MCSCF study of the cycloaddition reaction between ketene and ethylene

In this paper we describe the results obtained in an MCSCF study at the STO-3G and 4-31G computational levels of the cycloaddition reaction between ketene and ethylene to form cyclobutanone. For this reaction, we have explored both the possibility of a concerted supra-antara approach between the two molecules (in agreement with the most common assumption on the mechanism of this reaction) and the possibility of nonconcerted reaction paths involving diradical species. The results obtained at the two computational levels are in good agreement and show the following: (i) One can obtain the cyclobutanone product through two possible mechanisms that have very similar energies. The energetically more favored mechanism is a highly asynchronous process (two-stage) involving the formation of a short-lived diradical intermediate. This mechanism corresponds to the attack of the ethylene molecule on the carbon atom bonded to oxygen, with the approaching direction lying in the plane of the ketene molecule (parallel approach). The other possible mechanism is a two-step process corresponding to an attack of the ethylene on the same carbon atom but with the approaching direction lying in a plane orthogonal to the plane of the ketene molecule (perpendicular approach). (ii) No reaction path exists for the supra-antara approach. The only critical point located in this case is a second-order saddle point (SOSP) that has no chemical meaning. This finding contradicts the common assumption that ketene molecules add as antarafacial partners to suprafacial olefins.