THEORETICAL AND EXPERIMENTAL STUDIES OF THE DIELS-ALDER DIMERIZATIONSOF SUBSTITUTED CYCLOPENTADIENES

Diels-Alder dimerizations of a series of substituted cyclopentadienes have been studied. Experimentally, preparation of 1-oxaspiro[2.4]hepta -4,6-diene results in immediate formation of the Diels-Alder dimer wit h two adducts formed in a ratio of 3:1. Two substituted spiroheptadien e systems gave similar results. Theoretically, transition states for f our different isomers were located leading to the four dimer minima of 1-oxaspiro[2.4]hepta-4,6-diene. The transition state associated with the lowest of the four energetic barriers leads to the isomer with the same connectivity as the major isomer obtained in the dimerization of 2,2-dimethyl-1-oxaspiro[2.4]hepta-4,6-diene (structure confirmed by X -ray diffraction analysis). Unsymmetric transition states at the HF/6- 31G(d) level were predicted for three additional endo species with the energy barriers decreasing in the following order: spiro[2.4] hepta-4 , 6-diene > cyclopentadiene > 1-oxaspiro[2.4]hepta-4,6-diene > 1,2-dio xaspiro[2.4]hepta-4,6-diene. The barrier to forming the dimer of spiro [2.4]hepta-4,6-diene is predicted to be approximately 5 kcal/mol great er than that for forming the cyclopentadiene dimer and about 13 kcal/m ol greater than the barrier to formation of the dimer of 1-oxaspirol[2 .4]-hepta-4,6-diene.