Transition state structures aziridines

Because the transition state structure is tight, the steric effect may become an important factor influencing nucleophilicity. The reduced steric demands around the nitrogen atoms of aziridines and azetidines are responsible, at least in part, for the greatly enhanced nucleophilicity toward ester carbonyl as compared with other amines having similar basicities (28). 

Reaction of norbomadiene 74 (in excess) with 7V-benzyl aziridine 67 formed exclusively the all-sy l l-adduct 77. This stereochemistry, confirmed by NOE between Ha and Hb, resulted from attack at the underface of the dipole by the exo-face of the dipolarophile. Similarly, reaction of A -benzyl aziridine 67 with the diacetoxybenzonorbomadiene 30 gave a single adduct 78 (Scheme 11), the symmetrical structure of which was clearly apparent in the ll NMR spectrum. These stereochemical outcomes demonstrated that the transition state (TSa), in which the methano-bridge was adjacent to the (V-substituent, was favoured in the A -benzyl series (X and R small), and in accord with the semiempirical calculations. 

Nitrosyl halides add to alkenes references are scattered through the litnnture back to 1875 (ref. 194 and references cited therein). The adducts vary enormously in their stability, but when their structures allow they, like nonhalogenated nitroso compounds, isomerize to oximes or dimnize. The orientation of the reaction is consistent with an electrophilic medumism, in which the reagent is polarized as NO Hat. Bicyclic substrates and reaction media of low polarity favor syn addition, suggesting a four-center transition state (Scheme 81). Aziridine synthesis via NOCl/alkene adducts is discussed in Section 3.5.2.1. 

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