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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 ...
[1] [2] They can be described as cationic [1,2]-sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention. As such, a Wagner–Meerwein shift is a thermally allowed pericyclic process with the Woodward-Hoffmann symbol [ω 0 s + σ 2 s]. They are usually facile, and in many cases, they can take place at temperatures ...
[1,7] sigmatropic shifts are predicted by the Woodward–Hoffmann rules to proceed in an antarafacial fashion, via a Mobius topology transition state. An antarafacial [1,7] shift is observed in the conversion of lumisterol to vitamin D 2, where following an electrocyclic ring opening to previtamin D 2, a methyl hydrogen shifts. [9]
In the similar substitution of 1-chloro-3-methyl-2-butene, the secondary 2-methyl-3-buten-2-ol is produced in a yield of 85%, while that for the primary 3-methyl-2-buten-1-ol is 15%. Allylic shifts occur because the transition state is an allyl intermediate. In other respects they are similar to classical nucleophilic substitution, and admit ...
A 1,2-rearrangement 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. Skeletal isomerization is not normally encountered in the laboratory, but is the basis of large applications in oil ...
[10] [11] This places the σ C–C orbital of the methyl group parallel with the σ* C–O orbital of the activated alcohol. Before the activated alcohol leaves as H 2 O the methyl bonding orbital donates into the C–O antibonding orbital, weakening both bonds. This hyperconjugation facilitates the 1,2-methyl shift that occurs to remove water.
Methyl shifts are observed in the addition of tert-butyl cation to but-2-yne. The pentaallyl cation that is formed could be the result of a single 1,3-methyl shift or two consecutive 1,2-methyl shifts. Rearrangement via the double bond could also change the size of a cyclic system.
This result suggests that oxonium ylide formation is reversible, but that the subsequent step is irreversible. The symmetry-allowed [2,3] sigmatropic rearrangement must follow a pathway that is lower in activation energy than the 1,4-methyl shift, explaining the exclusive formation of the desired product.