Invertomers

NMR investigations in the diaziridine field also led to the problem of inversion stability at nitrogen. Further investigations paralleled those of oxaziridines NMR investigation in solution (67CB1178) was followed by preparative separation of invertomers and finally preparation of optically active individuals. 

The inversion barrier between (28a) and (28c) is 114kJmoF , the invertomers having the same free energy by chance. NMR data of oxaziridine carbon and substituent groups of the ring for compounds (29) and (30), taken from the above publications, are as shown. 

Aziridine, 2,3-diphenyl-l-(2,4,6-trinitrophenyl)-irradiation, 7, 61 Aziridine, 1,2-divinyl-rearrangement, 7, 539 Aziridine, 2,3-divinyl-rearrangement, 7, 42, 65, 539 Aziridine, N-ethyl-inversion, 7, 6 Aziridine, 2-halo-reactions, 7, 74 Aziridine, A/-halo-invertomers, 7, 6 Aziridine, 2-methyl- N NMR, 7, 11 Aziridine, methylene-ring-ring valence isomerizations, 7, 22 synthesis, 7, 92 Aziridine, iV-nitroso-reactions, 7, 74 Aziridine, iV-phosphino-inversion, 7, 7 Aziridine, 1-phthalimido-UV irradiation, 7, 62-63 Aziridine, l-(3-thienyl)-2-vinyl-rearrangement, 4, 746 Aziridine, 7V-trimethylsilyl-inversion, 7, 7 Aziridine, 1,2,3-triphenyl-irradiation, 7, 61 Aziridine, vinyl-isomerization, S, 287 Aziridinecarboxylic acid ring expansion, 7, 262 Aziridine-2,2-dicarboxylic acid, 1-methoxy-diethyl ester... 

Interestingly, both invertomers of the obtained M-chloroaziridines 16 were clearly observable in the H-NMR spectrum and they could even be separated by chromatography. The dehydrochlorination was investigated with a variety of bases however, the resulting yields were disappointingly low. Only for R = Ph, a yield of 39% of azirine 17 was obtained using DBU as the base, in all other cases the yields were lower [22]. Davis et al. [23] successfully applied the -elimination of the sulfinyl group in chiral non-racemic N-sulfinylaziridines (Scheme 9), whereby the eliminated sulfenate was trapped by an excess of methyl iodide, which facilitated the isolation of the desired product (18). 

A different situation existed when X and Y were not the same since two individual invertomers 87a and 87b were possible. In all cases studied, one invertomer was dominant (Figure 5). The invertomer preference was established using a combination of VT NMR and NOE spectroscopy. A bridge-substituent hierarchy was established in the N-benzyl series in... 

Molecular modelling (AMI) indicated that the bridge hetero-atom separations in 137 were as follows N-N = 8A, N-0 = 4.6A, 0-0 = 2.7 A. In addition, the invertomer preference of the N-benzyl groups position them over the aromatic rings thereby ensuring that the lone pairs on the heteroatoms are concentrated within the cavity section of the molecule. 

Preliminary studies of nitrogen substituent inversion processes have been reported for several naphthalen-l,4-imine derivatives. The syn and anti invertomers of the A-chloroamine (117) equilibrate in solution to a mixture in proportion 3 2. The process can be followed kinetically by NMR spectroscopy starting from the pure anti compound the inversion is relatively slow ki = 2.6 x 10 sec at 23°), and the free-energy barrier to inversion is as high AF = 23.5 kcal mole ) as values found for inversion in aziridines. (A-Chloroaziridine derivatives, for which the energy barrier is even higher, have also been resolved into diastereoisomeric invertomers. )... 

For the A-methyl compounds 107 and 108 the syn anti inversion is faster AF = 14 kcal mole ) the NMR spectrum of each compound at 22° consists of three time-averaged signals for the vinylic, bridgehead, and A-methyl hydrogen atoms, each of which broadens and splits into two as the temperature is lowered. The major component of the mixture was tentatively suggested to be the anti invertomer for each of 107 and 108. Similar results were described for the N-benzyl derivatives 102 and 123. ... 

A preliminary study by NMR spectroscopy has been made of the equilibration of N-invertomers for the anthracen-9,10-imine (195). ... 

B invertomer with s-cis conformation seems to be present (73CR(C)(276)511), while the compound with R = Me shows a temperature- and solvent-dependent NMR spectrum, owing to the presence of a mobile equilibrium between invertomers A and B. The s-trans and s-cis conformations were assigned (73CR(C)(276)511) to the A and B forms, respectively. 

In derivative 62, fast equilibration, on the NMR time scale, occurs between invertomers, which should both adopt (73CR(C)(276)511) the s-trans conformation. These results are not in a complete agreement with those from an IR study (74SA(A) 1471), which showed equal amounts, not solvent dependent, of the two rotational isomers for derivative 60 with R = R = Me and the presence of the B invertomer with predominant s-trans conformation for derivative 61 with R = Me and t-Bu. 

Substituents in the aziridine ring stabilize (75JA4692) one of the nitrogen invertomers, as is expected from steric effects in derivative 86, the predominant form is the one with the acetyl group anti with respect to the phenyl group. 

In C- and N-acyl derivatives of three-membered rings, the geometry of the conformers still seems rather undetermined and, as a result, the same may be said of conformer populations. This is true of simple acetyl derivatives and, of course, of the conformationally more complex aroyl derivatives. For the N-acylaziridines, assignment of nitrogen invertomers seems sufficiently reliable... 

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