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 sigma,pi-aromatizations. The cycloaddition of ethylene and 8 completes the same a-system as the latter, without the pi-system. Transition structures (4, 9np, and 11np) and second-order saddle points (9p and 11p) were 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 cc-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 pi-stabilization is weak. The transition structure 9np for the addition of acetylene (3) with 8 is nonplanar, indicating that the closed shell st-repulsion is greater than any ct-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 (11np) cycloaddition pathways, although at the MP4(SDTQ)/G-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.