A comparison of the Simmons-Smith reaction with carbenoids to nitrenoids and oxenoids

A computational study using both density functional and correlated wavefunction methods on the reaction between ethylene and model zinc carbenoid, nitrenoid and oxenoid complexes (L–Zn–E–X, E = CH2, NH or O, L = X = I or Cl) is reported. An investigation at DFT and MP2 levels of theory predicts that the epoxidation of ethylene with ClZnOCl oxenoid proceeds through a two-step mechanism involving an initial oxometalation and subsequent ring-closure, the latter being the rate determining step (RDS). Among all DFT methods applied in this study, BP86/CEP-31G(d) method produced the most similar geometries and energy barrier for the second step to those derived from MP2 simulations with correlation consistent basis sets. Interestingly, the mechanism of the cyclization reaction of ethylene with LZnEX is dependent on both E and X groups. Cyclopropanation of ethylene with IZnCH2I and aziridination of ethylene with IZnNHI proceed via a single-step mechanism with an asynchronous transition state. The reaction barrier for the aziridination with IZnNHI is 5.4 kcal/mol lower than that of cyclopropanation. Changing the leaving group of IZnNHI from I to Cl changes the mechanism of the aziridination reaction to a two-step pathway, with the second step as the RDS. The calculation results from the epoxidation with IZnOI and ClZnOCl oxenoids suggest a two-step mechanism for both oxenoids. The epoxidation reaction barriers for the RDS for both IZnOI and ClZnOCl is ∼15 kcal/mol, which is ∼6 kcal/mol less than that calculated for aziridination of ethylene with ClZnNHCl nitrenoid.