Hydro silylation asymmetric

It was described earlier that the diselenide 2 acted as chiral ligand for the Rh(I)-catalyzed asymmetric hydrosilylation of unfunctionalized alkyl aryl ketones with diphenylsilane in tetrahydrofuran [6]. When the reaction was carried out in methanol as solvent, it gave directly a chiral alcohol, not a hydro-silylated product [7] (Scheme 5). 

A proposed mechanism for the rhodium-catalyzed alcoholysis is represented in Scheme 49 (77). In the first step, activation of the hydrosilane occurs through oxidative addition. Formation of the alkoxysilane then takes place by nucleophilic attack of a noncoordinated alcohol molecule. The dihydro-rhodium complex 143 thus obtained liberates a hydrogen molecule upon reductive elimination. Nucleophilic cleavage of the silicon-rhodium bond, without prior coordination of the alcohol at the rhodium is supported by results obtained in asymmetric alcoholysis (cf. Sect. II-D). Optical yields were shown to be little dependent on the catalyst ligands (in marked contrast with the asymmetric hydro-silylation), indicating but weak interaction between alcohol and catalyst during the reaction. Moreover, inversion of configuration at silicon, which occurs in the particular case of methanol as solvent, is not likely to occur in a reaction between coordinated silane and alcohol. 

We have investigated (23,24) the potential of asymmetric hydrosilylation of various ketones. Methyl alkyl ketones (for example, 2-octa-none) gave about 44% e.e. if phenyl cyclohexyl silane was the hydro-silylating reagent (Table III). 

The first asymmetric synthesis at a prochiral silicon center in the sense of catalytic asymmetric reactions has recently been effected in the catalytic hydro-silylation of ketones with dihydrosilanes [45,65]. 

Yao S, Meng J-C, Siuzdak G, Finn MG (2003) New catalysts for the asymmetric hydro-silylation of ketones discovered by mass spectrometry screening. J Org Chem 68 2540-2546... 

The asymmetric reduction of a-keto-esters has been achieved via catalytic hydro-silylation followed by hydrolysis (Scheme 20). The rhodium catalysts used in these... 

The 0-TBS-hydro>y-diphenylprolinol silyl ethers 14, analogues of the Jorgensen-Hayashi catalyst 3a, were tested in 2008 by List and coworkers for the challenging catalytic asymmetric Michael reactions of acetaldehyde with nitroalkenes. While 4-hydroxy-proline derivative 14a displayed diminished activity, but slightly better enantioselectivity with respect to 3a, the 3-hydroxy-proline derivative 14b gave in this case very poor results (Scheme 11.12). 

Fleming and Betson reported that Ireland-Claisen rearrangement of either the E-or Z-silyl ketene acetals bearing a stereocenter at Cl possessing a TBDPS group gave moderate (R = i-Pr) to excellent (R = Me, Ph) 1,2-asymmetric induction (Scheme 4.32) [34]. The transition states were proposed to adopt a conformation with the CT hydro-... 

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