Azaallyl hydride

Interconversion of enantiomeric azaallyl hydrides like 40 and 40 is facile and probably involves an isomerization from an rf to an N-bound rj1 structure with a plane of symmetry (Eq. 42) [24]. This process is analogous to the well-documented 7T-[Pg.32]

Table 7 Activation parameters and kinetic data for azaallyl hydride inversion...

The interconversion of zirconaaziridine enantiomers is slower when there is not a primary or secondary alkyl on the zirconaaziridine carbon and isomerization to an azaallyl hydride is not possible. A mechanism that remains available involves the isomerization of each enantiomer to a planar rf complex (Eq. 43), such as that known to interconvert the enantiomers of aromatic aldehydes [16]. For the chelated zirconaaziridine 2d, high-level density functional theory (DFT) methods and a continuum solvation model have shown that enantiomer interconversion occurs through an rj1-imine intermediate (A) rather than through homolysis (B) or heterolysis (C) of the Zr-C bond [71] (Fig. 13). 

Azaallyl hydrides undergo many reactions as well as enantiomer interconversion. Heating 40a at 70 °C overnight led to the C-H activation product 41, identified by and 13C NMR (Eq. 44) [24]. Similar C-H activations (Eq. 45) have been observed by Andersen [73] and Scott [74]. 

Monitoring the progress of the reaction in Eq. 44 by H NMR revealed the formation of a transient species, assigned structure 42a on the basis of its H and 13C NMR spectra. The 13C NMR spectrum of 42a shows a resonance at 184.8 ppm, consistent with formation of a Zr-phenyl bond. Complete conversion to a related product 42b was obtained with the azaallyl hydride 40b... 

Lithium aluminium hydride reduces tosylacetonitrile to the primary enamine / -(/ -toluenesulphonyl)vinylamine 274. Treatment of the latter with sodium hydride in THF at — 60°C furnishes the stabilized a-azaallyl anion 275, which is converted into the dihydropyridine 276 by reaction with benzylideneacetophenone (equation 114)140. 

The previous cycloaddition reaction discussed is believed to proceed through an aldimine anion (19). Such delocalized anions can also be generated by treatment of suitable aldimines with a strong base. Subsequent cyclocondensation with a nitrile produces imidazoles [25-28]. The 2-azaallyl lithium compounds (19) are made by treatment of an azomethine with lithium diiso-propylamide in THF-hexane ( 5 1) (Scheme 4.2.9) [29. To stirred solutions of (19) one adds an equimolar amount of a nitrile in THF at —60°C. Products are obtained after hydrolysis with water (see also Section 2.3). If the original Schiff base is disubstituted on carbon, the product can only be a 3-imidazoline, but anions (19) eliminate lithium hydride to give aromatic products (20) in 37-52% yields (Scheme 4.2.9). It is, however, not possible to make delocalized anions (19) with R = alkyl, and aliphatic nitriles react only veiy reluctantly. Examples of (20) (Ar, R, R, yield listed) include Ph, Ph, Ph, 52% Ph, Ph, m-MeCeUi, 50% Ph, Ph, p-MeCeUi, 52% Ph, Ph, 3-pyridyl, 47% Ph, Ph, nPr, 1% [25]. Closely related is the synthesis of tetrasubstituted imidazoles (22) by regioselective deprotonation of (21) and subsequent reaction with an aryl nitrile. Even belter yields and reactivity are observed when one equivalent of potassium t-butoxide is added to the preformed monolithio anion of (21) (Scheme 4.2.9) [30]. 

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