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 (cis,cis, cis,trans, and trans,trans isomers) have been elucidated
by both NMR and X-ray crystallographic analyses. Most notably, an
X-ray crystallographic analysis has revealed that the trans,trans 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 trans,trans isomers 2a,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.