Allene and fluoroallenes as dienophiles in Diels-Alder reactions: An AM1 and PM3 study

Transition structures and energetics of Diels-Alder reactions of allene with cyclopentadiene and hexachlorocyclopentadiene, and fluoroallenes with cyclopentadiene and furan have been investigated using semiempirical MO methods at AM1 and PM3 levels. Allene and fluoroallenes react through an asynchronous transition structure in which the terminal carbon atom involved in the reaction pyramidalizes and the central carbon atom triangulates and this deformation accounts for almost half the reaction barrier. Allene is found to be a less reactive dienophile compared to ethylene, contrary to expectations, and its reactivity enhances with fluorine substitution up to 1,1-difluoroallene and declines on further substitution of fluorine. Tetrachloroallene is found to be less reactive than allene itself. The reactivity of furan in such cumulene 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 preferences of the above reactions.