11 - Rearrangements

Abstract

A rearrangement may involve the one-step migration of an H atom or of a larger molecular fragment within a relatively short-lived intermediate. However, a rearrangement may be a multistep reaction that includes the migration of an H atom or of a larger molecular fragment as one of its steps. The Wagner–Meerwein rearrangement of a carbenium ion and the Claisen rearrangement of allyl alkenyl ethers exemplify a rearrangement of the first type. In many rearrangements, the migrating group connects to one of the direct neighbors of the atom to which it was originally attached. Rearrangements of this type are the so-called [1, 2]-rearrangements or [1, 2]-shifts. These rearrangements can be considered as sigma-tropic processes, the numbers “1” and “2” characterizing the subclass to which they belong. The adjective “sigma-tropic” emphasizes that a σ-bond migrates in these reactions. [1, 2]-Rearrangements of carbenium ions occur quantitatively only: if the new carbenium ion is substantially better stabilized electronically by its substituents than the old carbenium ion or if the new carbenium ion is substantially more stable than the old carbenium ion because of other effects such as reduced ring strain or if the new carbenium ion is captured in a subsequent, irreversible reaction.