A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the Cope rearrangement of 1,5‐hexadiene and its cyano derivatives

A novel interpretation of the chameleonic and centauric models for the Cope rearrangements of 1,5‐hexadiene ( A ) and different cyano derivatives ( B : 2,5‐dicyano, C : 1,3,4,6‐tetracyano, and D : 1,3,5‐tricyano) is presented by using the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT) on the reaction paths calculated at the B3LYP/6‐31G(d,p) level. The progress of the reaction is monitorized by the changes of the ELF structural stability domains (SSD), each being change controlled by a turning point derived from CT. The reaction mechanism of the parent reaction A is characterized by nine ELF SSDs. All processes occur in the vicinity of the transition structure and corresponding to a concerted formation/breaking of C 1 C 6 and C 3 C 4 bonds, respectively, together with an accumulation of charge density onto C 2 and C 5 atoms. Reaction B presents the same number of ELF SSDs as A, but a different order appears; the presence of 2,5‐dicyano substituents favors the formation of C 1 C 6 bonds over the breaking of C 3 C 4 bond process, changing the reaction mechanism from a concerted towards a stepwise, via a cyclohexane biradical intermediate. On the other side, reaction C presents the same type of turning points but two ELF SSD less than A or B ; there is an enhancement of the C 3 C 4 bond breaking process at an earlier stage of the reaction by delocalizing the electrons from the C 3 C 4 bond among the cyano groups. In the case of competitive effects of cyano subsituents on each moiety, as it is for reaction D , seven different ELF SSDs have been identified separated by eight turning points (two of them occur simultaneously). Both processes, formation/breaking of C 1 C 6 and C 3 C 4 bonds, are slightly favored with respect to the parent reaction ( A ), and the TS presents mixed electronic features of both B and C . The employed methodology provides theoretical support for the centauric nature (half‐allyl, half‐radical) for the TS of D . © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1427–1437, 2005