Ruthenium complexes hydrosilylation

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

Other metal complexes such as titanium or ruthenium complexes can also be used to catalyze the olefin hydrosilylation reactions. Further information is provided elsewhere.30... 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

Ruthenium complexes including carbonyl derivatives used in the hydrosilylation of alkenes exceptionally give regular saturated products, but predominantly lead to unsaturated silyl olefins, which are the products of dehydro-genative silylation [1-4]. 

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. 

As we have already mentioned, ruthenium complexes predominantly catalyze the dehydrogenative silylation of alkenes but competitively with the hydrosilylation so the reaction usually gives a mixture of the dehydrogenative silylation and hydrosilylation products. Ru3(CO)12 appears to be a very active catalyst for the dehydrogenative silylation of styrene, para-substituted styrenes [ 19, 20],trifluoropropene and pentafluorostyrene [21] by trialkyl-, phenyldialkyl-silanes (but also triethoxysilane) (Eq. 10). 

Ruthenium complexes are known to be generally less reactive in hydrosilylation reactions when compared with platinum and rhodium ones. However, very selective ruthenium-based catalytic systems have been recently developed. The hydrosilylation of terminal alkynes generally tends to proceed through cis addition, resulting in trans adducts as the main products. 

Recently, the use of carbon dioxide as a carbon building block [152] has attracted increasing attention. The hydrosilylation of carbon dioxide catalyzed preferably by ruthenium complexes leads to the synthesis of silyl formate esters (Eq. 98) [153]. Results of the reaction of hydrosilylation in supercritical carbon dioxide as a solvent and substrate have recently been reported [154]. 

It is well known that hydrosilylation processes usually catalyzed by Pt and Rh complexes can be efficiently applied in polymer chemistry. Ru3(CO)12 was effectively used for the functionalization of polysiloxanes via hydrosilylation of allyl derivatives with polymethylhydrosiloxanes [175]. On the other hand, polymerization via coupling of activated aromatics with dienes occurs mostly in the presence of ruthenium complexes as catalysts (Eq. 111). For representative references see Ref. [176] and papers cited therein. 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

Cyclic seven-membered vinyl silanes 161 were obtained by regio- and stereoselective hydrosilylation of internal alkynes catalyzed by the ruthenium complex [Cp Ru(MeCN)3]PF6, as shown in Equation (33) <2005JA10028>. Hydrosilylation of 2,2-divinyladamantane with bis(hydrosilane) species 162 in the presence of Zeise s dimer [Pt2Cl4(CH2CH2)2] gave the disilacyclic 163 in high yields (Equation 34) <19980M4267>. 

Since 1957 and the discovery of the Speir s catalyst H2PtCl6/ PrOH, considerable efforts have been made to find new catalysts with high activity and selectivity. Along with the platinum-based catalysts, the Wilkinson s complex [103] Rh(Ph3P)3Cl is one of the most popular hydrosilylation catalysts. Ruthenium catalysts are also able to promote the addition of silanes to unsaturated carbon-carbon bonds, and several reports have shown during the past decade that the well-defined ruthenium complexes of type Ru(H)(Cl)(CO)L can provide excellent activity and selectivity [104—... 

Ruthenium complexes are not much employed for hydrosilylation, since they are not in general reactive enough. [RuCl2(PPh3)3] and [RuHCl(PPh3)3] are examples, and [(l,4-diaza-I,3-diene)RuHCl(COD)) specifically catalyzes the hydrosilylation of isoprene to give an allylsilane. ... 

Supported platinum, rhodium, and ruthenium complex catalysts have been used extensively in the reaction of trisubstituted silanes with acetylene in the gas phase, predominantly in a continuous-flow apparatus. Formation of a polymer layer on the surface after immobilization of the platinum complex has protected the catalyst against leaching in long-term hydrosilylation tests [91]. 

The cationic Ru complex 4 also promotes silylative dimerisation of aromatic aldehydes with hydrosilanes. For example, the reaction of benzaldehyde and triethylsilane in the presence of a catalytic amount of 4 affords the dimerisation product 19 along with a small amount of the hydrosilylation product PhCH20SiEt3 (Equation 7). This type of silylative dimerisation of aldehydes is relatively scarce in the literature common ruthenium complexes such as [RuCl2(PPh3)3] and [Ru3(CO)j2] give only the hydrosilylation products. 

Another very useful route to vinylsilanes is hydrosilylation of alkynes by means of chlorosilanes, trialkoxysilanes, methyldichloro- or dialkoxysilanes, respectively, as catalyzed by ruthenium-, rhodium- or platinum complexes (equation 43a)62. A well-known example is the reaction of 31 with 7 in the presence of a ruthenium complex (equation 43b)62. 

Effective disproportionation and co-disproportionation of vinylsilane with ruthenium complexes containing the Ru-H, Ru-Si bond, called subsequently silylative coupling or trans-si y aiion of olefins with vinylsubstituted silanes, was revealed in 1984 as a new synthetic route to substituted vinylsilanes and are commonly used as organic reagents. Subsequent extensive synthetic and catalytic study has shown that silylative coupling of olefins with vinylsubstituted silicon compounds occurs (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing the M-Si (silicometallics) and the M-H bond (where M = Ru, Rh, Ir, Co, Fe). The insertion of olefin into M-Si bond and vinylsilanes into M-H followed by elimination of vinylsilane and ethane respectively, are the key steps in this new process. 

In the recent years, all new mechanistic implications on the late transition metal-catalyzed hydrosilylation of olefins reported involve ruthenium complexes as model transition metal centers of molecular catalysis (86-88). 

There are a limited number of group VHI-X metal-based catalytic systems active and selective in asymmetric hydrosilylation of 0=0 bond. These few systems include Fe(OAc)2/DUPHOS active in hydrosilylation of aryl methyl ketones with (EtO)2MeSiH or PMHS (301,302), ruthenium complexes bearing oxazolinylferrocenephosphine ligand (303), or chiral bis(paracyclophane)-substituted (NHC) ligands in hydrosilylation of aryl alkyl ketones with H2SiPh2 (304) and iridium(I)/DIPOF system active in hydrosilylation of acetophenone with diphenylsilane. 

Multinuclear ruthenium complexes with azulene ligands were reported by Nagashima etal, [141], who investigated their catalytic behavior in hydrosilylation reactions. 

The silole skeleton can be constructed by double trans hydrosilylation of 1,3-diynes. It was realized by using a cationic ruthenium complex as the catalyst in 2007 [18], The reaction of l,4-diphenylbuta-l,3-diyne and diphenylsilane (PhaSiHa, 3 equiv) in... 

Hydrosilylation of multiple bonds has not often been catalyzed by cyclopenta-dienylmetal complexes. Nevertheless, several methods concerning hydrosilylation of terminal and internal alkynes catalyzed by the ruthenium complex 53 and [(/] -C5Me5)Ru(MeCN)3] PF6 70 have been recently reported [36]. Of special interest is the complex 70, which has a high preference for the formation of the branched 71 over the linear product 72 (Scheme 28). The reaction of the Si-H bond with the triple bond is stereoselectively frans-addition. In the case of alkynyl silanes the reaction proceeds through endo-dig hydrosil tion to the silacycles 73 with endocyclic double bond (Scheme 29) [36b]. 

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