EFFECTS ON THE POPULATION OF ATROPISOMERS OF CHARGED AND DIPOLAR DERIVATIVES OF 1,8-DI(2'-PYRIDYL)NAPHTHALENE

Dicationic 1',1'-dimethyl (6a), dipolar 1',1'-dioxide (6b), and 6',6'- dipyridone (7) derivatives of 1,8-di(2'-pyridyl)naphthalene were prepa red. Proton NMR failed to reveal the presence of individual anti-syn a tropisomers over a wide range of temperatures. By contrast, the 1',1 ' '-dimethyl derivative of 1-(2'-pyridyl)-8-(3 ''-pyridyl)naphthalen (8) , a positional isomer of 6a, existed in DMSO-d(6) as a 2:1 mixture of anti to syn atropisomers at ambient temperatures. Electrostatic repuls ion is believed to cause the anti atropisomer to be favored; especiall y in 6a where the charged regions are closer together than in 8, owing to the splayed-out arrangement of the rings. AM1 and PM3 computations confirm that the anti is highly favored over the syn isomer for 6a, 6 b, and 7.