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Organic Chemistry

30.7 Sigmatropic Rearrangements

Organic Chemistry30.7 Sigmatropic Rearrangements

30.7 Sigmatropic Rearrangements

30.7 • Sigmatropic Rearrangements

A sigmatropic rearrangement, the third general kind of pericyclic reaction, is a process in which a σ-bonded substituent atom or group migrates across a π electron system from one position to another. A σ bond is broken in the reactant, the π bonds move, and a new σ bond is formed in the product. The σ-bonded group can be either at the end or in the middle of the π system, as the following [1,5] and [3,3] rearrangements illustrate:

The notations [1,5] and [3,3] describe the kind of rearrangement that is occurring. The numbers refer to the two groups connected by the σ bond in the reactant and designate the positions in those groups to which migration occurs. For example, in the [1,5] sigmatropic rearrangement of a 1,3-diene, the two groups connected by the σ bond are a hydrogen atom and a pentadienyl group. Migration occurs to position 1 of the H group (the only possibility) and to position 5 of the pentadienyl group. In the [3,3] rearrangement of an allylic vinylic ether, the two groups connected by the σ bond are an allylic group and the vinylic ether. Migration occurs to position 3 of the allylic group and also to position 3 of the vinylic ether.

Like electrocyclic reactions and cycloadditions, sigmatropic rearrangements are controlled by orbital symmetries. There are two possible modes of reaction: migration of a group across the same face of the π system is suprafacial, and migration of a group from one face of the π system to the other face is antarafacial (Figure 30.12).

Figure 30.12 Suprafacial and antarafacial sigmatropic rearrangements.

Both suprafacial and antarafacial sigmatropic rearrangements are symmetry-allowed, but suprafacial rearrangements are often easier for geometric reasons. The rules for sigmatropic rearrangements are identical to those for cycloaddition reactions (Table 30.3).

Table 30.3 Stereochemical Rules for Sigmatropic Rearrangements
Electron pairs (double bonds) Thermal reaction Photochemical reaction
Even number Antarafacial Suprafacial
Odd number Suprafacial Antarafacial
Problem 30-7

Classify the following sigmatropic reaction by order [x,y], and tell whether it will proceed with suprafacial or antarafacial stereochemistry:

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