Aziridines theoretical calculations

Associated with the greater deviation from nitrogen planarity (compared to ammonia) is an increase in the barrier to inversion. We have not made a study of the Czv form (with planar nitrogen) but previous theoretical calculations 30,53,54) jg d to estimates in the range 15.5— 18.3 kcal/mol. Experimental values of inversion barriers in simple substituted aziridines 5) are in the range 18—21 kcal/mol. For comparison, the experimental inversion barrier in ammonia 35) is 5.8 kcal/ mol. 

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989-1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the C=C, C=Si and Si=Si bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at C=C bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII). 

There have been several previous ab initio molecular orbital studies of oxirane using the experimental geometry.30.3it34,35,38,39,63) These have been mostly concerned with ionization potentials and first order properties such as multipole moments. Some details are given in Table 10. As with aziridine, all the theoretical dipole moments except those calculated with the minimal Slater basis 33) are higher than the experimental value. 

The calculations predict that fMws-diaziridine is more stable than the cis isomer by 7.8 kcal/niol (4-31G). This difference is close to a previous theoretical estimate of 7.1 kcal/mol by Bonaccorsi, Scrocco and Tomasi ) who used assumed geometries and a minimal Slater basis. The lower energy for the trans form can be attributed partly to the more favorable interaction between nitrogen lone pair dipoles. The theoretical bond separation energies (Table 1) at the 6-31G level are slightly less negative than the values for cyclopropane and aziridine. 

The theoretical structure for this molecule was obtained assuming Cg symmetry. The angle between the N—H bond and the ring plane is 73.7°, somewhat larger than in aziridine (69.7°) and oxaziridine (72.3°). Bond lengths are similar to those in related molecules. Dioxaziridine is an isomer of nitrous acid, HO—N=0 and is calculated to be 84.3 kcal/mol less stable than the latter (6-31G ).57)... 

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>. 

Two theoretical studies on the structure of aziridine have been published, one relating to the effect of hyperconjugation on the barriers to the inversion of nitrogen and the other to the structures of aziridine-enamines. The reaction by which (270 R = H, Me, or NH2) are hydrolysed to cis- and to trans- 27 ) has been subjected to ab initio calculations. Theoretical predictions have been found to agree with experimental results. 

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