Comment on the "Nature of bonding in the thermal cyclization of (Z)-1,2,4,6-heptatetraene and its heterosubstituted analogues".

ReceiVed: May 7, 2004; In Final Form: September 24, 2004 In a recent paper by Chamorro and Notario (CN),1 the pericyclic or pseudopericyclic character of the thermal cyclization of (Z)-1,2,4,6-heptatetraene (C) and its heterosubstituted analogues (A and B) (see Scheme 1) was discussed by means of a topological electron localization function (ELF) analysis applied to the corresponding transition states (TSs). The aim of this work was to explore whether an ELF can provide further insight in the context of a recent controversy2-5 concerning the nature of the TSs involved in reactions A and B. The authors concluded that their ELF results, which were mainly based on the examination of electron density fluctuation, provide proof in support of a single disrotatory pericyclic bond interaction (mechanism 1 in Scheme 1) for reactions A, B, and C. In our opinion, the analysis of electron density fluctuation used to reach this conclusion is not solid enough to warrant such an assertion. To make our point, we have optimized the TS and computed the ELF (at the same B3LYP/6-31+G(d) level of theory using the HF/6-31G** optimized geometry of the TS and with the same programs and visualization tools used in the work by CN1) for a well-established pseudopericyclic reaction (mechanism 2 in Scheme 1), namely, the cyclization of 5-oxo2,4-pentadienal to pyran-2-one (reaction D in Scheme 1), that was not considered in the study of CN.1 The TS of reaction D was optimized at the HF/6-31G** level (its geometry can be found in the Supporting Information) because both the B3LYP/ 6-31G** (see ref 5) and B3LYP/6-31+G(d) methods give a reaction path without an energy barrier. This TS was characterized by the existence of a unique imaginary frequency corresponding to the ring closure, as well as by computing the intrinsic reaction path from the TS going downhill to reactants and products in mass-weighted Cartesian coordinates. Even though the original discussion was focused on the three reactions studied by CN,1-4 Rodrı́guez-Otero et al.5 introduced this fourth reaction, which is of great importance because it allows the comparison of the controversial A and B reactions with a reaction that has an unquestionable single disrotatory pericyclic TS (reaction C) and another that has an undeniable pseudopericyclic nature (reaction D). CN1 gave basically three reasons to assign a pericyclic nature to reactions A and B: (i) the lone pair of N in TS A or O in TS B has a low contribution to the whole process, so that it can be considered just a mere stabilization factor; (ii) the populations and fluctuations of the basins resemble those expected in pericyclic reactions; and more importantly, (iii) the measures of cyclic electron density fluctuations from one basin to its contiguous basins, following a clockwise or anticlockwise circulation around the ring reaction center, are essentially the same for reactions A, B, and C. Taking into account the results obtained for reaction D listed in Table 1, these are our remarks to the aforementioned three arguments: (i) In the pseudopericyclic reaction D, the contribution of the basin V(C2,C3)6 to the total electron delocalization of the O7 lone pair basin facing the C2 atom (basin number 10 in Table 1) is 3.4% (3.3-3.7%), which is even lower than the contributions of the basin V(C2,C3) for the equivalent electron pair delocalization in reactions A, B, and C.1 Therefore, a low contribution of the basin V(C2,C3) to the heteroatom electron lone pair delocalization is not a conclusive indication that the lone pair plays a mere stabilization role and that, as a consequence, the reaction is pericyclic.