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Thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially. Here the geometry of the transition state is prohibitive, but an alkyl group , due to the nature of its orbitals, can invert its geometry, form a new bond with the back lobe of its sp 3 orbital, and therefore proceed via a suprafacial shift.
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 ...
The actual mechanism of alkyl groups moving, as in Wagner–Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple discrete electron transfers. It is, nevertheless ...
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.
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.
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.
The attention directed towards the vinylcyclopropane rearrangement by Woodward and Hoffmann as a representative example for [1,3]-carbon shifts clearly enhanced the interest in this reaction. Furthermore, their analysis revealed potential experiments that would allow to distinguish between a concerted or stepwise mechanism.
The Arndt-Eistert homologation is a specific example of this use, wherein a carboxylic acid may be elongated by a methylene unit. Another common use is in ring-contraction methods; if the α-diazo ketone is cyclic , the Wolff rearrangement results in a ring-contracted product.