NR transfer reactivity of azo-compound I of p450. How does the nitrogen substituent tune the reactivity of the species toward C-H and C = C activation?

摘要

We studied electronic structures and reactivity patterns of azo-compound I species (RN-Cpd I) by comparison to O-Cpd I of, e.g., cytochrome P450. The study shows that the RN-Cpd I species are capable of CC aziridination and CH amidation, in a two-state mechanism similar to that of O-Cpd I. However, unlike O-Cpd I, here the nitrogen substituent (R) exerts a major impact on structure and reactivity. Thus, it is demonstrated that FeNR bonds of RN-Cpd I will generally be substantially longer than FeO bonds; electron-withdrawing R groups will generate a very long FeN bond, whereas electron-releasing R groups should have the opposite effect and hence a shorter FeN bond. The R substituent controls also the reactivity of RN-Cpd I toward CC and CH bonds by exerting steric and electronic effects. Our analysis shows that an electron-releasing substituent will lower the barriers for both bond activation reactions, since the electronic factor makes the reactions highly exothermic, while an electron-withdrawing one should raise both barriers. The steric bulk of the substituent is predicted to inhibit more strongly the aziridination reactions. It is predicted that electron-releasing substituents with small bulk will create powerful aziridination reagents, whereas electron-withdrawing substituents like MeSO2 will prefer C-H bond activation with preference that increases with steric bulk. Finally, the study predicts (i) that the reactions of RN-Cpd I will be less stereospecific than those of O-Cpd I and (ii) that aziridination will be more stereoselective than amidation.