Pericyklisenä

Pericyklisenä is a Finnish term that translates to "pericyclic" in English. It refers to a type of chemical reaction that involves a cyclic transition state where bonds are rearranged simultaneously in a concerted manner. These reactions are characterized by the movement of electrons in a closed loop, typically involving pi systems. Pericyclic reactions are often photochemical or thermal, meaning they can be initiated by light or heat, respectively.

A key principle governing pericyclic reactions is the Woodward-Hoffmann rules, which predict the stereochemical outcome and reactivity of these transformations based on the symmetry of the molecular orbitals involved. These rules distinguish between thermally allowed and photochemically allowed pericyclic reactions. For example, a [4+2] cycloaddition like the Diels-Alder reaction is thermally allowed, proceeding readily with heat. In contrast, a [2+2] cycloaddition is thermally forbidden but photochemically allowed.

Common examples of pericyclic reactions include cycloadditions (such as Diels-Alder and [2+2] cycloadditions), electrocyclic reactions (where a ring opens or closes), and sigmatropic rearrangements (where a sigma bond migrates across a pi system). These reactions are fundamental in organic synthesis, allowing for the formation of new rings and carbon-carbon bonds with predictable stereochemistry. Their concerted nature often leads to high selectivity and atom economy.