15sigmatropic

15sigmatropic rearrangement is the informal name for what is correctly called a 1,5-sigmatropic rearrangement, a class of pericyclic reactions in organic chemistry. In these reactions a sigma bond migrates across a five-atom, conjugated chain (a pentadienyl fragment), moving from one terminus of the system to the other. The migrating substituent can be a hydrogen atom or another substituent such as an alkyl or aryl group, and the reaction often proceeds intramolecularly.

Mechanism and stereochemistry: The process is typically concerted, occurring through a cyclic, six-electron transition state. According to the Woodward–Hoffmann rules, when the migrating group and the pi system have odd i and j values (as in 1,5-sigmatropic shifts), the thermally allowed pathway is usually suprafacial on both ends. Photochemical activation can alter the allowedness or stereochemical outcome in some systems. The rearrangement results in a new sigma bond between the migrating group and the far end of the pentadienyl unit, accompanied by a reorganization of the adjacent pi system.

Scope and applications: 1,5-sigmatropic rearrangements occur in a wide range of substrates containing allylic or pentadienyl motifs. The reaction can involve hydrogen shifts or the migration of alkyl, aryl, or heteroatom substituents. Rates and selectivity depend on substitution pattern, stabilization of the conjugated system, and, in some cases, external catalysts or chiral environments.

Relation to other sigmatropic processes: This rearrangement is part of a broader family that includes the Claisen ([3,3]-) and Cope ([3,3]-) rearrangements, distinguished by the i and j values of the migrating fragment.