The thio-Claisen rearrangement is the sulfur analogue of the Claisen rearrangement: a thermal or promoted [3,3]-sigmatropic reorganization of an allyl vinyl sulfide or a related allyl sulfur substrate. In the classical form, the six-atom cyclic transition structure reorganizes the allyl–sulfur and vinyl π bonds to create a new carbon–carbon σ bond and a homoallylic thiocarbonyl product ^[1]. Allyl aryl sulfides can undergo a related thio-Claisen process that transfers the allyl group to an ortho carbon of the aromatic ring and initially forms a dearomatized sulfur-containing intermediate, which then rearomatizes to an ortho-allyl thiophenol derivative ^[2]. The reaction is distinguished from the oxy-Claisen rearrangement by sulfur at the heteroatom position and from ionic allylic substitution by its concerted pericyclic bond topology. Substrate geometry and chair- or boat-like transition-state conformation govern regio- and stereochemical transfer.