AN AM1 CALCULATIONAL STUDY OF THE DIELS-ALDER ADDITION OF MALEIC-ANHYDRIDE TO C5-SUBSTITUTED PENTAMETHYLCYCLOPENTADIENES AND 2,5-DIMETHYLTHIOPHENE OXIDE - AN ATTEMPT TO ASCERTAIN THE FACTORS CONTROLLING THE PI-FACIAL SELECTIVITIES AND RELATIVE REACTIVITIES

The Diels-Alder reactions of maleic anhydride (1) with a group of CS-substituted cyclopentadienes and 2,5-dimethylthiophene oxide (5) have been studied with the semi-empirical method AM1. We find that the facial selectivities and relative reactivities found experimentally for six dienes (2a-2f) are accurately reproduced at this level of theory. The results of the calculational study provide no concrete support for the proposal that sigma-sigma* hyperconjugation in the transition state - an interaction between a sigma bond at C5 of the cyclopentadiene and the incipient sigma bonds anti to it (the Cieplak effect) - is the major source of the syn facial selectivity observed for 2a-2d. In addition, the relative reactivities and facial selectivities are predicted for a number of cyclopentadienes which will be synthesized and studied experimentally.