WagnerMeerweinomarrangemang

The Wagner–Meerwein rearrangement is a class of carbocation-mediated 1,2-shifts in which a migrating group—usually an alkyl or aryl group, or a hydride—moves to the adjacent carbocation center. This migration results in a new carbocation at the previous location and often leads to a rearranged, more stable cation that can be trapped by a nucleophile, eliminated to form an alkene, or undergo further transformations. The reaction is named after Wilhelm Wagner and Hans Meerwein, who contributed to its early elucidation in the 20th century.

Mechanistically, the process proceeds through a 1,2-shift: the migrating group accompanies its bonding electrons to the cationic center, while the positive charge shifts to the donor carbon. The outcome is a structural rearrangement of the substrate. Migrating groups can include alkyl, aryl, or hydride, with hydride shifts frequently occurring when they yield a more stable carbocation. The rearrangement is common under acidic or solvolytic conditions and can occur in various phases, including solutions and enzyme- or catalyst-assisted environments.

Scope and significance, the rearrangement helps explain many natural and synthetic processes. It plays a key role in terpenoid biosynthesis, carbocation rearrangements observed during solvolysis of tertiary halides, and ring-expansion or contraction events in cyclic systems. In synthetic organic chemistry, the Wagner–Meerwein rearrangement is exploited to access rearranged skeletal frameworks that are otherwise challenging to prepare. Related concepts include the semi-pinacol rearrangement, which involves neighboring group migration adjacent to a cationic center and shares mechanistic features with Wagner–Meerwein processes.

History and naming: the term covers a family of related 1,2-shifts rather than a single reaction, reflecting contributions from Wagner and Meerwein to carbocation rearrangement theory.