Mechanistic study of styrene aziridination by iron(<scp>iv</scp>) nitrides

Abstract

A combined experimental and computational investigation was undertaken to investigate the mechanism of aziridination of styrene by the tris(carbene)borate iron(iv) nitride complex, PhB( ^<i>t</i> BuIm)₃Fe[triple bond, length as m-dash]N. While mechanistic investigations suggest that aziridination occurs <i>via</i> a reversible, stepwise pathway, it was not possible to confirm the mechanism using only experimental techniques. Density functional theory calculations support a stepwise radical addition mechanism, but suggest that a low-lying triplet (<i>S</i> = 1) state provides the lowest energy path for C-N bond formation (24.6 kcal mol^-1) and not the singlet ground (<i>S</i> = 0) state. A second spin flip may take place in order to facilitate ring closure and the formation of the quintet (<i>S</i> = 2) aziridino product. A Hammett analysis shows that electron-withdrawing groups increase the rate of reaction <i>σ</i> _p (<i>ρ</i> = 1.2 ± 0.2). This finding is supported by the computational results, which show that the rate-determining step drops from 24.6 kcal mol^-1 to 18.3 kcal mol^-1 when (<i>p</i>-NO₂C₆H₄)CH[double bond, length as m-dash]CH₂ is used and slightly increases to 25.5 kcal mol^-1 using (<i>p</i>-NMe₂C₆H₄)CH[double bond, length as m-dash]CH₂ as the substrate.