Transition states for the [2+2] addition of CH2=CH2, CH2=O, and [M]=O across the C=C double bond of ketene:: Electronic structure and energy decomposition

Organic molecules with double bonds, such as ethylene and formaldehyde, are well-known to undergo [2 + 2] cycloadditions across the Cdouble bondC moiety of ketene. Surprisingly, the same pathway was recently predicted to be kinetically favored in the reaction of CpReO3 with ketene, in contrast to the additions of other metal oxides to the heterocumulene. Since the origin of the activation energies of [2 + 2] cycloadditions was poorly understood, a comparative density-functional study on the [2 + 2] addition of Xdouble bondY (CH(2)double bondCH, CH(2)double bondO, and CpO(2)Redouble bondO) across the Cdouble bondC bond of ketene has been carried out. The electronic structure of the transition states has been analyzed in terms of interactions between the ketene and Xdouble bondY fragments. The analysis reveals that, besides stabilizing orbital interactions, additional contributions such as electrostatics and repulsion due to the Pauli principle determine the activation energies of cycloadditions. The stabilization arising from orbital interactions is weakest in the transition state for the metal-oxide addition to the heterocumulene, although this reaction has the lowest activation barrier.