Frontier-orbital analyses of ketene [2+2] cycloadditions

Twelve kinds of ketene [2 + 2] cycloadditions have been investigated by ab initio calculations. They are composed of four ketenes (Y-HC=C=O, Y=H, NH2, Cl, and CN) and three isoelectronic ketenophiles (ethylene, methylenimine, and formaldehyde). All the transition state geometries obtained here are n ot different significantly, but the extent of formation of two covalent bon ds differs appreciably. The difference is attributable to the degree of the charge transfer interactions. One is the interaction from the pi orbital a nd:or the lone pair orbital of a ketenophile to the LUMO of a ketene (domin ant charge transfer, CT1). The other is that from the HOMO of the ketene to the pi* orbital of the ketenophile (second dominant charge transfer, ct1). CT1 contributes to the formation of only one covalent bond, and ct1 does t o the formation of the other. This independent function is characteristic o f ketene [2 + 2] cycloadditions. They are not concerned with the orbital ph ase. We also have examined Fukui's postulate that the deformation of partic ular frontier orbitals causes the reaction progress. The role has been veri fied both by configuration analyses along the intrinsic reaction coordinate of the ketene-ethylene reaction and by the examination of distortions of f rontier-orbital shapes along the low-frequency vibrational modes.