On the Claisen Rearrangement Mechanism

It is believed that the key stage of the reaction is a concerted [3,3] sigmatropic shift accompanied by inversion of the allyl fragment [3]. However, there are data that the thermal ortho rearrangement can produce alkenylphenols whose structure indicates the lack of inversion of the allyl fragment ( α rearrangement) [4]. The reaction products in some instances involve dialkenylphenols [5, 6], and their formation is explained by intermolecular reactions via dissociation of C–O bond. para -Substituted phenols are assumed to result from ortho migration at the first reaction stage and allyl group migration through a new cyclic transition state to the para position at the second stage [3]. A dissociative mechanism for the formation of these products has also been supposed [5]. Thus, there some contradictions in the notion on the Claisen rearrangement mechanism.