Transition states aziridine intermediates

Analogous methyl azidoformate forms with norbornene a thermal unstable triazoline.251 The decomposition products are 40% aziridine and 55% imide. Furthermore it has been observed that the rate of nitrogen evolution of the triazoline from methyl azidoformate increases threefold when triglyme and 20-fold when dimethyl sulfoxide are substituted for 1,1-diphenylethane as solvents. This fact supports a betaine intermediate in the thermal decomposition reaction. The triazoline from 2,4-dinitrophenyl azide and norbornene could just be isolated, but from picryl azide only the aziridine was obtained.252-254 Nevertheless, the high negative value of the activation entropy (—33.4 eu) indicates a similar cyclic transition state for both reactions. 

For example, Jacobsen has studied the asymmetric aziridination of alkenes using (diimine)-copper(I)-catalysts 85. The results support the intermediacy of a discrete Cu(III)-nitrene intermediate and thus suggests mechanistic similarity (particularly regarding transition state geometry) to asymmetric cyclopropanation [95JA5889]. 

Heine ° has pointed out that this reaction represents an example of the reverse of the reaction on pyrolysis of imido esters. Heine has recently shown that the pyrolytic isomerization of cis- and rra j-l-p-nitrobenzoyl-2,3-diphenylaziridines into 2-p-nitrophenyl-4,5-diphenyl-2-oxazolines is a stereospecific process. These results are consistent with a mechanism that involves either a four-membered transition state or a short-lived tight ion-pair intermediate that collapses to the oxazoline before racemisation can occur . The pyrolysis of l,3-diaroyl-2-aryl-aziridines results in a different kind of reaction, in which a-benzamidobenzal-acetophenones are produced, viz. 

Mechanistic approaches to asymmetric aziridine synthesis have been carried out systematically using a variety of p-substituted benzaldehydes (Table 4.16). Two kinds of reaction mechanism, controlled by the nature of the p-substituent of aryl aldehydes, are proposed an S i-like mechanism, via cationic-like transition state for the fragmentation of intermediate adducts to aziridine products (step 2) by intramolecular nucleophihc substitution, when EDG-substituted benzaldehydes are used and an Sielectron-withdrawing group (EWG) substituted benzaldehydes are used [99]. 

The reaction is cis-steieospecific when esters of acetic acid are used while, in the case of a-bromopropionic esters, a mixture of cis-trans aziridines is formed. The cu-selectivity shown by esters of acetic acid is not surprising. In fact, it may be explained by assuming an E geometry for both the enolate and the imine and a closed chair-like Zimmerman-Traxler transition state in which the imine side chain is in an axial position while the halogen atom is in an equatorial location. The subsequent nucleophilic di lacement oi the halogen atom in the resulting intermediate teads directly to the formation of the cis-aziridine (Scheme 19). 

The ccnnplete diastereoselectivity, induced and simple, shown by this reaction is not unexpected if one considers the corresponding use of these imines in the preparation of p-lactams. Here, in fact, the formation of the P-amido esters occurs via a closed boatlike transition state that appears to be more stable than the corresponding chair-like one. The aldol intermediate then und goes cyclization via a nucleophilic substitution to give the final aziridine product (Scheme 21). 

Treatment of an aziridine with nitrosyl chloride, S-nitro-TV-nitrosocarbazole or methyl nitrite leads to the formation of an alkene and nitrous oxide (equation 76) " an intermediate iV-nitroso derivative was isolated at low temperatures. The deamination of cis-and frflBJ-2,3-dimethylaziridine gave the corresponding ds- and fra J-2-butenes with complete stereospecificity in each case. The reaction was found to be first order with respect to the A nitroso intermediate and the transition state was suggested to involve simultaneous cleavage of both C—bonds. A similar decomposition... 

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