Influence of the Position of an Annular Nitrogen Atom on the Magnitude of the Rotational Barriers in Atropisomers of 1,8-Dihetarylnaphthalenes

J Org Chem. 1997 May 16;62(10):3215-3219. doi: 10.1021/jo961825n.

Abstract

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.