A series of 1,4-di-[2-aryl-1-diazenyl]-2-methylpiperazines (4a–n) have been synthesized by the reaction of 2-methylpiperazine with 2 equiv. of the appropriate diazonium salt. The products have been characterized by IR and NMR spectroscopy, and the molecular composition has been verified by HR-EIMS, with accurate mass measurement of the molecular ion. The presence of a chiral centre at C2 of the piperazine ring in the bistriazene 4 creates a multitude of diastereotopic protons in the methylene groups of the piperazine ring, as evidenced by the complexity of the NMR spectra, which nevertheless can be fully assigned in some cases, such as the tolyl- (4h) and phenyl- (4j) derivatives. These two compounds also show a discrimination between the two N-arydiazenyl groups, as evidenced by the doubling of several aromatic-carbon signals in the 13C NMR spectra. The assignment of the proton and carbon signals in 4h and 4j has been aided by the use of 2D NMR spectroscopy. A DEPT spectrum of 4j clearly discriminates the methylene carbons and also indicates the methine carbons of the piperazine ring. COSY spectra provide clear information about the interactions between diastereotopic protons; when these results are combined with the results of HSQC spectroscopy, the proton and carbon signals can be fully correlated, leading to an unequivocal assignment of the proton and carbon atoms of the piperazine ring. The HSQC spectrum of 4j also gives a complete correlation of the aromatic-proton and -carbon signals. These results compare favorably with the previously reported assignments of proton and carbon signals for triazenes of type 1 and bistriazenes of type 3.Key words: triazene, bistriazene, piperazine, 2-methylpiperazine, diastereotopic protons, diazonium coupling, 2D NMR, COSY, DEPT, HSQC.
Assignment of the 1H and 13C spectra of aspidocarpine and assignment of the structure and stereochemistry of the von Braun reaction product of aspidocarpine by 2D nmr spectroscopy
