Bis-silylation reactions

Although no efficient bis-silylation reaction for aldehydes and ketones with acyclic disilanes has been established, highly strained cyclic disilanes add to C=0 bond in a 1,2-fashion in the presence of nickel and platinum catalysts (Equations (44) and (45)).83 128... 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

Bis-silylation of bicyclopropylidene in the presence of the palladium-isocyanide catalyst with hexamethyldisilane and phenylpentamethyldisilane gives 1,2-addition products in good yields at 70 °C vide supra). No G-G bond cleavage takes place with the bis-silylation reaction. In contrast to this example, in which both the cyclopropane rings are... 

Addition of disilanes to isocyanides is catalyzed by palladium complexes, giving A-substituted bis(silyl)imino-methanes (Equation (53)).132 A wide range of isocyanides including aryl isocyanides and alkyl isocyanides can take part in the reaction. However, it is important to note that tert-alkyl isocyanides hardly undergo the bis-silylation reaction. This low reactivity of / r/-alkyl isocyanides allows their use as spectator ligands in the catalytic bis-silylations. 

Nickel tetracarbonyl undergoes a rapid oxidative addition of the Si-Si bond of 1, highly strained fluorinated disilane, at room temperature to give ffve-membered cyclic bis(organosilyl)nickel(II) complex 2, which then reacts with terf-buty-lacetylene to give six-membered disilacyclohexadiene derivatives 3 as a mixture of the regioisomers (Eq. 1) [10]. A similar bis-silylation reaction of alkynes with bis(organosilyl)nickel(II) complex has been reported in the reaction of bis(trichlorosilyl)(bipy)nickel(II) (bipy 2,2 -bipyridyl), which is prepared by dialkyl(bipy)nickel(II) with trichlorosilane [11]. 

Oxidative addition of Si-Si bonds onto palladium(O) has long been presumed to be involved in a number of palladium-catalyzed bis-silylation reactions of unsaturated carbon compounds. The oxidative addition and its reverse reaction, i.e., reductive elimination, may be in rapid equilibrium, whose direction is influenced by the structure of disilanes and ligands on the palladium atom. In spite of early reports on the formation of bis(organosilyl)palladium(II) complexes [14,15], a well-characterized complex was first synthesized in 1992 by reaction of hydro disilanes with hydridepalladium complex, probably through initial activation of Si-H bond followed by silylene migration (see Sect. 2.3) [16]. Since... 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

The silicon-carbon bonds having at least one hetero-atom substituent on the silicon atom are oxidized by H202 with a fluoride source to give the corresponding alcohols with retention of the configuration at the carbon atoms [103,104]. The Si-C oxidation is successfully combined with bis-silylation reactions, providing new access to stereo-defined alcohols. Optically active (3-hydroxy ketones 108 are synthesized by enantioselective 1,4-bis-silylation of a,(3-unsaturated ketones followed by transformations including the Si-C oxidation (Scheme 9) [64]. 

Divalent titanium has been implicated as a catalyst in several reactions, including silane polymerization (1) and hydrosilation (2). Considerable theoretical effort has therefore been expended recently in analyzing the role of divalent titanium as a catalyst in the hydrosilation and bis-silylation reactions. 

The bis-silylation reaction has found application in the synthesis of the antifungal metabolite (-)-avenaciolide [139] and in the enantioselective preparation of ( )-allylsi-lanes [140]. The use of chiral isocyanides prepared from the terpenoid (-H)-ketopinic acid has also been investigated in an enantioselective bis-silylation of olefins, although the levels of enantioselectivity are, in most cases, modest [141]. 

The chemistry of titanium is of considerable importance, primarily because of its roles as a catalyst in various chemical reactions (e.g silane polymerization (1), hydrosilation (2), and Ziegler-Natta (3) polymerization), as materials and materials precursors, and as the basis for electronic and magnetic devices. In the past several years, the interest in titanium chemistry in this group has focused on its fundamental molecular and electronic structure in a variety of chemical environments, on its function as a catalyst in the hydrosilation and bis-silylation reactions, and on the nature of the structure, bonding, and mechanism of formation of metallocarbohedrenes. 

Reactions of silylenes with alkynes present an alternative approach for the silylation of alkynes without the aid of transition-metal catalysts (4). The bis-silylation reaction has been accomplished in a stereospecific manner via a 1,4-silyl migration with the easily available NHC-stabilized silylaminosilylene Ar SiMe3)N(Cl)Si- hPr) (Ar = 2,6-iPr2C6H3, hPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) under metal-free conditions (5), representing the first successful approach for the selective bis-silylation of alkynes with a donor-supported silylene as the silylation reagent. Furthermore, the alcoholysis of these bis-silylated alkenes gave trimethoxylsilyl-substituted alkenes. 

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