Stereochemical Investigation of the Products of the Photoinduced Perfluoroalkylation–Dimerization of Anthracene

The photoinduced perfluoroalkylation of anthracene has been shown to provide 9,9′,10,10′-tetrahydro-10,10′-diperfluoroalkyl-9,9′-bianthracenes in over 70% yields, each perfluoroalkyl iodide producing three diastereomers. The structures of all three diastereomers (<i>cis</i>,<i>cis</i>, <i>cis</i>,<i>trans</i>, and <i>trans</i>,<i>trans</i> isomers) have been elucidated by both NMR and X-ray crystallographic analyses. Most notably, an X-ray crystallographic analysis has revealed that the <i>trans</i>,<i>trans</i> diastereomer having the two trifluoromethyl groups in 10,10′-positions adopts a 74° twisted relationship of the two dihydroanthracene rings. Furthermore, each of the two <i>trans</i>,<i>trans</i> isomers <b>2a</b>,<b>b</b> has been shown to exist as a mixture of new types of atropisomers and the energy barrier for the interconversion of the two atropisomers has been estimated to be 12 kcal/mol by variable-temperature NMR analysis.