szigmatropikus

Szigmatropikus refers to a class of pericyclic reactions involving the concerted migration of a sigma bond. In these reactions, a sigma bond cleaves on one atom within a conjugated pi system and a new sigma bond forms on another atom of the same pi system. The pi system itself is conserved throughout the reaction.

These reactions are characterized by the movement of a sigma bond through a conjugated system of pi electrons. The migratory group, which is attached via a sigma bond, moves across the conjugated system. The number in the term "szigmatropikus" indicates the number of atoms the sigma bond travels across. For example, a [3,3]-szigmatropikus rearrangement, such as the Claisen rearrangement, involves the sigma bond moving across three atoms on one side and three atoms on the other side of the transition state. Similarly, a [1,5]-szigmatropikus rearrangement involves the sigma bond moving across one atom and then five atoms.

Szigmatropikus rearrangements are typically photochemical or thermally induced. The stereochemistry of these reactions is often predictable based on orbital symmetry rules, such as the Woodward-Hoffmann rules. These rules dictate whether a reaction will proceed thermally or photochemically. Common examples include the Cope rearrangement, Claisen rearrangement, and the Wittig rearrangement. These reactions are important in organic synthesis for the construction of complex molecules.