الحلقي

تفاعلات المركبات الحلقية

تحضير الهكسان الحلقي

تحضير البنتان الحلقي ومشتقاته

تحضير البيوتان الحلقي ومشتقاته

تحضير البروبان الحلقي ومشتقاته

Curtius Rearrangement

Hofmann Elimination

Hofmann rearrangement

Preparation of acyl chlorides

Synthesis of banana oil

Preparation of esters by Fischer esterification