C-AZIDODIAZIRINES IN THE S(RN)1 REACTION OF AZIDE ION WITH ARYLCHLORODIAZIRINES - FURTHER INSIGHTS INTO REACTION-MECHANISM

Mixtures of arylchlorodiazirines and sodium azide in DMSO form visible charge transfer complexes. Irradiation of these solutions with fluorescent room light leads to S(RN)1 displacement of chloride and the transient formation of C-azidodiazirines. Relative reactivity studies (using competition experiments) show that nitro-substituted arylchlorodiazirines are substantially more reactive than other arylchlorodiazirines. This is attributed to facile electron transfer in the propagation cycle, involving the nitro-substituted aromatic ring. C-Azidodiazirines can be isolated in solution and spectroscopically characterized when the S(RN)1 reaction is initiated by addition of catalytic amounts of the sodium salt of 2-nitropropane. These azidodiazirines readily decompose at room temperature by first order processes to give molecular nitrogen and benzonitriles. Solvent and substituent effects on decomposition rates are minimal. Computational studies on potential intermediate carbenes in the decomposition of azidodiazirines have been carried out at the HF/6-31G* level. Singlet alpha-azidocarbenes RCN3, where R = NH2, OH, F, vinyl, phenyl, and CH3, are energy minima at this computational level. Isodesmic calculations show that the azido group is comparable to OH in its carbene stabilizing ability. Subsequent loss of N2 from alpha-azidocarbenes, leading to nitriles, is a highly exothermic process (126 kcal when R = vinyl and 128 kcal when R = phenyl).