Computational study on the electrocyclic reactions of [16]annulene

The electrocyclic reactions of [16]annulene have been investigated by ab initio and DFT calculations. Among the six conformers of [16]annulene identified, 3, with Cs symmetry, is taken as the starting reactant for the cyclization reactions of [16]annulene, even though it is 31.4 kJ mol(-1) less stable than the most stable conformer, 1. The pathways of the electrocyclic reactions from reactant 3 to two tricyclic products, 5 and 6, have been found. All pathways identified are stepwise, i.e., the two ring closure processes occur one after the other. Among the pathways found, the ones with the lowest overall barrier for reactions 3 --> 5 and 3 --> 6 have the same rate-determining step and hence the same overall barrier, 131.0 kJ mol(-1). Thus, based on the barriers calculated, it is not possible to determine whether 5 or 6 is the dominant product in the cyclization reaction of 3.