Aziridines, pyramidal inversion

The dipole moments of aziridine (1) and 1//-azirine (3) were calculated to be 1.92-1.94 D and 2.29-2.31 D, respectively, using ab initio MO methods (87JPC6484, 88JST(165)99, 89JCC468). These studies also estimated the dipole moments of the planar transition states attained during pyramidal inversion to be 0.21 D and 0.56 D, respectively, for (1) and (3). The value for aziridine compares with the known experimental value of 1.89 D. The dipole moment of 2//-azirine has also been estimated by theoretical methods <84CHEC-I(7)47>. Dipole moment measurements have been used to determine the preferred conformation of A-arylaziridines <7UCS(B)2104>. 

The consequence is that although the two possible pyramids are nonidentical mirror images, their lifetime at room temperature is so short that they cannot be detected by conventional means, and amines are never optically active. However, if structural features can retard the rate of pyramidal inversion, then the two forms could be detectable, which is exactly what happens in aziridines. Stability of the pyramid in a cyclic amine means that cis,trans isomers can exist if another substituent is present on a ring carbon. In 1,2-dimethylaziridine, the pyramidal inversion rate is slow enough at —40°C that both the cis and trans forms can be detected by NMR spectroscopy. 

Tertiary amines with three different alkyl groups are chiral but cannot be resolved because pyramidal inversion causes racemization at room temperature. Nevertheless, chiral aziridines can be resolved and stored at room temperature. 

Table 2. Activation parameters for pyramidal nitrogen inversion from NMR data Aziridines and substituted aziridinesa)...

Incorporation of d-functions on the inverting N site is especially important. Such functions contribute more to the pyramidal GS than to the TS and thus stabilize the GS with respect to the TS. In the absence of -functions on N, the inversion barrier in NH3 is found to be much too small and H2N—CN and H2N—SiHs are found to be more stable in the planar form inclusion of -functions (and p on H) leads to the correct NH3 barrier 158> and to pyramidal nitrogen sites in HaN—CN and H2N— SiH3 (Table 8 144>). Of course, the relative error introduced by the absence of -functions is smaller the higher the barrier. This may explain in part why the barriers calculated for aziridine and oxaziridine without including -functions are in satisfactory agreement with experimental values. In PH3, the phosphorus inversion barrier changes from 30.9 to 37.2 kcal/mole on inclusion of two sets of -functions 159>. 

The inversion barrier in aziridine is attractive dominant 144,i6D the same result has been obtained in a model system derived from NH3 by fixing one H—N—H angle at 80° during inversion 170>. The angle strain effect (discussed in Section 3.1) may be ascribed to the fact that the high compactness of the pyramidal form, due to the presence of a small... 

Oxaziridines For oxaziridines the N-inversion barrier is considerably higher than that for similar aziridines. A-Alkyloxaziridines show a high degree of configurational stability of the pyramidal nitrogen with inversion barriers... 

Trisubstituted atoms with a lone pair, such as amines, may undergo the process of pyramidal atomic inversion on the NMR time scale. The resonances of the two methyls in the aziridine 5-15 become equivalent at elevated temperatures through rapid nitrogen inversion. 

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