Silyl trick

Fig. 1.1.1 a-Silyl-controlled asymmetric synthesis (the silyl trick ). 

In removing the oxazoline auxiliary from the products, Richards and coworkers have demonstrated the use of the diastereoselective ferrocenyloxazoline lithiation in the synthesis of conformationally constrained amino acid derivatives (Scheme 146) °. Amination of 305 was achieved by nitration, reducing to the amino group after removal of the oxazoline under standard conditions. Using the trick of silylating the more reactive diastereotopic site, it was possible to make either enantiomer of 321 from the same oxazoline starting material. 

The same product could be made by an aldol condensation between the enolate of a ketcir the same aldehyde (EtCHO) but some other method would have to be used to ensure enolizc the ketone on the right side and to prevent self-condensation of the aldehyde. A silyl eru produced via the kinetic enolate would do the trick. 

Hydrosilylations by complexed CuH have been applied to several substrate types (Scheme 1-17). As illustrated by the following examples, the stereochemical outcomes from both 1,2-additions (to aryl ketones and aryl imines ) and 1,4-conjugate additions (cyclic ketones, P-aryl and/or P-silyl enoates, and unsaturated lactones) can be controlled by these ligand-accelerated reactions. One of the key tricks to this chemistry is to take advantage of the tolerance of CuH complexes to alcohols and water.In fact, several methods rely on the presence of a bulky alcohol (e.g., t-BuOH) to significantly enhance reaction rates. It takes relatively little added alcohol (volume-wise) to accelerate the hydrosilylation, usually on the order of 1-3 equivalents. The role of this additive is usually ascribed to the more rapid quenching of an intermediate copper alkoxide or enolate, which necessarily generates a copper alkoxide, an ideal precursor to rapid reformation of CuH in the presence of excess silane. Thus, the rate increase is presumably due to... 

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