Stereoisomerism in the series of isoindole-based quaternary salts

Abstract

Steric structure of quaternary ammonium salts of the 3-[alkyl(aryl)amino]-1-[alkyl(aryl)iminio]-1H-isoindole series was studied by ¹H NMR spectroscopy and X-ray analysis. The examined salts are characterized by E,E configuration of substituents with respect to the exocyclic C-N bonds, and the presence of ortho- and meta-substituents in the benzene rings on the nitrogen atoms gives rise to atropisomers. Sterically hindered N-(2,4,6-trimethylphenyl) derivative was found to exist as Z,Z isomer which undergoes irreversible thermal isomerization into the more stable E,E isomer through intermediate Z,E structure. The reactions of 1,1,3-trichloro-1H-isoindole with secondary aromatic amines having an electron-withdrawing substituent in the ortho position in the presence of organic bases (triethylamine, N,N-dimethylbenzylamine) are accompanied by decomposition of the latter and formation of unsymmetrically substituted salts of the 3-[alkyl(aryl)amino]-1-(dialkyliminio)-1H-isoindole series, which contain both arylamine residue and dialkylamino group and are also characterized by atropisomerism.