Addition reactions disilanes

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

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. 

Silylated triphosphanes and triphosphides, synthesis, 31 188-194 yields, 31 194 Silylenes, 29 2-6 addition reactions, 29 4-6 to butadiene, 29 4 to ethylene, 29 4 to hexadienes, 29 5 mechanism, 29 4 nitric oxide scavenging, 29 4 complexes, 25 37, 51, 116, 118 as catalyst intermediates, 25 118 extrusion from disilanes, 25 114, 118 halides, 3 225 from hydridosilanes, 25 14 insertion into element-hydrogen bonds, 29 3-4... 

Isomerization of the product to the (E) isomer eventually occurs when the reaction is carried out at raised temperature, but at 25 °C the pure (Z) isomer is obtained. Complex (103) also catalyzes the addition of disilanes such as (MeO)Me2SiSiMe2(MeO) and (MeO)2MeSiSiMe(MeO)2 to acetylene (equation 171).622 Again, the (Z) isomer is formed with high selectivity. Scheme 64 gives the mechanism outlined to explain the regioselectivity of the reaction. 

Addition reactions of the Si-Si cr-bonds of disilanes 121, 131, and 133 to the C=C bonds of various arynes were found to be promoted by a palladium-1,1,3,3-tetramethylbutyl isocyanide complex. Diverse 1,2-disilylated arenes 130, 132, and 134 were obtained from five-membered and benzo-condensed six-membered cyclic disilanes (Equations 21-23). The H, 13C, and z9Si NMR spectroscopic data as well as X-ray crystallographic analysis were used to confirm the above structures <20050M156>. 

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. 

Addition Reactions.- The irradiation through Pyrex of the electron-acceptor alkenes (20) in acetonitrile solution with phenanthrene and hexamethyl disilane brings about a regio-... 

The [2a + 27t] addition reaction of disilanes and acetylenes is closely related to hydrosilation. The prototype of the reaction is shown in the next example (equation 27). The cis addition is established similar to hydrosilation. 

C,0-Bis-silylation. Bis-silylationofa,/3-unsaturatedcarbonyl compounds can be achieved by palladium-TMSOTf-catalyzed addition of disilanes to enones, enals, or aromatic aldehydes via an jj -silyloxyallylpalladium intermediate. The scope of the reaction is wide and it generally leads to 1,4-bis-silylated compounds. 

Palladium complexes carrying chiral BINAP ligands catalyze the addition of disilanes to a,)8-unsaturated ketones these are subsequently converted into either j8-silylketones or -hydroxyketones (Scheme 29). Silylketones are obtained in up to 92% ee. Complexes made in situ from [(COD)RhCl]2 and the new chiral ligands (42) (R = H, Me) catalyze the reaction of Ph2SiH2 with aromatic ketones to give, after hydrolysis, chiral secondary alcohols in good optical yields (>80% ee). 

Although the chemistry of the organic substituted disilanes has been recently reviewed (2), this section will briefly discuss their chemistry as it bears on the reactions of the higher polysilanes, followed by a consideration of the reactions of these latter compounds. In addition, reactions of the disilanes reported subsequent to this review (2) will be included. In general, we will consider first a particular facet of the chemistry of the disilanes, followed immediately by a discussion of the related reactions of the higher polysilanes. 

In the last few years the design and use of various disilane compounds has gained importance because of the reactivity of the Si-Si bond and the large potential for organic synthesis involved with it. Many publications offer us numerous examples of possible reactions at the silicon-silicon bond such as addition reactions with C-C double bonds or C-C triple bonds [1, 2], addition reactions with C-element multiple bonds (e.g. aldehydes, quinones, isocyanides) [3-5] or metathesis [6, 7] and cross-metathesis [8]. In the most cases the existence of a catalyst (palladium, platinum or nickel complexes) for activation of the silicon-silicon a bond is indispensable for a successful transformation [9-11]. 

In contrast to the rare occurrence of the oxidative addition of C-C bonds, the oxidative addition of the nonpolar bonds between two main group atoms, such as boron and silicon, can be facile. The oxidative addition of Si-Si bonds in disilanes and the B-B bond in diborane(4) reagents is likely to be a step in a variety of catalytic reactions, including the additions of disilanes, - diborane(4) reagents, and silaboranes - across olefins and alkynes, the silylation and borylation of arene C-H bonds, - - the borylation of alkane C-H bonds, and the conversion of aryl halides to arylsilanes and arylbo-ronate esters. " ... 

The oxidative addition of disilanes occurs to palladium complexes of isonitrile ligands and platinum complexes of trialkylphosphine ligands as part of tiie catalytic silylation of alkynes and aryl halides. The addition of stannylboranes to Pd(0) complexes has also been reported,and the addition of diboron compounds to many metal systems, such as Pt(0) complexes (Equation 6.67), is now common. These reactions all occur with metal complexes that do not undergo intermolecular reactions with alkane C-H bonds, let alone C-C bonds. Thus, the Lewis acidic character of these reagents must accelerate the coordination of substrate and cleavage of the E-E bonds. 

B. PALLADIUM-CATALYZED ADDITION REACTIONS OF DISILANES B.i. Bis-silylation of Acetylenes... 

Palladium-catalyzed diboration can be achieved with bis(pinacolato)diboron for strained 1,2-disubstituted alkynes. ° Analogously, palladium-catalyzed addition reactions of diboron compounds provide 1,2-bisborylalkanes from l-alkenes ° or cis-l,4-bisboryl-2-alkenes from 1,3-dienes.2,3-Bisboryl-l-propenes are accessible from allenes (Scheme 5-174). The asymmetric allene diboration was examined by Burks et ai, who showed that monodentate phosphoramidite ligands gave excellent enantioselectivities (up to 97% ee) (Scheme 5-174). The (asymmetric) metal-catalyzed reactions of diboron, silylboron and disilane compounds have been reviewed.Recently, the addition across two triple bonds to give dienes has been explored by Singidi et... 

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

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