ijsigmatropic

ij-sigmatropic rearrangements, usually written as [i,j]-sigmatropic, are a class of pericyclic reactions in organic chemistry in which a sigma bond migrates across an adjacent or remote conjugated π-system. This migration reorders bonding without adding or removing atoms. The indices i and j indicate the span of the π-system that the migrating fragment traverses between the two ends of the sigma bond, and they determine the type of rearrangement. Common examples include [1,5]-sigmatropic shifts and the widely studied [3,3]-sigmatropic rearrangements.

Mechanistically, these processes are typically concerted, proceeding through cyclic transition states that couple sigma- and pi-electron reorganization. Orbital symmetry considerations, encapsulated in the Woodward-Hoffmann rules, govern whether a given [i,j]-sigmatropic rearrangement is allowed thermally or requires photochemical activation. In many cases the migrating fragment moves suprafacially on both sides, with stereochemical outcomes that can be predicted from the topology of the transition state.

Notable examples include the [3,3]-sigmatropic rearrangements known as the Cope rearrangement (in 1,5-dienes) and the Claisen rearrangement (in allyl vinyl ethers). The [1,5]-sigmatropic hydrogen shift is another classic case, enabling migration of a hydrogen across a five-atom π-system. More extended [i,j]-shifts occur in specialized substrates and are used strategically in synthesis.

Overall, ij-sigmatropic rearrangements are important tools in organic synthesis for constructing complex carbon frameworks and for rearranging skeletons in natural products. Limitations include reliance on favorable orbital overlap, substitution patterns, and competing reaction pathways, which can restrict scope and necessitate careful control of conditions.