Phosphoramides enantiopure

The first example of an enantiopure Lewis base promoted allylation was reported by Denmark and coworkers in 1994 [46]. Stoichiometric amounts of the enantiopure phosphoramide (R,R)-20 promoted the enantioselective allylation (Scheme 16.13). There was a complete stereochemical correlation between the geometry (E/Z) of allylsilane and the diastereomeric ratio (syn/anti) of the products. 

The early phosphoramides 21.9 and 21.10 (Figure 21.1), developed by Denmark, exhibited modest enantioselectivity in the allylation reaction but played an important role in the mechanistic elucidation and development of the second generation of catalysts. Thus, kinetic measurements and the observation of a nonlinear relationship between the enantiopurity of 21.9/21.10 and the product 21.7 indicated that two molecules of the catalyst are coordinated to the silicon centre.However, when the concentration of the catalyst is low, a second mechanism may compete, namely that with only one molecule of the catalyst coordinated, which apparently attenuates the enantioselectivity. As a logical progression from these mechanistic observations, bidentate catalysts, such as 21.12 and 21.13, were designed. The latter exhibited higher enantios-electivities and also was more reactive, so that the loading could be reduced from 10-20 to 5 Excellent diastereoselectivity was... 

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