1,2-rearrangement

From Wikipedia, the free encyclopedia

A 1,2-rearrangement or 1,2-shift or Whitmore 1,2-shift ^[1] is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. In the example below the substituent R moves from carbon atom C¹ to C².

The rearrangement is intramolecular and the starting compound and reaction product are structural isomers. The 1,2-rearrangement belongs to a broad class of chemical reactions called rearrangement reactions.

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 a 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 S_N1 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.

An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds ^[2]. 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/mole). In alkene radicals proton abstraction to an alkyne is preferred.

[edit] 1,2 Rearrangements

The 1,2-rearrangement mechanism is found in the

• Wagner-Meerwein rearrangement
• Pinacol rearrangement
• Hofmann rearrangement
• Curtius rearrangement
• Lossen rearrangement
• S_N1 reaction (generally)
• Halogen dance rearrangement
• 1,2-Wittig rearrangement
• Beckmann rearrangement
• Fritsch-Buttenberg-Wiechell rearrangement
• Criegee rearrangement
• Dowd-Beckwith ring expansion reaction
• Brook rearrangement
• Benzilic acid rearrangement
• Favorskii rearrangement
• Wolff rearrangement
• Stevens rearrangement

[edit] 1,3-Rearrangements

1,3-rearrangements take place over 3 carbon atoms. Examples:

• the Fries rearrangement
• a 1,3-alkyl shift of verbenone to chrysanthenone

[edit] References

1. ^ Whitmore, Frank C. (1932). "The common basis of molecular rearrangements". J. Am. Chem. Soc. 54 (8): 3274–3283. DOI:10.1021/ja01347a037.
2. ^ Brooks, Michele A., Lawrence T. Scott (1999). "1,2-Shifts of Hydrogen Atoms in Aryl Radicals". J. Am. Chem. Soc. 121 (23): 5444–5449. DOI:10.1021/ja984472d.