Organomagnesium reagents reactions with imines

The use of organomagnesium reagents as bases leads to complexation of the nitrile imines (e.g., 141), which has been found to have a strong effect in promoting syn selectivity in reactions with methyl 2-(l-hydroxyalkyl)acrylates via coordination of the metal atom with the alcoholic oxygen (e.g., leading to the formation of 142). Lithium complexation had little effect (78). 

Chiral amines have been transformed into chiral imines RCH=NG, which are usually in equilibrium with the tautomeric enamines. These enamines undergo asymmetric alkylations, and the best results are often obtained with ethers 1.58 or with valine derivatives 1.59 (R = i-Pr, R = tert-Bu) [169, 173,253] in the presence of bases. Enamines, lithioenamines and zinc enamines derived from imines are very potent Michael donors that often participate in highly stereoselective reactions [161, 162, 169, 173, 254, 257, 260, 262, 267], Chiral imines can suffer very selective addition reactions of organomagnesium reagents [139, 253, 254] and allyl-metals [154, 258]. They also suffer stereoselective Ti-catalyzed silylcyanation [268], Strecker reaction [266], and [2+2] or [4+2] cydoadditions [131, 256, 263], When the reaction produces an imine product, the chiral auxiliary is recovered after acidic hydrolysis. However, when an amine is obtained as the product, as is often the case from phenethylamine derivatives, the chiral residue is cleaved by hy-drogenolysis. In such cases, the chiral amine is not, strictly speaking, a chiral auxiliary. But these processes will be discussed anyway because of their importance in asymmetric synthesis. 

Similarly, reaction of enamines such as 218 with benzotriazole affords a-aminoalkyl-benzotriazoles 219, which react smoothly with organomagnesium compounds, giving tertiary alkyl carbylamines 220 (equation 149) . The whole sequence can therefore be considered as the addition of Grignard reagents to imines. 

A few examples of stereoselective additions of organomagnesium or -cerium reagents to imines bearing a chiral arenechromium tricarbonyl or dieneiron tricarbonyl residue have been described [539, 1197], Reactions of allylmetals with an oxime bearing a chiral ether functionality can be stereoselective provided that the oxime displays the E configuration [1198],... 

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