Conformational studies by dynamic nuclear magnetic resonance. Part X. Stereodynamics and conformations of hindered triazenes

Abstract

Rotational barriers about the N(1)–N(2) bond in a number of 1,1-dialkyl-3-aryltriazenes (R₂N·N:NAr) have been measured by line-shape analysis of the n.m.r. signals. It has been observed that the barrier is only slightly affected by the bulkiness of the alkyl groups but is substantially lowered when the triazenes contain the piperidyl ring with two (DMP) or four (TMP) methyl groups in positions 2 and 6. This feature is explained in the DMP derivatives as being due to the repulsion of the two axial cis-methyl groups; this conformation has been confirmed by X-ray diffraction. In TMP derivatives, the lowering of the barrier can be explained by assuming a non-chair (twisted) conformation for the piperidyl ring in solution. ¹³C N.m.r. spectra also seem to support this conformation.