Ring Opening Reactions of Iminoaziridines and Aminoazirinium Ions. Is Electrocyclic Ring Opening of 2H‐Azirinium Ions a Myth?

Abstract

AbstractTreatment of the 3,3‐dimethyl‐2‐iminoaziridine 7 with hydrogen bromide yields similar amounts of the α‐bromo amidine 10a · HBr and the α,β‐unsaturated amidinium bromide 11. Ring opening reactions of the 3‐tert‐butyl‐2‐iminoaziridines 8 by nucleophiles (potassium methoxide, Pirkle's alcohol 13, and 4‐nitrophenol) are sluggish affording the α‐amino imidates 12, 14, and 15, respectively. Compound 15a isomerises via the Meisenheimer‐type intermediate 16 to furnish the α‐amino amide 17. Methyl triflate converts (R)‐8 into the 2‐tert‐butyl‐3‐amino‐2H‐azirinium triflates (R)‐18 which do not undergo racemisation as shown after hydrolysis to yield the α‐amino amides (R)‐20. In a similar sequence, (R)‐8a and b are converted into the α‐amino amides (R)‐26 by the action of hydrogen halides or 4‐toluenesulfonic acid hydrate followed by hydrolysis of the intermediate imidoyl derivatives (R)‐24. Retention of configuration is observed after treatment of (R)‐8a with hydrogen chloride and of (R)‐8b with hydrogen bromide. Some racemisation found in the other cases is supposed to involve (R)‐24 but not the aminoazirinium ions (R)‐8 · HX. In a side reaction, 8a and b decompose into methyl isocyanide (22) and the imines 23 at low temperatures, probably under general acid catalysis. The results are not consistent with the hypothesis of an electrocyclic ring opening of 3‐amino‐2H‐azirinium ions to afford 2‐amino‐1‐azaallyl cations (= α‐imino carbocations). – The heat of protonation of the parent compounds (E)‐27, (Z)‐27, 30 and the structures and energies of the resulting cations (E)‐28, (Z)‐28, 29, and 31 were calculated by ab initio methods. Complete geometry optimisations were performed with the RHF/6‐31 + G** basis set. Energies were calculated on the RHF, MP2, MP4SDTQ, and CCSD(T) levels. According to the geometric parameters, the amidine delocalisation of the free bases is abondened in the cations (E)‐28, (Z)‐28, and 31 while it is considerably strengthened in 29. In accord with these geometric changes, 29 is by far the most stable protonated species followed by (E)‐ and (Z)‐28. Azirine 31 is much higher in energy. The calculated difference in (gas phase) basicity between the two nitrogen atoms of iminoaziridines appears to exclude the ring‐protonated cations as intermediates also for reactions in solution.