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 for mylketene (8) are two examples of sigma,pi-aromatizations. The cycload dition of ethylene and 8 completes the same a-system as the latter, wi thout the pi-system. Transition structures (4, 9np, and 11np) and seco nd-order saddle points (9p and 11p) were calculated for these reaction s using ab initio molecular orbital theory at the MP2/6-31G level wit h MP4(SDTQ)/6-31G + ZPE relative energies. It is argued that cc-inter actions may contribute to the planarity of the transition structure (4 ) for the trimerization of 3. However, the very low frequency out-of-p lane vibrations (MP2/6-31G) indicates that any pi-stabilization is we ak. The transition structure 9np for the addition of acetylene (3) wit h 8 is nonplanar, indicating that the closed shell st-repulsion is gre ater than any ct-stabilization (aromatic or otherwise) in this cycload dition. The potential energy surface for the addition of ethylene to 8 is closely balanced between the planar (lip) and nonplanar (11np) cyc loaddition pathways, although at the MP4(SDTQ)/G-31G + ZPE level the planar one is favored by 0.6 kcal/mol. Pseudopericyclic orbital intera ctions favor the planar pathway, but angle strain in the 8 moiety at t he transition structure disfavors it.