| Abstract: | Defined as the formation of two new bonds of equal length at the same time, synchronism has been shown to be not possible for the Diels-Alder reaction when the products are not of strict Cs symmetry. This has been done in two steps. First, the SCF -MO transition-state structures for the cyclization reactions of ethylene with cyclopentadiene and furan have been calculated. The activation energies (and heat of reaction) with the STO -3G, 3-21G, and 4-31G basis sets are as follows: with cyclopentadiene 35.0 (?80.7), 30.0 (?32.0), 38.4 (?19.9) kcal/mol; with furan 33.4 (?66.4), 34.6 (?16.5), 42.1 (?8.8) kcal/mol. Then, using a bond-order-bond-length relationship, a value for the tendency toward asynchronism has been calculated for substituted reactants where the two new bonds are arbitrarily held equal. This tendency has been shown in all cases which break strict Cs symmetry. Further, inspection of the atomic-orbital coefficients showed that the substituents cannot be considered as mere perturbations on the pi system in many cases. |
