1,2-rearrangement

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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.

1,2-shift

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.

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 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.

Radical 1,2-rearrangements

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

Radical 1,2-rearrangement

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 ^[3].

An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds ^[4]. 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.

Aryl radical 1,2-shift in a helicene