Mechanistic pathways for ketene dimerization

A semiempirical MO method was used to calculate potential energy surfaces for ketene dimerization. One supra-supra and two supra-antara pathways were investigated. Six parameters were used to characterize the relative motion of individual groups. Two levels of approximation for the motion were considered. First, synchronous reaction pathways were followed, and a set of single-dimensional potential curves generated. Then, deviations from these synchronous reaction pathways were considered for each parameter and five two-dimensional potential surfaces for each pathway generated. Agreement with the available experimental data is satisfactory. However, we conclude that contrary to the Woodward-Hoffmann rules 1] the supra-supra pathway is allowed and the supra-antara pathways essentially forbidden. In the former case the local symmetry of the orbitals involved in the ring formation is not conserved during the reaction, in the latter case the nuclear repulsion is dominating.