Silyl metals, addition

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

Employing ketones or aldehydes as starting materials, the corresponding silylethers are obtained. Thereafter, the oxidation or hydrolysis of the obtained silylethers gives the corresponding alcohols (Scheme 17). In most cases, a hydride (silyl) metal complex H-M-Si (M = transition-metal), which is generated by an oxidative addition of H-Si bond to the low-valent metal center, is a key intermediate in the hydrosilylation reaction. 

A mechanism for catalysis by platinum compounds was proposed in 1965 by Chalk58) and has since been supported by increasing knowledge about silyl-metal systems and by the direct detection of Pt-Si211) and Rh-Si61,18s) complexes in the reaction mixtures. The suggested mechanism requires olefin coordination to the Pt(II) species (in the case of H2PtCl6 formed by reduction by the silicon hydride), oxidative addition of the silane, formation of an intermediate in which silicon and alkyl are both bonded to the platinum center, and reductive elimination of alkylsilane, probably assisted by coordination of more olefin ... 

Consider, for example, the interaction of a silyl hydride R3SiH with a zero-valent metal species M [M could be L2 Pt(0)]. An oxidative addition gives a silyl metal hydride [Eq. (135)] (77). A simple series of oxidative... 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Some of these redistributions may be initiated either by homolysis or catalytic activation or even by conventional acidic or basic catalysts, but a number of them proceed more readily in the presence of olefins. These reactions are fast and yield a large number of products that also contain adducts formed during the preliminary redistribution of the silane substrate(s) prior to their hydrosilylation. Redistributions of silicon compovmds catalyzed by transition metals occur when at least one Si—H bond is present in the molecule, since the Si—H bond is the most labile of those undergoing oxidative addition to yield a silyl metal hydride. 

Movassaghi and Hill developed a ruthenium-catalyzed cycloisomerization of 3-azadienynes to the corresponding pyridines [11]. The alkynyl imines were produced from a variety of iV-vinyl and iV-aryl amides by amide activation and nucleophilic addition of copper(I) (trimethylsilyl) acetylide sequence reaction. Then by Ru-catalyzed protodesilylation and cycloisomerization, the desired pyridine derivatives were formed selectively in good to excellent yields (Scheme 2.7). For the reaction mechanism, C-silyl metal vinylidene was found to be the key intermediate. 

Variations and Improvements on Alkylations of Chiral OxazoUnes Metalated chiral oxazolines can be trapped with a variety of different electrophiles including alkyl halides, aldehydes,and epoxides to afford useful products. For example, treatment of oxazoline 20 with -BuLi followed by addition of ethylene oxide and chlorotrimethylsilane yields silyl ether 21. A second metalation/alkylation followed by acidic hydrolysis provides chiral lactone 22 in 54% yield and 86% ee. A similar... 

A great number of articles related to the mechanism of this reaction has been published. It can be considered as certain that the silanes react with the platinum center by an oxidative addition to the metal with formation of a silylplatinum hydride and subsequent transfer of the silyl group to the coordinated alkene. 

Pure cis-71 could be isolated in 65% yield and was characterized by means of x-ray structure analysis (Fe-Si 2.270(1)/2.272(1) A). This ds-conformer is exclusively formed. The formation of 71 requires a photoinduced silyl exchange followed by a decarbonylation reaction and further steps of an oxidative addition to the metal [176]. 

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

In addition to routine spectroscopic characterization, transition metal silyls have been examined by a variety of physical methods, principally to determine (1) the definite presence of an Si—M bond, (2) the manner in which such a bond is influenced by other ligands, (3) whether such a bond possesses any w-component, and (4) the trans influence of the silyl ligand. 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

---