Mechanism of the Thermal Rearrangement of Substituted Cyclopentadiene - Maleic Anhydride Diels-Alder Adducts
In this report, a new internal mechanism is proposed which fits the available kinetic data and explains the nonoccurrence of the rearrangement in the C7-cyc1opentyl substituted adduct, while still accounting for the data which was previously held to support an external pathway. The suggested mechanism involves a sigmatropic [1,3] carbon migration, as defined by Woodward and Hoffmann, and explicitly mathematically derived for certain systems by Fernandez. This type of mechanism has been directly observed for the rearrangement of 2-bicyclo[2.1.1]hexene to 2-bicyclo[3.1.0]hexene and that of 2-bicyclo[3.2.0]heptene to 2-bicyclo[2.2.1]heptene· The reactions previously reported to undergo these [1,3] carbon migrations have been shown to proceed with predominant inversion of configuration. Selection rules for sigmatropic reactions have been described by Woodward and Hoffmann.