Ring strain aziridine reactions

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. 

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... 

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. 

Starting from a mixture of cis 75 and trans 74 2,3-divinylazmdines (derived from the thermal decomposition of azide 73), cis material 75 selectively underwent thermal rearrangement with significantly reduced reaction temperature [15ej. The additional loss of ring strain allowed conversion of the reactant aziridine 74 at 140 °C in xylenes to the product 76 even though a boat-Uke transition state had to be passed. Up to 40% overall yield of 76 was obtained (Scheme 10.18). 

Poly(ethylene imine). Poly(ethylene amine) (PEI) is the simplest polybase. It can be prepared directly via the acid-catalyzed polymerization of ethyleneimine (aziridine, 24C) (Fig. 40). 24C may be prepared via a number of routes with ethanolamine being a convenient precursor (223). The cationic polymerization of 24C is very rapid due to the release of the ring strain associated with the monomer. However, the synthesis of PEI under these conditions leads to highly branched structnres dne to chain transfer reactions involving the —NH— species in the polymer backbone. Cyclic products can also be prodnced nnder these conditions. Copol5nnerization with an appropriate comonomer can rednce the degree of chain branching (224). 

The cycloaddition of 6-la with isocyanates without any catalyst led to tetracyclic imidazolidinone derivatives 6-8a-6-8c in moderate yields. Note that all reported reactions of simple aziridines with isocyanates require a catalyst such as transition metal salt [12], indicating that the C-N bonds in NSBV molecule might be more reactive than simple aziridine due to the enhanced ring strain caused by rigid ring system as well as rapid aza-Cope rearrangement. Single-crystal structures of 6-7a and 6-8a were determined by X-ray diflBraction (Fig. 6.2). 

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