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
|
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:
