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 π-facial selectivities and relative reactivities

The Diels–Alder reactions of maleic anhydride (1) with a group of C5-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 σ–σ* hyperconjugation in the transition state – an interaction between a σ bond at C5 of the cyclopentadiene and the incipient σ 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.

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 π-facial selectivities and relative reactivities Journal Articles