Aziridines ring inversion

Substituted 2-haloaziridines are also known to undergo a number of reactions without ring opening. For example, displacement of chlorine in (264) with various nucleophilic reagents has been found to occur with overall inversion of stereochemistry about the aziridine ring (65JA4538). The displacements followed first order kinetics and faster rates were noted for (264 R = Me) than for (264 R = H). The observed inversion was ascribed to either ion pairing and/or stereoselectivity. 

Finally, the substituent effects on the geometry of the -substituted aziridine ring, the -inversion energy barriers, and the ring opening reactions of aziridines by cyanide ion53 have been discussed... 

The oxonium ions are isoelectronic with amines and both have similar structures. The activation energy for the inversion of the isopropyloxiranium cation (AE" " =10 + 2 kcal mole ) is close to that of N-phenylaziridine (AE = 12 kcal mole ) X The angle between the aziridine ring and the phenyl substituent is 0 = 125 It agrees well with CNDO-2 calculations (0 = 124") for the methyloxiranium cation... 

The distribution of invertomers and the thermodynamic parameters for conformational inversion of C-unsubstituted aziridines have been determined by 250 MHz H n.m.r. spectroscopy. The steric hindrance of the aromatic group in C-aryl-aziridines with other ring substituents was found to be consistent with some form of conjugation between the aromatic system and the aziridine ring. 

The final stage on the route to the aziridine product involves an internal Sn2 ring closure in which the primary amine group attacks the a-carbon atom holding the remaining bromine from the backside to close an aziridine ring and displace bromide with inversion of configuration at the acarbon. This displacement has been shown to go exclusively by this mechanism in the case of the p-amino-a-bromoketones an S l reaction is less likely because of the instability of an a-keto carbocation. For a detailed discussion of the mechanism of the classic Sn2 substitution reaction, refer to Experiment [22],... 

The stereochemistry of aziridine ring substitution, as pointed out above, is controlled by the product distribution in the p-amino-a-bromoketone intermediate, which in this case favors the erythro configuration. Thus, inversion of configuration during ring formation leads to the trans-substituted aziridine... 

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