Silyl-transition metal complexes synthesis

The chemistry of organosilicon compounds has expanded with the use of transition metals. Both catalytic processes (179) and the chemistry of silicon transition metal compounds (180) have been developed. Catalytic reactions of organosilanes and more recently, reactions of silyl-transition metal complexes have already found interesting applications in organic synthesis (80,181,182). 

Transition metal complexes have been widely investigated as catalysts for the synthesis of alkoxysilanes via alcoholysis of hydrosilanes. The system provides a convenient method for the protection of hydroxy groups in organic synthesis and the synthesis of silyl ethers. The general reaction is shown in Eq. (59). 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Some years ago Hengge et al. published a feasible synthesis of KSi6Men (1) and did basic research on the reactivity of this silyl anion including the preparation of transition metal complexes [1-3]. Our group contributed two crystal structures of such compounds [4, 5]. In an effort to get further insight into the reactivity of 1 and the structures of its transition metal conq>lexes, we reacted it with various transition metal conq>ounds. 

The synthesis and reactivity of transition metal complexes, including titanium derivatives with functionalized silyl-substituted Cp and related ligands, have been reviewed.378... 

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. 

Tetraethynylmethane (39), a potential monomer for a three-dimensional superdiamonoid carbon network [1], was elusive for many years [51, 52], until its synthesis was accomplished in 1993 by Feldman and co-workers [53]. The key step in the synthesis was the acid-mediated Johnson orthoester variant of the Claisen rearrangement, which provided the central quaternary methane C-atom with suitable functional groups for the ultimate transformation into 39 [Scheme 13-9(b)]. Solid 39, like tetraethynylethene (20), decomposes rapidly at room temperature in either the presence or absence of oxygen. The earlier efforts to prepare tetraethynylmethane had yielded the peralkynylated derivatives 40-42 [Scheme 13-9(c, d)] [51, 52]. Tetraethy-nylallene represents another potential precursor for a three-dimensional carbon network [1], but remains elusive of the perethynylated [K]cumulenes, so far only the silyl-protected [3]cumulenes 43a and 43b [Scheme 13-9 (e)] have been prepared [54]. With 44 [Scheme 13-9 (f)], the first transition metal complex of a perethynylated ligand is now available [55]. 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

Although a lot of data have been reported on such competitive reactions, the number of examples of selective dehydrogenative silylation remains limited. Over the past two decades, the research efforts have been focused on the search for new selective catalysts of dehydrogenative silylation ensuring efficient generation of vinylsilanes and other vinylsilicon compounds as well as on mechanistic implications of transition metal complexes as real intermediates of these complicated processes. Dehydrogenative silylation has become a useful method for synthesis... 

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]. 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

Silyl and germyl complexes with suitable functionalities were successfully used as precursors for the synthesis of compounds with corresponding transition metal element multiple bonds.137 145 Due to the aforementioned... 

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. 

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