A 15N NMR study of tautomerism in dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate

Dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate can exist either in 1,2- or 1,4-dihydro tautomeric forms. The 15N NMR spectra of dimethyl dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate were measured at the 15N natural abundance level as well as in 15N doubly labelled selectively and in 15N completely labelled compounds (20% 15N). The J(15N,15N) value was determined in 15N completely labelled compounds (20% 15N) using 1D 15N INADEQUATE and was found to be 12.2 ± 0.2 Hz in deuteriochloroform, acetonitrile-d3, DMSO-d6 and CD3OH. Very similar 15N chemical shifts and 1J(15N,1H) values were also observed in all the solvents. This indicates that compound 1 exists completely in the 1,4-dihydro tautomeric form (i.e., as dimethyl 1,4-dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate) in all the solvents tested.