On the Development of Aromaticity in Cycloadditions: Ab Initio Transition Structures for the Trimerization of Acetylene and for the Addition of Ethylene and Acetylene to Formylketene

The trimerization of acetylene (3) and the cycloaddition of 3 with formylketene (8) are two examples of o,π-aromatizations. The cycloaddition of ethylene and 8 completes the same o-system as the latter, without the π-system. Transition structures (4, 9np, and llnp) and second-order saddle points (9p and lip) weee calculated for these reactions using ab initio molecular orbital theory at the MP2/6-31G* level with MP4(SDTQ)/6-31G* + ZPE relative energies. It is argued that π-interactions may contribute to the planarity of the transition structure (4) for the trimerization of 3. However, the very low frequency out-of-plane vibrations (MP2/6-31G*) indicates that any π-stabilization is weak. The transition structure 9np for the addition of acetylene (3) with 8 is nonplanar, indicating that the closed shell π-repulsion is greater than any jr-stabilization (aromatic or otherwise) in this cycloaddition. The potential energy surface for the addition of ethylene to 8 is closely balanced between the planar (lip) and nonplanar (llnp) cycloaddition pathways, although at the MP4(SDTQ)/6-31G* + ZPE level the planar one is favored by 0.6 kcal/mol. Pseudopericyclic orbital interactions favor the planar pathway, but angle strain in the 8 moiety at the transition structure disfavors it.