sigmatropiske

Sigmatropic is a term used in organic chemistry to describe a concerted pericyclic reaction where a sigma bond migrates across a conjugated pi system. The numbering of the sigmatropic shift refers to the positions in the conjugated system between which the sigma bond migrates. For example, in a [3,3]-sigmatropic rearrangement, a sigma bond moves from the third atom of one pi system to the third atom of another, involving a six-membered cyclic transition state. These reactions are characterized by the simultaneous breaking of one sigma bond and the formation of another, with no intermediate species.

Sigmatropic rearrangements are often driven by the formation of a more stable product or the relief of steric strain. Common examples include the Claisen rearrangement and the Cope rearrangement, both of which are [3,3]-sigmatropic shifts. These reactions are important tools in organic synthesis, allowing for the efficient formation of new carbon-carbon bonds and the construction of complex molecular architectures. The stereochemistry of sigmatropic rearrangements is often predictable and governed by orbital symmetry rules, such as the Woodward-Hoffmann rules. These rules dictate whether a reaction will proceed in a thermally or photochemically allowed manner. Understanding sigmatropic rearrangements is crucial for predicting reaction outcomes and designing synthetic strategies.