WagnerMeerwein

Wagner–Meerwein rearrangement is a class of carbocation rearrangements in organic chemistry in which a neighboring group migrates to the carbocation center, causing a shift of the positive charge and a skeletal rearrangement of the molecule. The key step is a 1,2-migration of a substituent, typically an alkyl group or a hydride, from an adjacent carbon to the electron-deficient carbon. The result is a new carbocation, often more stable, which is then trapped by a nucleophile or deprotonated to furnish a rearranged product. This process is commonly observed after ionization steps such as solvolysis or dehydration and can lead to ring expansion or contraction in cyclic substrates as well as broader skeletal changes in acyclic systems.

In practice, Wagner–Meerwein rearrangements explain many rearrangements seen in natural product biosynthesis and synthetic organic chemistry, particularly in terpene and steroid frameworks. Variants of the mechanism include direct 1,2-hydride shifts and 1,2-alkyl shifts, with the driving force typically the formation of a more stable carbocation or the stabilization imparted by allylic or resonance effects.

Historical notes: The rearrangement is named after German chemists Victor Wagner and Hans Meerwein, who described this class of 1,2-shifts in the early 20th century. Since then, it has become a fundamental concept in carbocation chemistry and is frequently invoked to rationalize rearranged products encountered in both laboratory synthesis and natural product biosynthesis.