Inversion, nitrogen

The fully saturated pyrazolidines have been utilized as models for the study of the nitrogen inversion of hydrazines. For instance, (75), a 2,3-diazabicyclo[2.2.1]heptene derivative, presents a consecutive inversion process at two nitrogen atoms with an activation barrier... 

Conformation about a Csp —Csp single bond (Conformation of the ring (Nitrogen inversion... 

The conformation of heteroethylenic rings. A - and A -pyrazolines, and the nitrogen inversion and preferred orientation of the IV-methyl groups in pyrazolidines (71T123) have been discussed in connection with their NMR spectra (Section 4.04.1.3.3). 

Diaziridines also show slow nitrogen inversion, and carbon-substituted compounds can be resolved into enantiomers, which typically racemize slowly at room temperature (when Af-substituted with alkyl and/or hydrogen). For example, l-methyl-3-benzyl-3-methyl-diaziridine in tetrachloroethylene showed a half-life at 70 °C of 431 min (69AG(E)212). Preparative resolution has been done both by classical methods, using chiral partners in salts (77DOK(232)108l), and by chromatography on triacetyl cellulose (Section 5.08.2.3.1). 

Table 1 Barriers to Nitrogen Inversion in Substituted Aziridines ...

Diazetidines applications, 7, 483 electrophilic reaction, 7, 460 nitrogen inversion, 7, 10 nucleophilic reactions, 7, 462 photochemical reactions, 7, 456-457 reductive cleavage, 7, 465 spectroscopy, 7, 451-454 structure, 7, 451... 

In S-N compounds containing H bonded directly to nitrogen, inverse detection 2D NMR spectroscopy can be applied to give N... 

The barriers for nitrogen inversion in 1,3,4-oxadiazolidine derivatives have been determined by the introduction of chiral substituents on the nitrogen atoms and monitoring diastereomerization by NMR and by following enantiomer-ization by multidimensional gas chromatography <2000AGE2938>. 

Besides, cyclic nitroso acetals have attracted considerable interest because [3+ 2]-cycloaddition of nitronates to olefins provides a simple and efficient procedure for the synthesis of five-membered cyclic nitrosals (see Scheme 3.153). Hence, knowledge of the configurations of these products allows conclusions about the mechanism of this very important process. However, it is impossible to distinguish kinetic and thermodynamic products of [3+ 2]-cycloaddition of nitronates to olefins without studying such a fundamental problem as the stereodynamics of five-membered cyclic nitroso acetals, that is, the ring and nitrogen inversions. 

Subsequently, this interpretation has been commonly accepted however, it does not account for the factors responsible for a decrease in the rate of the fast classical ring inversion. Hence, in our opinion, it is more reasonable to consider the stereodynamics of isoxazolidines as a result of superposition of two processes, a fast ring inversion (IR) and slow nitrogen inversion (shown in the lower portion of Scheme 3.166) (418). 

In conclusion, the barrier of nitrogen inversion in cyclic nitrosoacetals is very high. This circumstance allows one to distinguish kinetic and thermodynamic... 

Detailed configurational and conformational analyses of the starting six-membered cyclic nitronates (356) of cationic intermediates (357), and (decoupling products (357 + Nu), taking into account slow nitrogen inversion (Ijv) in nitroso acetals (see Section 3.4.3.4.4), made it possible to propose a mechanistic model based on the stereochemical outcome of the C,C-coupling process (Chart 3.23). 

Therefore, X-ray diffraction data provide unambiguous evidence that the structural characteristics of BENA are substantially more similar to those of nitroso acetals (418) (Table 3.26) than to enamines and, consequently, the dynamic process observed by NMR spectroscopy is most likely a slow nitrogen inversion. 

Quantum chemical calculations by the DFT PBE/TZP method for model nitroso acetals and related compounds (467) are also indicative of nitrogen inversion as the slowest dynamic process in the ene nitroso acetals studied (see Fig. 3.4, Table 3.27). 

Table 3.28 The barriers of nitrogen inversion of BENA and related compounds determined with dynamic NMR...

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