Carbonyl asymmetric hydrosilylation

While it is beyond the scope of this chapter to cover the asymmetric hydrosilylation of ketones and imines in any detail, a number of the more catalytically active ML combinations will be mentioned here. A full review of the area has recently appeared.138 Asymmetric hydrosilylation of carbonyl groups is usually performed with rhodium or titanium catalysts bearing chelating N- or P-based ligands. Representative results for some of the most active Rh/L combinations (Scheme 32) for addition of Si H to acetophenone are given in Table 11. 

Until 1968, not a single nonenzymic catalytic asymmetric synthesis had been achieved with a yield above 50%. Now, barely 15 years later, no fewer than six types of reactions can be carried out with yields of 75-100% using amino acid catalysts, i.e., catalytic hydrogenation, intramolecular aldol cyclizations, cyanhydrin synthesis, alkylation of carbonyl compounds, hydrosilylation, and epoxidations. 

Asymmetric hydrosilylation of prochiral carbonyl compounds, imines, alkenes and 1,3-dienes has been extensively studied and continues to be one of the most important subjects in the hydrosilylation reactions. This topic has been reviewed at each stage of its development as a useful synthetic method based on asymmetric catalytic processes1,3,187-189. In the last decade, however, substantial progress has been made in the efficiency of this reaction. Accordingly, this section summarizes the recent advances in this reaction. 

Ferrocenyl ligands, via zinc reagents, 9, 120 Ferrocenylmethyl phosphonium salts, with gold(I), 2, 274 Ferrocenylmonophosphine, in styrene asymmetric hydrosilylation, 10, 817 Ferrocenyl oxazolines, synthesis, 6, 202 Ferrocenylphosphines with chromium carbonyls, 5, 219 in 1,3-diene asymmetric hydrosilylation, 10, 824-826 preparation, 6, 197 various complexes, 6, 201 Ferrocenylselenolates, preparation, 6, 188 Ferrocenyl-substituted anthracenes, preparation, 6, 189 Ferrocenyl terpyridyl compounds, phenyl-spaced, preparation 6, 188 Ferrocifens... 

Although -3-pinanyl-9-borabicyclo[3.3.1]nonane and related substances have also been developed as efficient asymmetric reducing agents for carbonyl compounds (Volume 8, Chapter 1.3), we discuss here only asymmetric reductions using chirally modified metal hydride reagents. The asymmetric hydrosilyl-ation of a carbonyl group catalyzed by a chirally modified transition metal is mentioned briefly. 

Asymmetric hydrosilylation of prochiral carbonyl compounds, alkenes, 1,3-dienes, and imines has been extensively studied and remains one of the most important subjects in the field. This reaction is strongly affected by the nature of the catalyst (metal, type of chiral ligand) and the substrate as well as the reaction conditions (solvent, temperature, etc.). In recent years, many papers have been published on asymmetric hydrosilylation, describing new catalytic systems (mainly new optically active ligands) and new synthetic applications of the reaction [4, 24]. 

Rhodium-phosphine complexes are usually active and effective in the asymmetric hydrosilylation of olefins, ketones, and aldehydes, allowing for the virtual synthesis of optically active alkoxysilanes and organic compounds of high purity. Chiral rhodium-phosphine catalysts predominate in the hydrosilylation of pro-chiral ketones. This subject has been comprehensively reviewed by several authors who have made major contributions to this field [52-54]. A mechanism for the hydrosilylation of carbonyl groups involving the introduction of asymmetry is shown in Scheme 3 [55]. 

The hydrosilylation of carbon-heteroatom multiple bonds had received little attention until it was found in 1972 that Rh(PPh3)3Cl is an extremely effective catalyst for the hydrosilylation of carbonyl compounds. This is a new and unique reduction method since the resulting silicon-oxygen bond can easily be hydrolyzed. Other transition metal complexes including platinum, ruthenium , and rhodium also have good catalytic activity in the selective and asymmetric hydrosilylation of carbonyl compounds "". 

