INFLUENCE OF THE POSITION OF ALL ANNULAR NITROGEN ATOM ON THE MAGNITUDE OF THE ROTATIONAL BARRIERS IN ATROPISOMERS OF 1,8-DIHETARYLNAPHTHALENES

Five new atropisomerically chiral 1,8-dihetarylnaphthalenes were prepa red by Pd(O)-catalyzed coupling reactions. Variable-temperature proton NMR spectra show those compounds with a 2'-pyridyl or 2'-pyrazinyl ri ng have a much lower energy barrier for rotation to interconvert confo rmational isomers than those with a 3'-pyTidylring. Coalescence temper atures may differ by as much as 100 degrees C. The results of AM1 and PM3 computations indicate the preferred transition state for a-bond ro tation places the annular nitrogen atom in the 2' or ortho position to ward the face of the second hetaryl ring and not toward the naphthalen e ring.