13(C) NMR and X-Ray Crystallographic Determination of the Structures of Some Isomeric Phenylquinoxalines

摘要

The good thermal and hydrolytic stability of polyphenylquinoxalines has prompted many investigations for characterization and property correlations for these polymers. The presence of isomers in polyphenylquinoxalines as a result of the ring forming polymerization reaction has been noted. The existance of isomers contributes to the amorphous form of these polymers and inhibits crystalline polymer formation. Unequivocal structural assignments were made for the two isomeric quinoxalines isolated from the reaction of 3-bromobenzil with 4-iodo-O-phenylenediamine as well as for their respective acetylene terminated derivatives. The higher melting isomers were assigned the structures 3-(3-bromophenyl)-2-phenyl-6-iodoquinoxaline and 3-(3-ethynylphenyl)-2-phenyl-6-ethynylquinox-aline, and the lower melting isomers were assigned the structures 2-(3-bromophenyl)-3-phenyl-6-iodoquinoxaline and 2-(3-ethynylphenyl)-3-phenyl-6-ethynylquinoxaline. Both x-ray crystallographic analysis and 13C NMR analyses were used and were in complete agreement. For carboxy the 13C NMR study, isotropically enriched versions of the quinoxaline isomers were prepared starting from the 3-bromobenzil-alpha-13C. Utilizing substituent effects, all of the 13C absorptions in the molecules were assignable.