“1,2-rearrangement”的意思、由来-开放百科全书

A 1,2-rearrangement or 1,2-migration 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 C² to C³.

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 rearrangement involving a hydrogen atom is called a 1,2-hydride shift. If the substituent being rearranged is an alkyl group, it is named according to the alkyl group's anion: i.e. 1,2-methanide shift, 1,2-ethanide shift, etc.

Reaction mechanism

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

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.

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/mol or 340 kJ/mol). In alkene radicals proton abstraction to an alkyne is preferred.

1,2 rearrangements

1,3-Rearrangements

References

1. ^{{cite journal | title = The common basis of molecular rearrangements | first = Frank C. | last = Whitmore | journal = J. Am. Chem. Soc. | year = 1932 | volume = 54 | issue = 8 | pages = 3274–3283 | doi = 10.1021/ja01347a037}}
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. ^{{cite journal | title = 1,2-Shifts of Hydrogen Atoms in Aryl Radicals | first = Michele A. | last = Brooks |author2= Lawrence T. Scott | journal = J. Am. Chem. Soc. | year = 1999 | volume = 121 | issue = 23 | pages = 5444–5449 | doi = 10.1021/ja984472d}}