Silyls metal

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

The first silyl-metal complex was synthesized in 1956 with the compound CpFe(CO)2SiMe3 [93, 94]. Since then, methods for the formation of M-Si bonds have been investigated systematically and developed to an effective set of tools. Numerous articles covering this subject give evidence for the continuous development of this area [64, 95],... 

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. 

These compounds form an interesting contrast to the Lewis base adducts of silyl-metal carbonyls (Section II,C,4), where silicon is thought to be present in a cation. 

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. 

Carreira and Kruger reported facile transmetallation of silicon enolates to other soft metal enolates including Gu derivatives.499 They reasoned that the use of soft metal fluoride complexes enabled silyl metal transmetallation with catalytic use of a soft metal source. The concept is illustrated in Scheme 103. Normal Lewis acid-catalyzed reactions of silicon enolates with aldehydes proceed via activation of aldehydes by carbonyl oxygen coordination to Lewis acids, as shown in the upper equation of Scheme 103. A key step for catalytic turnover is the desilyation of 233 by the... 

Silyl radicals have been produced by one-electron oxidation of silyl metals [11]. This is found to be the method of choice for the generation of persistent silyl radicals and allowed the preparation of the first isolable silyl radical (see later in this chapter). Reactions (1.5) and (1.6) show two sterically hindered silyl anions with Na+ as the counter-cation, and their oxidation by the nitrosyl cation [12] and the complex GeCh/dioxane [13], respectively. 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

Less reactive silyl metal species such as lithium bis(triphenylsilyl) cuprate react with a variety of acyl chlorides to give the corresponding acyl silanes in moderate to good yields75. Dilithium bis(trimethylsilyl) cyanocuprate is particularly effective for the preparation of sterically hindered acyl silanes (Scheme 8), and appears to provide a good general... 

When the salt [BU4NHAUX2] (X = Cl, Br) was reacted with 2 equiv of the dppm-substituted iron silyl metallate K[Fe Si(OMe)3 (CO)3(dppm)], instead of the expected [Fe—Au—Fe]- silyl derivative anion chain, the complex [AuFe Si(OMe)3 (CO)3(/r-dppm)]2 CH2Cl2 (30) was obtained with a Au—Au and two Au—Fe bonds. It is interesting to point out that complex 30 forms a 10-membered-ring structure in which the Au—Au interaction is reminiscent of the transannular interactions that occur in organic cycles of middle size (8-12). The similar reaction with HgCl2 yielded the expected Fe—Hg—Fe complex (31)96. 

Various types of partially arylated alkylpolysilanes are most conveniently obtainable by use of the silyl metallic reagents. Since a number of examples of the synthesis of such a class of compounds have been given in recent reviews (48, 51, 73, 76a, 212), only a few are illustrated below. 

While silyl metal carbonyl derivatives are unreactive towards neutral metal carbonyls (7), metal carbonyl anions give rise to various displacement processes (entries 1-3). The order of displacing ability seems generally similar to that noted earlier for nucleophilicities (Section II, A,1) ... 

Much more promising results have been obtained with a flow system shown diagrammatically in Fig. 6 this allows the vapor of a silyl metal-carbonyl derivative to be rapidly pyrolyzed in a stream of helium... 

In the present case, conditions are adjusted so that helium and silyl metal-carbonyl vapor (typically with a mole ratio of 30 1) have a total pressure of about 1 mmHg. Decomposition is very rapid on entering the furnace crystalline solids may form around the exit nozzle, while a matt grey or mirror-like thin film is deposited on a small plate about 5 mm from the nozzle. These thin films have been analyzed by X-ray powder and electron microprobe methods, and directly by atomic absorption spectroscopy (33,121, 187). 

Die 2-stufige Reaktion, das heiBt die Isolierung einer Silyl-Metall-Verbindung und ihre Umsetzung mit einem Halogen-silan bietet vor allem... 

Triorgano-alkoxy-silane konnen in ahnlicher Weise mit Silyl-Metall-Ver-bindungen reagieren wie die -chlor-silane (180)... 

Route (a) is severely limited by the availability of the silyl alkali reagents. KSiH3i4>, LiSiph3a> 27> 50> 155>1S6), LiSimeph262,64), LiSime2ph64>, and KSiph3129 have been used in the synthesis of Ti-, Zr-, Hf-, Fe-, Pt-, and Au-Si compounds. The reactions are carried out in cooled etheral solvents, the yield of the silyl metal derivative varying between 13 (Eq. (1)) and 90% (Eq. (2)) ... 

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 ... 

In spite of the many known silicon-transition metal complexes (15,16), little systematic work has appeared on the reaction mechanisms of silyl metal complexes. This state of affairs is in marked contrast to the current work on cr-alkyl transition metal complexes, where much emphasis is placed on determining detailed decomposition mechanisms (75-76). The reason for the interest in decomposition mechanisms is that the products of a transition metal-catalyzed reaction are released by the decomposition of the product-metal complex. Thus, to understand a catalytic process, one must have knowledge not only of the interaction of the reactants with the metal (leading to a substrate-metal complex), but also of the mechanisms whereby substrates are transformed on the metal and the manner in which the products are released. 

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... 

S.2.9.3. t] o-Alkyl-, -Aryl-, Acyl- and Silyl Metal Complexes by Reaction of Metal-Atom Vapors. 

Unusual Double Silylation Reactions of Bis(silyl)metal Complexes with o-Carboranyl Unit... 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

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