13C N.m.r. Investigation on the Nitrogen Met hylation of some Azabenzenes

The 'H and 13C n.m.r. spectra of N-methylated pyridine, pyridazhe, pyrimidhe and pyrazhe and Nfldimethylated pyrimidine and pyrazine have been recorded and analysed. The change in the "C chemical shifts Mder the influence of N-methylation (As) in the diazabenzenes could be predicted by the AS values of pyridine. A comparison of the A6 values of N-methylation with those of N-protonation showed that both reactions have a similar effect. ETRODUCTION Several n.m.r. investigations on nitrogen protonation in aza-aromatics have been re orted previous1y.ld It appeared that the change in 'C chemical shift (A6) under the influence of nitrogen protonation in diaza compounds can be described in terms of protonation parameters obtained from the A6 values in one or more mono-aza In this work we investigate whether this procedure is also applicable for nitrogen methylation. Moreover we compare the influence of N-protonation on the I3C chemical shifts with the influence of N-methylation. There is an important difference between N-protonated and N-methylated aza compounds; a pro-ton is in a rapid exchange between the nitrogen atoms while a methyl group is fixed on one of the nitrogen atoms. As a consequence of this, monoprotonation of a diaza compound does not affect the symmetry of the molecule in the n.m.r. experiment, whereas monomethylation of a symmetric diaza compound re-sults in a lowering of the symmetry (cf. Ref. 7). For the n.m.r. investigation on N-methylated aza-aromatics we have chosen the following azabenzenes: pyridine (l), pyridazine (2), pyrimidine (3) and pyrazine (4). The N-methylated compounds are N