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Photoirradiation would require an antarafacial shift of hydrogen. Although rare, there are examples where antarafacial shifts are favored: [5] Antarafacial [1,5] hydride shift. In contrast to hydrogen [1,5] shifts, there have never been any observed [1,5] alkyl shifts in an open-chain compound. [4]
The actual mechanism of alkyl groups moving, ... Examples are hydride shifts. and the Claisen rearrangement: [5] Olefin metathesis. Olefin metathesis is a formal ...
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 C 2 ...
When a pinacol is not symmetrical, there is a choice for which hydroxyl group will leave and which alkyl shift will occur. The selectivity will be determined by the stability of the carbocations. In this case although both choices are tertiary, the phenyl groups result in significantly higher stabilization of the positive charge through resonance.
A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. [1] [2] They can be described as cationic [1,2]-sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention.
Berson's classic (1967) example of a [1,3]-sigmatropic alkyl shift proceeding with stereochemical inversion (WH symbol [σ 2 a + π 2 s]) A [1,j]-sigmatropic rearrangement is also a two component pericyclic reaction: one component is the π-system, the other component is the migrating group.
1,2-Methyl shifts also occur in vinyl cations, and like 1,2-hydride shifts, they have higher activation barriers compared to their alkyl cation equivalents. In the protonation of alkynes, both 1,2-hydride and 1,2-methyl shifts may take place. The preference depends on the alkyl substituent since it will dictate the resulting allyl cation product.
A 1,2-Wittig rearrangement is a categorization of chemical reactions in organic chemistry, and consists of a 1,2-rearrangement of an ether with an alkyllithium compound. [1] The reaction is named for Nobel Prize winning chemist Georg Wittig.