ALLENE AND FLUOROALLENES AS DIENOPHILES IN DIELS-ALDER REACTIONS - ANAM1 AND PM3 STUDY

Transition structures and energetics of Diels-Alder reactions of allen e with cyclopentadiene and hexachlorocyclopentadiene, and fluoroallene s with cyclopentadiene and furan have been investigated using semiempi rical MO methods at AM1 and PM3 levels. Allene and fluoroallenes react through an asynchronous transition structure in which the terminal ca rbon atom involved in the reaction pyramidalizes and the central carbo n atom triangulates and this deformation accounts for almost half the reaction barrier. Allene is found to be a less reactive dienophile com pared to ethylene, contrary to expectations, and its reactivity enhanc es with fluorine substitution up to 1,1-difluoroallene and declines on further substitution of fluorine. Tetrachloroallene is found to be le ss reactive than allene itself. The reactivity of furan in such cumule ne reactions is found to be almost the same as that of cyclopentadiene . While PM3 predictions in certain cases are not reliable AMI reaction barriers explain reasonably the reactivities and stereochemical prefe rences of the above reactions.