Asymmetric Hydrosilylation with Dihydrosilanes. Chiral alkoxysilanes have also been obtained in the rhodium catalyzed hydrosilylation of carbonyl compounds (68, 78, 79) (eq. [29]). 

Two kinds of selective asymmetric hydrosilylation of a,/ -unsaturated carbonyl compounds have been performed. The 1,4-addition induces asymmetry on a y-carbon to give optically active saturated carbonyl compounds150,116 (equation 76), while the 1,2-addition gives optically active allylic alcohols151,128 (equations 77 and 78). 

Both ligand 171 and its parent, (iS)-P-Phos (Ar = Ph), as their complexes with in 5/tM-generated CuH, have been studied in asymmetric hydrosilylations of halogen-containing aryl ketones. That is, a variety of ketones bearing either chloride or bromide in the a-, 3-, or y-positions undergo 1,2-carbonyl reduction in toluene at -20 C, using phenylsilane as the source of hydride. Enantiomeric excesses t5 ically exceed 90%, as illustrated by the examples below. 

Synthetic and catalytic aspects of asymmetric hydrosilylation of C=C bonds were covered by Nishiyama and Itoh (19) and Hayashi (20,21). Catalyzed hydrosilylation of carbonyl compounds and/or imines were reviewed by Gronzalez and Nolan (22), Nishiyama (23,24), Carpentier and Bette (25), and Riant and co-workers (26). Copper catalyzed 1,2- and 1,4-hydrosilylation were concisely reviewed by Rendler and Oestreich (27). 

This review deals with recent advances in catalytic asymmetric hydrosilylation of olefins, carbonyl and imino compounds in the presence of transition metal complexes of chiral phosphine ligands with particular emphasis on the asymmetric reduction of prochiral carbonyl compounds, which has been extensively studied in the last few years by several research groups and proved to provide an effective reduction method for organic syntheses. 

In Section 4, it is described that chlorotris(triphenylphosphine)rhodium(I) (7) is quite an effective catalyst for the hydrosilylation of carbonyl compounds. For this reason, extensive studies on asymmetric hydrosilylation of prochiral ketones to date have been based on employing rhodium(I) complexes with chiral phosphine ligands. The catalysts all prepared in situ are rhodium(I) complexes of the type, (BMPP>2Rh(S)a (8) [40] and (DIOP)Rh(S)Cl (6) [41], and a cationic rhodium(III) complex, [(BMPP)2lUiH2(S)2] Q04 (5) [42], where S represents a solvent molecule. An interesting polymer-supported rhodium complex (V) [41], and several chiral ferrocenylphosphines [43], recently developed as chiral ligands, have also been employed for asymmetric hydrosilylation of ketones. Included in this section also are selective asymmetric hydrosilylation of a,0-unsaturated carbonyl compounds and of certain keto esters. 

Selective asymmetric hydrosilylation of a,p-unsaturated carbonyl compounds... 

In 1958, Russian chemists [31] reported that chloroplatinic acid-catalyzed hydrosilylation of a,/3-unsaturated carbonyl compounds takes place in a 1,4-fashion. Recently, it has been disclosed [35] that highly selective 1,2- as well as 1,4-addition of hydrosilanes to ajS-unsaturated terpene ketones can be achieved by using chloro-tris(triphenylphosphine)rhodium(I) (7), the selectivity depending markedly on the nature of the hydrosilane employed as described in Section 4.1. This achievement has resulted in studies on two kinds of selective asymmetric hydrosilylation of a, unsaturated carbonyl compounds by making use of either selective 1,4-addition or 1,2-addition the 1,4-addition induces asymmetry on a jS-carbon to afford optically active saturated carbonyl compounds, while the 1,2-addition gives optically active allylic alcohols. 

In the preceding Sections it was described that chiral phosphine-rhodium complexes are effective in causing stereoselective addition of a hydrosilane to a variety of prochiral carbonyl compounds to give silyl ethers of the corresponding alkanols with fairly high enantiomeric bias at the carbon atom. The present section describes an application of the catalytic asymmetric hydrosilylation of ketones to the preparation of some new asymmetric bifunctional organosilanes. 

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