Opening the aziridinimine ring: a theoretical study

Abstract

The opening of a model aziridinimine has been studied by ab initio molecular orbital methods. Geometries of stationary points were optimized at the MP2/6-31G(d,p) level, while relative energies were estimated using configuration interaction (CISDQ) and quadratic configuration interaction [QCISD(T)] methods with the 6-311G(d,p) basis set and corrected for zero-point energies. The [2 + 1] cycloreversion of aziridinimine giving an imine plus an isocyanide is consistently favoured over its isomerization yielding other alternative rings. The calculated enthalpy of activation for cycloreversion of the unsubstituted molecule amounts to ΔH^‡= 150 kJ mol^–1 at 0 K. While methyl substitution at the exocyclic nitrogen activates this fragmentation, methyl substitution at the ring nitrogen atom slightly deactivates it. For trimethylaziridimmine, we estimate a value of ΔH^‡= 120 kJ mol^–1. While the latter can be compared with the experimental values of 112–128 kJ mol^–1 measured for alkyl substituted species, the entropy of activation for alkylated aziridinimines is calculated to be positive, ΔS^‡(calc)≃ 10 J mol^–1 K^–1, in conflict with available experimental results, ΔS^‡(exptl)=–13 to –24 J mol^–1 K^–1. The energy barrier for ring–ring rearrangement giving an alternative ring with an exocyclic CC bond amounts to ca. 160–170 kJ mol^–1. The stereochemistry of the [2 + 1 ] cycloreversion can be rationalized by a stereoelectronic effect. Protonation occurs at the exocyclic nitrogen. The aziridinimine radical cation ion still has a cyclic structure; the adiabatic ionization energy is estimated to be E_i,a(aziridinimine)= 9.4 ± 0.3 eV. The cyclic ion easily undergoes ring-opening yielding a more stable open distonic radical cation, H₂C˙–NH–CNH⁺, which lies 128 kJ mol^–1 below the cyclic ion.