Influence of the Position of an Annular Nitrogen Atom on the Magnitude of the Rotational Barriers in Atropisomers of 1,8-Dihetarylnaphthalenes.
Five new atropisomerically chiral 1,8-dihetarylnaphthalenes were prepared by Pd(0)-catalyzed coupling reactions. Variable-temperature proton NMR spectra show those compounds with a 2'-pyridyl or 2'-pyrazinyl ring have a much lower energy barrier for rotation to interconvert conformational isomers than those with a 3'-pyridyl ring. Coalescence temperatures may differ by as much as 100 degrees C. The results of AM1 and PM3 computations indicate the preferred transition state for sigma-bond rotation places the annular nitrogen atom in the 2' or ortho position toward the face of the second hetaryl ring and not toward the naphthalene ring.
Zoltewicz
John A.
JA
Institut für Organische Chemie, Karl-Franzens Universität, A-8010 Graz, Austria.
Maier
Nobert M.
NM
Fabian
Walter M. F.
WM