Chemical consequences of fluorine substitution. Part 1. Experimental and theoretical results on Diels-Alder reactions of alpha- and beta-fluorostyrenes

Vinyl fluorides such as alpha- and beta -fluorostyrenes 2 and 3 are poor di enophiles for Diels-Alder reactions. Under thermal conditions these compoun ds do not react with usual dienes, but with the highly reactive 1,3-dipheny lisobenzofuran (4) to give mixtures of the corresponding endo- and exo-prod ucts. Kinetic measurements show that the fluorostyrenes 2 and 3 are less re active than the parent styrene (la). Additional electron withdrawing as wel l as electron donating substituents (p-Cl, p-F, m-Me) on the phenyl ring of 2 and 3 slightly accelerate the reaction rate by a maximum factor of 4. DF T calculations (UB3LYP/6-31G(d)) of activation energies for furan and isobe nzofuran as model dienes reflect the order of the reaction rates of the kin etic measurements. The charge difference in the double bond carbon atoms of the different dienophiles pre-determines the activation energies for the e ndo- and exo-transition states. The optimised structures of the transition states show that the reactions are concerted but asynchronous without birad ical character. This asynchrony is determined by the differences in the p(z ) orbital coefficients of the double bond carbon atoms of the fluorostyrene s. The frontier molecular orbitals reveal that all the reactions are cycloa dditions with normal electron demand. The calculated small endolexo prefere nces are in qualitative agreement with the experimental values. In the tran sition states of the reactions of 2a and (E)-3a where the fluorine atom and the oxygen atom of furan are syn-orientated with respect to the reaction c enter, electrostatic repulsion determines the endo/exo selectivity. Semiemp irical calculations show that the reactions of 4 are more exothermic with r espect to the parent furan and more endothermic with respect to isobenzofur an. However, regarding relative reactivity and diastereoselectivity, the se miempirical methods are less reliable in comparison to either experiment or DFT.