Aziridine ring strain

Aziridine ring strain promotes ring opening... 

Another circumstance that increases leaving-group power is ring strain. Ordinary ethers do not cleave at all and protonated ethers only under strenuous conditions, but epoxides are cleaved quite easily and protonated epoxides even more easily. Aziridines and episulfides, three-membered rings containing, respectively, nitrogen and sulfur, are also easily cleaved (see p. 458). ... 

A melting point sample exploded at 150°C. Larger quantities showed signs of autocatalytic decomposition from -30°C. This presumably involves release of strain in the aziridine ring. 

The reaction of azetidines with dinitrogen pentoxide is found to reflect the reduced ring strain in this system compared to aziridines.Accordingly, while the carbamate and V-alkyl... 

The strained ring in aziridines makes them reactive, with ring opening under attack from any nucleophile to create a new nucleophUe-carbon bond. Aziridine rings can be formed from intramolecular substitution of a halogen atom by the nitrogen in nitrogen mustards. 

The triazoline adducts from benzvalene (Scheme 21)162 and diphospha-benzvalene (Scheme 22) photolyze to yield novel tetracyclic aziridine ring systems165 that are valence isomers of azepines,162 whereas that from De-war thiophene (Scheme 20) gives a novel tricyclic aziridine that desulfurizes with triphenylphosphine to yield the trifluoromethylated Dewar pyrrole (Scheme 153).15 9,160 The stabilization of these strained molecules is attributed to the perfluoroalkyl effect.159... 

Calculations at the MP2(Full)/6-31++G(d,p)//MP2(Full)/6-31+G(d) level of theory were used to investigate the SN reactions between ammonia and aziridine, aze-tidine, methylethylamine, and four fluorinated derivatives of aziridine.52 The results show that aziridine and azetidine have strain energies of 27.3 and 25.2 kcalmol-1, respectively, and that as a consequence they react 7.76 x 1023 and 2.30 x 1017 times faster with ammonia than does the methylene group of methylethylamine. However, even after subtracting the effect due to the release of ring strain, aziridine still reacts much faster than the other two substrates. This is because the electrostatic attraction of the charges in the product-like dipolar transition state are much greater for aziridine. 

Woerpel and Nevarez postulated a stepwise mechanism for the formation of 185 (Scheme 7.54).127 Coordination of benzaldehyde to the Lewis acidic silicon atom of 169g86 triggers an attack by the pendent A-aryl group to form zwitterion 189. Isomerization of 189 to azomethine ylide 190128 enabled a 4n-electrocyclization to produce aziridine 185. The thermodynamic driving force for this reaction was attributed to the relief of the silaaziridine s ring strain. 

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