Hydrosilylation rhodium-catalysed

Another approach in the use of chiral S/P ligands for the hydrosilylation reaction of ketones was proposed more recently by Evans et Thus, in 2003, these workers studied the application of new chiral thioether-phosphinite ligands to enantioselective rhodium-catalysed ketone hydrosilylation processes. For a wide variety of ketones, such as acyclic aryl alkyl and dialkyl ketones as well as cyclic aryl alkyl ketones and also cyclic keto esters, the reaction gave high levels of enantioselectivity of up to 99% ee (Scheme 10.44). 

In addition. Taller and Chase have reported the use of chiral tridentate S/N/P ligands for the rhodium-catalysed hydrosilylation of ketones.The best ligand, which provided an enantioselectivity of up to 64% ee, was that bearing the shortest reach to the metal to give a tridentate ligand, as shown in Scheme 10.51. 

Asymmetric hydrosilylation of ketones. Chiral alkoxysilanes have also been obtained in the rhodium-catalysed hydrosilylation of carbonyl compounds50-52 (equation 6). 

Alkyl/alkyl ketones are challenging substrates for rhodium-catalysed asymmetric hydrosilylation and are generally reduced with low enantioselectivities using oxazoline-based hgands. However a number of alternative ligand systems have been used successfidly in the rhodium-catalysed hydrosilylation of alkyl/alkyl ketones. For example, the 8-keto ester (3.158) undergoes enantioselective... 

As well as rhodium-catalysed hydrosilylation, asymmetric ruthenium and titanium-catalysed hydrosilylation have also been reported. Amongst these, Buchwald s report of the hydrosilylation of ketones using titanocene catalysts and inexpensive polymethyUiydrosiloxane (PMHS) appear to be the most general. 

Rhodium-catalysed intramolecular hydrosilylation of homoallyl silyl ethers (52), mediated by norbornene, has been shown to be controlled by ligands thus, with bis(diphenylphosphino)hexane (dpph), the reaction produces oxasilacyclohexanes (53), whereas frani-oxasilacyclopentanes (54) are formed in the presence of BINAR Norbornene is believed to serve as a hydride shuttle. " ... 

Aspects of the rhodium-catalysed hydroformylation of olefins have been reviewed. " Copper(ii) acetate catalyses the highly stereoselective solvolysis of -alkenylpentafluorosilicates to -alkenyl ethers under an atmosphere of air. Since the pentafluorosilicates can be obtained via hydrosilylation of acetylenes, the sequence represents a regio- and stereo-selective transformation of acetylenes into carbonyl precursors in moderate yield. The reaction of vinylmercurials and mercury carboxylates catalysed by palladium(ii) acetate provides a stereospecific route to enol carboxylates, which are valuable precursors of specific enolates. ... 

A rhodium(I)-diphosphine complex which is P-chiral catalyses enantioselective hydrosilylation of a range of simple ketones.263 After acidic workup, the corresponding alcohols are obtained in high yields and ees up to 99%. 

The performance of this ligand class in different catalytic reactions depends not only on the individual reaction, but also on the metal used. The rhodium NHC complex in Figure 4.23 catalyses the hydrosilylation of acetophenone with 98% conversion and... 

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

Stereoselective intramolecular alkene hydrosilylation followed by Si-C cleavage is a valuable route to diols both relative - and absolute stereochemistries may be controlled. The rates of the fundamental steps in the [Rh(diphosphine)] catalysed reactions are controlled to some extent by the nature of the diphosphine. From deuterium-labelling studies a silyl insertion mechanism becomes apparent. Whether such mechanisms are applicable to C=0 hydrosilylation versions of this reaction is not yet known. Other highly enantioselective (83-96% ee) C=0 silylations use 13,162 but attempts to use readily available (-)-sparteine as a rhodium ligand are much less successful (5-8% ee).i Further details of the spectacularly effective MOP ligand 14 have appeared.i Optical purities of 93-96% ee are realised using 0.01 mol% Pd2( x-Cl)2( n-C3H5)2 and 0.02 mol% 14 for terminal alkenes and norbomene. 

During a synthesis of pederol dibenzoate, a key step required formation of a regiospecific enol silyl ether in a highly oxygenated and base-sensitive substrate. The reaction was uniquely achieved by a rhodium(i)-catalysed hydrosilylation sequence (Scheme 47). ... 

Mechanistic aspects of hydrosilylation catalysed by rhodium have been outlined, and the recently introduced silylene-based mechanism has been discussed. ... 

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