1- Azaallyl system

Bergbreiter and Newcomb have shown that the benzylamine imine of 3-pentanone is deprotonated by lithium diisopropylamide selectively at the benzylic position of a 2-azaallyllithium system that slowly isomerizes by a protonation-deprotonation sequence to afford the 1-azaallyl system (equation 46). ... 

Cycloadditions of this type do not occur with isolated non-activated C-C double bonds, the 2-azaallyl system (Scheme 3) is only capable of existence with stabilizing aromatic groups and the allyl-lithium stem (Scheme 3) has no tendency at all to undergo cycloaddition, if a group is lacking which can stabilize the negative charge in the 2-position ... 

Vinylmagnesium bromide reacts as a Michael acceptor with a zincated hydrazone, yielding a 1,1-bimetallic species, which in some cases can be sequentially trapped with two different electrophiles.246 2463 The reaction proceeds via metalla-aza-Claisen rearrangement. The dimethyl hydrazone anion behaves as an azaallylic system (Scheme 81).247 Vinylphosphonium salts add Grignard reagents, forming alkylphosphonium ylides. These ylides... 

The explanation of this observation is obscure. However, photolysis does effect the transformation of the conformation from trans.trans to cis.trans. No fluorescence has been observed from solutions of this azaallyl ion. A further difference from the diphenylallyl system is that the photo-stationary state is insensitive to temperature. 

This cycloaddition can be performed with either the amino acid, the aldehyde, or the electron-poor alkene linked to the support. Intramolecular azaallyl anion cycloadditions have been used to prepare polycyclic systems on solid phase (Entries 5 and 7, Table 15.5). 

Monitoring the isomerization process in deuterated solvent allows the determination of the mechanism operative in the model system. For instance, in the concerted mechanism, that proceeds through a symmetric transition state, for each deuteron incorporated from the solvent, an isomerization must occur. On the other hand, via the azaallylic intermediate, it is possible for a deuteron to be incorporated without a subsequent isomerization. It is this latter process which has been observed, thus strongly supporting the existence of the azaallylic intermediate (307, 308). 

In conclusion, the tautomerization is intramolecular and the suprafacial 1,3 proton shift occurs across a azaallylic anion. The model differs slightly, however, from the biological system by providing competing stereochemical and isotope-labeling reactions pathways. Therefore, coenzymes carry out stereospecific reactions due to their apoenzymes, while nonenzymatic model reactions are not as stereospecific (310). 

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