a sigmatropic reaction in organic chemistry is a pericyclic reaction. a sigmatropic reaction does not use a catalyst and involves a single molecule (an uncatalyzed intramolecular process). it changes one σ-bond into a different σ-bond. the name "sigmatropic" is the result of a compounding of the long-established "sigma" name for single carbon-carbon bonds and the greek word "tropos", meaning turn. this is a rearrangement reaction which means that the bonds in a molecule shift between atoms without any atoms leaving or new atoms added to the molecule. in a sigmatropic reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. true sigmatropic reactions usually do not need a catalyst. some sigmatropic reactions are catalyzed by a lewis acid. sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. the most well-known of the sigmatropic rearrangements are the [3,3] cope rearrangement, claisen rearrangement, carroll rearrangement and the fischer indole synthesis.