A Theoretical Study of the Reaction of Ketene Radical Cation with Ethylene: Nucleophilic Addition or Concerted [2+1] Cycloaddition?

A thorough molecular orbital study of the potential energy surface of ionized cyclobutanone reveals that, in line with ESR data, this species does not appear to be a minimum on the MP2/6-31G*//6-31G* + ZPVE potential energy surface. While ring-opening by cleavage of the C(1)/C(2) bond (a-cleavage) has no barrier, the alternative reaction, i.e., cleavage of the C(2)/C(3) bond (β-cleavage), is predicted to be associated with a significant barrier. This is indicated by the fact that the β-cleavage product 4 is higher in energy than the sum of the heats of formation of the fission products C2H4 and C2H2O•+. The reverse reaction, i.e., the formal [2+1] cycloaddition of ionized ketene with ethylene, has been analyzed in detail. It has been found that this process should not be classified as a cycloaddition reaction but rather as a nucleophilic addition of C2H4 to the terminal carbon atom of the ketene radical cation. This type of reaction is governed by a 3-electron/2-orbital interaction as is the case with the recently described15 nucleophilic addition of NH3 to ionized ketene. The MO results present for the first time a coherent description of all experimental findings, including ESR studies at 77 K and ion/molecule reactions of C2H4O•+with C2X4 (X = H, D) in the gas phase. © 1988, American Chemical Society. All rights reserved.