1,2,4,5-Tetrazines, hexahydro-, conformation

The conformation of a diaryl-dialkyl-hexahydro-l,2,4,5-tetrazine by X-ray crystallographic analysis has been reported for the first time <00SL137>. 

Fig. 26. Conformational sets and types of hexahydro-s-tetrazines. (Reprinted with permission from Ref. 374. Copyright 1976 American Chemical Society.)...

Variable temperature 13C NMR spectroscopy has shown that the 1,2,3,4,5,6-hexamethyl-hexahydro-l,2,4,5-tetrazine (57) is the trans isomer with the two C-methyl groups equatorial and the N-methyl groups symmetrically diaxial and diequatorial. The observed dynamic 13C NMR effects are consistent with (57a) and (57b) as the major conformation and probably (57c) as the minor one. The first coalescence represents the freezing out of (57c) while the (57a) (57b) interconversion remains fast. The nitrogen inversion barrier was found to be 32.2 kJ moF1 <79JCS(P2)98l). The He(I) photoelectron spectrum of (57) confirmed the NMR data (80JCS(P2)9l). 

Controversy continues over the conformations favoured by hexahydro-1,2,4,5-tetrazines. On the basis of n.m.r. spectra, at varying temperatures, of a variety of model compounds, Nelsen and Hintz concluded that the... 

---