Chiral silyl-transition-metal complexes

C. Chiral Silyl-Transition Metal Complexes Nucleophilic Displacement of... 

Chiral organosilanes have been shown to undergo stereospecific catalytic reactions leading to the preparation of optically active silyl-transition metal complexes. We first discuss the stereochemistry and mechanism of transition metal catalyzed reactions of organosilicon compounds. Then the stereochemistry of chiral organosilyl-transition metal complexes are described. The chemistry of optically active silyl- and germyl-transition metals has been the subject of a recent review (12), and we concentrate here on mechanistic implications, especially in the field of homogeneous catalytic reactions. 

Synthesis of Planar Chiral Aromatic Compounds The Dotz benzannulation reaction initially affords arene complexes, which are somewhat unstable and difficult to isolate. These complexes possess a chirality element that is lost upon decomplexation. The instability originates with the free phenol group, and a relatively stable complex that can easily be isolated using standard organic chemistry techniques is obtained if the free phenol is converted to an ester or silyl ether prior to isolation. Planar chiral arene/transition metal complexes have been used extensively for asymmetric synthesis [27]. 

Ferraris et al.108 demonstrated an asymmetric Mannich-type reaction using chiral late-transition metal phosphine complexes as the catalyst. As shown in Scheme 3-59, the enantioselective addition of enol silyl ether to a-imino esters proceeds at —80°C, providing the product with moderate yield but very high enantioselectivity (over 99%). 

Addition of the elements of Si—H to a carbonyl group produces silyl ethers which are synthetically equivalent to chiral secondary alcohols since the silyl groups are easily hydrolyzed. Hydrosilylation can be catalyzed by acids or transition metal complexes. Enantioselective hydrosilylation of prochiral ketones has been extensively studied using platinum or rhodium complexes possessing chiral ligands such as BMPP (86), DIOP (87), NORPHOS (88), PYTHIA (89) and PYBOX (90)." ... 

Since the middle of the 198O s remarkable progress has been achieved in the development of asymmetric aldol reactions of silyl enolates. In the beginning of this evolution, chiral auxiliary-controlled reactions were extensively studied for this challenging subject [106]. As new efficient catalysts and catalytic systems for the aldol reactions were developed, much attention focused on catalytic enantiocontrol using chiral Lewis acids and transition metal complexes. Thus, a number of chiral catalysts realizing high levels of enantioselectivity have been reported in the last decade. 

In 1986, Reetz et al. reported that chiral Lewis acids (B, Al, and ll) promoted the aldol reaction of KSA with low to good enantioselectivity [115]. The following year they also introduced asymmetric aldol reaction under catalysis by a chiral rhodium complex [116]. Since these pioneering works asymmetric aldol reactions of silyl enolates using chiral Lewis acids and transition metal complexes have been recognized as one of the most important subjects in modern organic synthesis and intensively studied by many synthetic organic chemists. 

The search for a catalyst suitable to promote addition of the less reactive silyl enol ethers of ketones has identified a novel class of cationic transition metal complexes in two independent laboratories. The use of a chiral palladium(II) di-aquo complex in the catalytic asymmetric addition of silyl enol ethers to aldehydes (first demonstrated by Shibasaki, Sodeoka et al. [52a, 52b]) provided a clear precedent for their subsequent use with a-imino esters [53] (Scheme 27). Initial experiments focused on the reaction of various a-imino esters 82a-c with silyl enol ether 83 (1.5equiv) in the presence of the Pd diaquo complex 80a (10 mol %) in DMF. Extensive experimentation led to the formation of 84c in 67% ee, and also underscored the importance of suppressing the generation of tetrafluoroboric acid during the course of the reaction. 

The chemistry of silicon, germanium, and tin transition metal compounds has been the subject of several reviews (12, 180). Optically active silyl ligands have been introduced in a transition metal complex by reaction of chiral functional organosilanes. However chiral silyl ligands containing complexes are limited to a few metal centers we shall discuss in turn iron, cobalt, platinum, and manganese complexes. 

Reactions of 2-alkenes, 3-alkenes, etc., with monohydrosilanes lead predominantly to alkylsilanes with terminal silyl group, which means that in the presence of transition metal complexes, hydrosilylation is accompanied by the isomerization of olefins. The formation of adducts with an internal silyl group is also possible, especially in the presence of chiral platinum and palladium complexes (8). 

Catalytic alcoholysis of silanes by a variety of transition metal based catalysts is a useful method to form silyl ethers under mild conditions (Scheme 19). The process is atom-economical hydrogen gas is the only byproduct. This mild method has not been fully exploited for the preparation of unsymmetrical bis-alkoxysilanes. A catalytic synthesis using silicon alcoholysis would circumvent the need of bases (and the attendant formation of protic byproducts), and eliminate the need for excess silicon dichlorides in the first silyl ether formation. We sought catalytic methods that would ultimately allow formation of chiral tethers that are asymmetric at the silicon center (Scheme 20). Our method, once developed, should be easily transferable for use with high-value synthetic intermediates in a complex target-oriented synthesis therefore, it will be necessary to evaluate the scope and limitation of our new method. 

Silyl Anion Equivalent. Silylboronic ester 1 reacts as a silyl anion equivalent in the presence of transition metal catalysts. Cyclic and acyclic a,/3-unsaturated carbonyl compounds serve as good acceptors of the silyl groups in conjugate addition of 1 catalyzed by rhodium and copper complexes, giving /3-silylcarbonyl compounds (eq 30). The silylation takes place with high enan-tioselectivity when Rh/(5)-BINAP or Cu/chiral NHC catalysts are used. Three-component coupling of 1, a,/3-unsaturated carbonyl compounds, and aldehydes affords 8-hydroxyketone stereoselec-tively in the presence of a copper catalyst (eq 31). The copper enolate 32 is presumed as an intermediate of the reaction. 

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