Sigmatropic rearrangements are a type of chemical reaction where a molecule undergoes a structural change involving the migration of a sigma bond and the reorganization of π bonds. This process typically occurs in a concerted manner, meaning that the bond-breaking and bond-forming steps happen simultaneously without intermediates. These rearrangements are classified based on the number of atoms involved in the migration, such as Cope rearrangement (a [3,3]-sigmatropic shift) and Claisen rearrangement (a [3,3]-sigmatropic shift involving allyl vinyl ethers). Sigmatropic rearrangements are important in organic synthesis for creating complex molecular structures efficiently.