The trifluoromethoxy group as a fluorine twin in the Diels-Alder reactions of halogenated quinones

We describe here a study devoted to the comparison of the relative influence of chlorine, fluorine, and trifluoromethoxy substituents on the regiochemical outcome of the Diels-Alder reaction. For this purpose, we examined the behavior of mixed 'halogenated' quinones bearing these groups in their cycloadditions with simple dienes. Contrary to the expectation based on its known electronic properties, the trifluoromethoxy group behaves very much more like a fluorine than a chlorine atom in such reactions. On the basis on an endo transition state demonstrated here for these additions, we tentatively suggest that non-bonded interactions are the main factor controlling the regiochemistry.