Triaziridines. Synthesis of eis-2,3-Diisopropyltriaziridine-l-carboxylic Esters
Irradiation of the (2)-azimines la, b in Et,O with a Hg high pressure lamp through Corex yielded (besides 30% of the previously described trans-triaziridines 3a, b) 15% of the new cis-triaziridines 4a, b. The same irradiation of the (E)-azimines 2a,b afforded only 15-18% of 3a,b but 2&23% of 4a,b. Thus, these azimine photocyclizations show some stereospecificity. The triaziridines 3a, b and 4a, b formed in this way were always accompanied by the same three types of by-products, namely 1&15% of the 'triazones' 5a,b, 11-20% of the carbamic esters 6a, b, and 5-10% of the ether/nitrene insertion products 7a, b. The constitution and configuration of the new cis-triaziridines 4 followed from their spectral properties. Of particular interest are the symmetry properties of 4 derived from the 'H-, I3C-, and "N-NMR spectra: The stereoisomers 3 and 4 differ only in that the isochronicity of the two constitutionally equivalent molecular halves is temperature dependent in 3 but independent in 4. Both triaziridines 3 and 4 exhibit the IR CO band at (for carbamates) remarkably high frequency. The results confirm that the alkyl-substituted N-atoms of triaziridines are pyramidally stable, that the corresponding acyl-substituted N-atoms (N(1)) are also pyramidal, but can invert more readily, and that rotation around the N(l),C(=O) bond is rapid. Thus, there can be only little amide-type delocalization between a triaziridine N-atom and an acyl substituent of the carbamate type attached to it.