Azetidine. strain energy

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. 

Polymerization of cyclic amines proceeds usually with high activation energies, due to the high strength of the C—N bond. To attain significant rates (k 10-3 mol-1 1 s 1) requires temperatures from 30 to 100 °C for azetidines and about 200 °C for the less strained bicyclic six-membered amines quinuclidine and triethylenediamine 9). Cyclic sulfides polymerize more rapidly than amines but much slower than ethers of a comparable structure (e.g. for 3,3-dimethylthietane k, = 2 10-2 mol-1 1 s at 35 °C, which is at least 104 times lower than for 3,3-dimethyloxetane (cf. Adv. Polymer Sci. 37 (1980)). 

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