Synthesis and thermal reorganisations of iminoaziridines. Ab initio calculations of their transition states, of diazatrimethylenemethanes, and some cyclic members of the C2H4N2 potential‐energy hypersurface
Authors: Quast Helmut , Aldenkortt Sven , Schäfer Peter , Schmitt Edeltraud , Würthwein Ernst‐Ulrich 
Published: July 2007

2‐Iminoaziridine 19 and the pairs of isomers 22/23 and 32/33 can be obtained in high yields by base‐induced 1,3‐dehydro‐halogenation of the corresponding α‐halo amidines. Regio‐selectivity of the ring closure reaction of 21 is achieved by treatment with potassium hydride as base in the presence of 18‐crown‐6 to afford almost exclusively 23. At low temperatures, the cyclisation of 21a under the influence of potassium tert‐butoxide occurs diastereoselectively yielding (Z)‐22 and (Z)‐23 (87:13). Resolution of racemic 21a can be attained by single recrystallisation of the nicely crystallised mandelates of the like configuration, thus providing an efficient route to (R)‐ and (S)‐23 of high enantiomeric purity. Chiroptical data are reported for 21a and 23. – (E)‐ and (Z)‐22 equilibrate faster by more than one order of magnitude than (E)‐ and (Z)‐23 but almost the same preference for the E diastereomer is found in both cases. – Only first‐order decomposition into isocyanides 6 and imines is observed on thermolysis of 19, 22, 23, and 32 with activation parameters depending on the substitution pattern. In contrast, the thermal valence isomerisation 35 → 36 is fast enough to compete with the [2 + 1] cycloreversion of 35. Thermal racemisation of (R)‐23 does not occur.

Quantum‐chemical calculations were performed on the parent iminoaziridines (E)‐ and (Z)‐38, 3‐amino‐2H‐azirine (39), the methylenediaziridines cis‐ and trans‐40, and diastereomeric closed‐shell and open‐shell planar (41) and (open‐shell) „mono‐orthogonal”︁ diazatrimethylenemethanes 42, 43. Complete geometry optimisations were appropriately performed with the RHF/6‐31 + G** and the UHF/6‐31 + G** basis sets. Energies of the closed‐shell states were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels, those of open‐shell states on the UHF, UMP2, UMP4SDTQ, and CCSD(T)/UHF levels. – (E)‐38, being lowest in energy of the parent iminoaziridines (Erel = 0.0), equilibrates with (Z)‐38 via an almost linear transition state (Erel = 113.4 kJ mol−1) and decomposes into hydrogen isocyanide and formaldimine in a one‐step cheletropic process [Erel = 179.8 kJ mol‐1, CCSD(T)/RHF] with a highly unsymmetrical transition state in which the N1–C2 bond is almost completely broken whereas the C2–C3 bond is still strong. The same is true for the cheletropic decomposition of (Z)‐38 the transition state of which (Erel = 168.2 kJ mol−1) is even lower by 11.6 kJ mol−1. – Energy‐rich zwitterionic transition states are found with the RHF method among which 1A′‐(E,Z)‐41 is lowest in energy [Erel = 203.0 kJ mol−1, CCSDT(T)]. Planar singlet (= transition states of CN‐bond rotation) and triplet diazatri‐methylenemethane diradicals possess energies in the range of Erel = 140–160 and 100–120 kJ mol−1 [CCSD(T)], respectively. Complete UHF optimisation of the singlet structures without symmetry constraint yielded five minima 42, 43 of mono‐orthogonal geometry with similar or slightly lower energies than the planar UHF singlets. In the transition state of the valence isomerisation of (E)‐38 [Erel = 170 kJmol−1, CCSD(T)/UHF], the N1–C3 bond is elongated while the plane of the methylene group is still orthogonal to the C(2) = NH plane. An activation energy of (185 ± 20 kJmol−1) is estimated for ring opening of 38 involving species with high diradical character.