Wagner-Meerwein Rearrangement Origin
The discovery of the Wagner-Meerwein rearrangement is closely tied to terpene chemistry: it was first observed during studies of bicyclic terpenes, such as the conversion of isoborneol to camphene. Georg Wagner (1899) and Hans Meerwein (1922) each made key contributions to this class of carbocation 1,2-shift reactions, and the rearrangement is named after them.
Wagner-Meerwein Rearrangement Overview
The Wagner-Meerwein rearrangement uses catalytic acid to convert an alcohol into an olefin. The alcohol is protonated and released as water to form a carbocation. A [1,2]-shift of an adjacent carbon-carbon bond generates a more stable carbocation, followed by loss of a proton to afford the alkene. Both E/Z products are possible in some cases.
- Reagents: Catalytic Acid
- Reactant: Alcohol
- Product: Olefin
- Type of Reaction: Carbocation Rearrangement
- Bond Formation: C=C
Lab Tips
- The initial carbocation can be generated via treatment of an alkyl halide with a Lewis acid, solvolysis of a secondary or tertiary alkyl halide, protonation of an alkene, etc.
- The 【1,2】-shift is not limited to alkyl shifts. Aryl or hydride shifts are also possible.
- Several consecutive 【1,2】-shifts may be necessary to generate the most stable carbocation.
- Several products may arise if more than one rearrangement pathway is possible.
- The stereochemistry of the migrating group is retained (Woodward-Hofmann rules).
- A nucleophile present in the reaction mixture (i.e. the solvent or the conjugate base of the acid used to promote the rearrangement) may capture the carbocation forming a substituted product instead.
Kürti, L., Czakó, B. (2005). Strategic Applications of Named Reactions in Organic Synthesis; Background and Detailed Mechanisms. Burlington, MA: Elsevier Academic Press.
Mechanism
Step 1: Formation of the carbocation
The starting material (an alcohol, alkyl halide, or alkene) undergoes acid-catalyzed loss of a leaving group (such as water or a halide ion), or is activated by protonation/ a Lewis acid, to generate the initial carbocation.
Step 2: [1,2]-Shift
An alkyl group, aryl group, or hydrogen atom on the adjacent carbon migrates with its bonding electron pair to the carbocation center, accompanied by cleavage of the original carbon–carbon bond to the migrating group. This generates a new, generally more stable carbocation at the adjacent position. The driving force is the formation of a more stable carbocation (e.g., from a secondary to a tertiary carbocation); in some cases, multiple successive shifts are required to reach the most stable state.
Step 3: Deprotonation or nucleophilic capture
The new carbocation can lose a proton to form an alkene, or it can be captured by a nucleophile present in the reaction system (such as the solvent or a counterion) to give a substituted product.
Top Citations
Original Paper
Related Reactions
Related Compounds
Catalytic Acid
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