1.3.5- Thiadiazines, tetrahydro-, conformation

By analogy with the tetrahydro-l,3,5-oxadiazine system, the ae conformer 466 of 3,5-dialkyltetrahydro-l,3,5-thiadiazines (465) should predominate in the conformational equilibrium. Investigation of a series of 3,5-dialkyl derivatives by H-NMR disclosed ring-inversion barriers that decrease with increasing size of the substituent (465 R = Me, AG 12.1 + 0.3 R = Et, AG 12.0 + 0.2 R = iPr, AG 10.5 + 0.4 kcal mol ). No further changes in the H-NMR spectra on cooling were found. Dipole-moment data indicated that all the compounds existed predominantly in the ae conformation.356 It is difficult to draw conclusions for Jtem in this series. 

The structure of the 2,6-dimesyl-3,4,5,6-tetrahydro-2//-l,2,6-thiadiazine manganese penta-carbonyl complex (47) (Section 6.16.9.2.1.1) has been confirmed by x-ray analysis <860M846>. The thiadiazine ring is in the chair conformation and the S—Nz and S—bond lengths lie between the normal S—N single and double bond values. 

There have been no further reports on the conformation of 1,3,4-oxadiazine and 1,3,4-thiadiazine systems since the early studies on tetrahydro-3,4-dimethyl-1,3,4-oxadiazine <76JCS(P2)I86I, 77JCS(P2)1816> and the corresponding dimethyl 1,3,4-thiadiazines <78CHE1281,80JCS(P2)279>. 

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