1,2-rearrangement

Reaction mechanism

A 1,2-rearrangement is often initialised by the formation of a reactive intermediate such as:

• a carbocation by heterolysis in a nucleophilic rearrangement or anionotropic rearrangement
• a carbanion in an electrophilic rearrangement or cationotropic rearrangement
• a free radical by homolysis
• a nitrene.

The driving force for the actual migration of a substituent in step two of the rearrangement is the formation of a more stable intermediate. For instance a tertiary carbocation is more stable than a secondary carbocation and therefore the SN1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether.

Carbocation rearrangements are more common than the carbanion or radical counterparts. This observation can be explained on the basis of Hückel's rule. A cyclic carbocationic transition state is aromatic and stabilized because it holds 2 electrons. In an anionic transition state on the other hand 4 electrons are present thus antiaromatic and destabilized. A radical transition state is neither stabilized or destabilized.

The most important carbocation 1,2-shift is the Wagner–Meerwein rearrangement. A carbanionic 1,2-shift is involved in the benzilic acid rearrangement.

Radical 1,2-rearrangements

The first radical 1,2-rearrangement reported by Heinrich Otto Wieland in 1911 was the conversion of bis(triphenylmethyl)peroxide 1 to the tetraphenylethane 2.

The reaction proceeds through the triphenylmethoxyl radical A, a rearrangement to diphenylphenoxymethyl C and its dimerization. It is unclear to this day whether in this rearrangement the cyclohexadienyl radical intermediate B is a transition state or a reactive intermediate as it (or any other such species) has thus far eluded detection by ESR spectroscopy.

An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds. The energy required in an aryl radical for the 1,2-shift can be high (up to 60 kcal/mol or 250 kJ/mol) but much less than that required for a proton abstraction to an aryne (82 kcal/mol or 340 kJ/mol). In alkene radicals proton abstraction to an alkyne is preferred.

1,2-Rearrangements

The following mechanisms involve a 1,2-rearrangement:

• 1,2-Wittig rearrangement
• Alpha-ketol rearrangement
• Beckmann rearrangement
• Benzilic acid rearrangement
• Brook rearrangement
• Criegee rearrangement
• Curtius rearrangement
• Dowd–Beckwith ring expansion reaction
• Favorskii rearrangement
• Friedel–Crafts reaction
• Fritsch–Buttenberg–Wiechell rearrangement
• Halogen dance rearrangement
• Hofmann rearrangement
• Lossen rearrangement
• Pinacol rearrangement
• Seyferth–Gilbert homologation
• SN1 reaction (generally)
• Stevens rearrangement
• Stieglitz rearrangement
• Wagner–Meerwein rearrangement
• Westphalen–Lettré rearrangement
• Wolff rearrangement

References

References

1. Whitmore, Frank C.. (1932). "The common basis of molecular rearrangements". [[Journal of the American Chemical Society]].
2. ''Über Triphenylmethyl-peroxyd. Ein Beitrag zur Chemie der freien Radikale'' Wieland, H. [[Chem. Ber.]] '''1911''', 44, 2550–2556. {{doi. 10.1002/cber.19110440380
3. ''Isomerization of Triphenylmethoxyl and 1,1-Diphenylethoxyl Radicals. Revised Assignment of the Electron-Spin Resonance Spectra of Purported Intermediates Formed during the Ceric Ammonium Nitrate Mediated Photooxidation of Aryl Carbinols'' K. U. Ingold, Manuel Smeu, and Gino A. DiLabio [[J. Org. Chem.]]; '''2006'''; 71(26) pp 9906–9908; (Note) {{doi. 10.1021/jo061898z
4. Brooks, Michele A.. (1999). "1,2-Shifts of Hydrogen Atoms in Aryl Radicals". Journal of the American Chemical Society.