Energy strain energies, aziridines

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. 

Three membered rings are all strained and the parent systems may be endothermic (AH°f. cyclopropane 35 oxirane -78 aziridine 92 thiirane 52 kJ/mole). Some molecules with small distorted rings (of high strain energy) are explosively unstable. Individual entries are ... 

N-Allylation of aziridines is often complicated by side reactions. The classical solution to this problem, reductive amination, can also be problematic due to the increased strain energy of the aziridinium intermediate. A way to avoid this difficulty was developed by Yudin and co-workers <2005JA17516, 2004JA5086>. The results obtained showed that NH-aziridines such as 197 or 198 underwent a palladium-catalyzed allylic amination with various allyl acetates affording the desired allylated product 199 and 200 with high levels of regioselectivity and in high isolated yields (Scheme 54). 

From the known heat of formation, the bond separation energy of oxirane can be obtained. This is — 14.0 kcal/mol after zero-point correction and is partly a measure of the ring strain. By this criterion, there is a steady reduction in strain energy from cyclopropane to aziridine to oxirane. This trend is reproduced by the theory at the 6-31G level (Table 5). However, even with 7-functions included, the theoretical value for oxirane is still about 5 kcal/mol too negative. 

The chemistry of aziridine is dominated by the strained three-membered ring structure. Reliable estimates give a value of 14 kcal.mole for the ring strain energy . The ring strain in aziridine leads to an increase in the polarity of the N-H bond reflected in the molecular dipole moment of 1.73 0.1 Z) . The microwave spectrum of the vapour has given the following structural data ... 

Aziridination of olefins has received particular interest due to the enormous synthetic potential of aziridines. Aziridines have a high strain energy... 

The heat of formation of aziridine is known, so it is possible to obtain an experimental bond separation energy. This is negative (— 19.3 kcal/ mol after correction for zero-point vibrations) suggesting some strain but rather less than in cyclopropane. Again, this bond separation energy is fairly well described (Table 5) at the 6-31G level. 

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