Stereoselectivity in addition of N-nitrenes to olefins

Abstract

Additions of phthalimidonitrene and 2,3-dihydro-2-oxobenzoxazol-3-ylnitrene to αβ-unsaturated esters, styrenes, and dienes have been examined by n.m.r. at temperatures (< – 20 °C) low enough for kinetically-formed invertomer ratios at nitrogen in the aziridine products to be observed. In many cases, addition is (a) stereospecific; only that invertomer having the olefin substituent (CO₂Me, Ph, or CR:CH₂)cis to the heterocycle (phthalimide, benzoxazolone) being observable and (b) contrathermodynamic, as can be shown by n.m.r. at temperatures >10 °C when thermodynamic equilibration by inversion at the aziridine ring nitrogen is complete. Some evidence is presented which suggests that the nitrenes add to the s-cis- conformations of the dienes, and a possible explanation for the high Stereoselectivity is offered.