Sigmatropic reaction

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A Sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one sigma bond changed to another sigma bond ^[1]. In this type of rearrangement reaction, a substituent moves from one part of a pi-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the pi system. True sigmatropic reactions are usually uncatalyzed, although lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions.

Sigmatropic rearragements are classified by the substituent that moves and the order of the rearrangement, which is given in brackets [i,j] with i and j the number of atoms that each sigma terminus has moved. For example, in a [1,5]hydride shift, a proton moves over 5 carbon positions.

The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangement, Claisen rearrangement, Carroll rearrangement and the Fischer indole synthesis. The Gassman indole synthesis contains a [2,3]sigmatropic rearrangement.

The hydride shifts and methyl shifts are also sigmatropic reactions. Hydride shifts occur at 4n+1 positions in a suprafacial fashion, Hydride shifts can also occur at 4n+3 positions in an antarafacial fashion, but this is not observed in the 3 case due to geometrical constraints.

[edit] MO picture

Most notably, cyclopentadiene undergoes a [1,5]hydride shift:

[1,3]Hydride shifts would be antarafacial and are not observed due to geometric constraints. Antarafacial [1,7]hydride shifts are observed in the conversion of lumisterol to vitamin D.

Methyl shifts occur at 4n+3 positions in a suprafacial fashion with inversion of stereochemistry: