Silanes catalyzed reactions

Other crosslinking reactions may be triggered by a catalyzed reaction between different units of a copolymerized functional monomer, such as A -methylol acrylamide or a copolymerized silane compound [86]. 

Like many homogeneously catalyzed reactions, the overall cycle (or cycles) in these polymerization reactions probably contains too many steps to be easily analyzed by any single approach. Both kinetics and model compound studies have thrown light on some of the steps. However, as indicated above, many of the model compounds isolated from the reactions of primary silanes with metallocene alkyls and hydrides are too unreactive to explain the polymerization results. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Recently, a palladium-catalyzed reaction has been reported which provides exclusive cA-addition and good selectivity for the terminal silane similar to platinum-based catalysts. The catalyst system, Pd2(dba>3+ 4PCy3, is unreactive to internal alkynes and succeeds with a range of aryl and akyl terminal alkynes.45... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

Roberts and co-workers have employed a number of chiral carbohydrate-derived thiols as polarity reversal catalysts in the radical hydrosilylation of electron-rich prochiral alkenes [68-70]. In these thiols, the SH group is attached to the anomeric carbon atom. Scheme 21 demonstrates the non-catalyzed reaction and in step b, the hydrogen atom transfer from the silane... 

The Pd-catalyzed reaction was applied to the synthesis of (allenylmethyl) silane derivatives [98], A series of 4-substituted-l-trimethylsilyl-2,3-butadienes 110 were prepared in 64—91% yields from easily accessible (3-bromopenta-2,4-dienyl)trimethylsi-lane 109 and soft nucleophiles 102 in the presence of 2 mol% of a Pd catalyst generated in situ from [PdCl(jT-aHyl)2]2 and dpbp (Scheme 3.55). 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

As with the silanes, some of the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C—Sn bond and it is a weaker bond.103 104 There are also useful synthetic procedures in which organotin compounds act as carbanion donors in palladium-catalyzed reactions, as discussed in Section 8.2.3 Organotin compounds are also very important in free-radical reactions, which will be discussed in Chapter 10. 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

In contrast to the reactivity of 6-dodecene-1,11-diynes, rhodium-catalyzed reaction of l-dodecene-6,11-diynes with silane led not to cascade cyclization/hydrosilylation but rather to carbonylative tricyclization. For example, reaction of 87 [X = G(G02Me)2] and dimethylphenylsilane catalyzed by Rh(acac)(GO)2 in THE at room temperature under GO gave the cyclopenta[e]azulene 88 in 92% yield as the exclusive product (Scheme 22). Although the protocol was... 

Ojima has reported a rhodium-catalyzed protocol for the disilylative cyclization of diynes with hydrosilanes to form alkylidene cyclopentanes and/or cyclopentenes. As an example, reaction of dipropargylhexylamine with triethyl-silane catalyzed by Rh(acac)(GO)2 under an atmosphere of CO at 65 °G for 10 h gave an 83 17 mixture of the disilylated alkylidene pyrrolidine derivative 92b (X = N-//-hexyl) and the disilylated dihydro-1/ -pyrrole 92c (X = N-//-hexyl) in 76% combined yield (Equation (60)). Compounds 92b and 92c were presumably formed via hydrosilyla-tion and hydrosilylation/isomerization, respectively, of the initially formed silylated dialkylidene cyclopentane 92a (Equation (60)). The 92b 92c ratio was substrate dependent. Rhodium-catalyzed disilylative cyclization of dipro-pargyl ether formed the disilylated alkylidene tetrahydrofuran 92b (X = O) as the exclusive product in low yield, whereas the reaction of dimethyl dipropargylmalonate formed cyclopentene 92c [X = C(C02Et)2] as the exclusive product in 74% isolated yield (Equation (60)). 

Pt2(dba)3-catalyzed reaction of a series of substituted benzenes with o-bis(dimethylsilyl)benzene.95 This intriguing process gives high yields of o-(aryldimethylsilyl)(dimethylsilyl)benzenes for a variety of arenes, but appears limited to this particular silane [Eq. (33)]. 

Murai and co-workers reported the silylformylation of aliphatic aldehydes in 1979.116 In this version of the transition metal-catalyzed reaction of HSiR3 and CO with various substrates, a formyl moiety is always present in the final product of the reaction. Murai utilized the Co2(CO)8 complex with a triphenylphosphine cocatalyst to catalytically form a-siloxy aldehydes from aliphatic aldehydes. An excess of reactant aldehyde is required to obtain the formyl products if silane is in excess, l,2-bis(siloxy)olefins are produced.117... 

The kinetic order in water for the spontaneous hydrolysis reaction, n, and the hydronium ion catalyzed reaction, m, varies depending on the structure of the silane ester and the solvent conditions [36, 40]. The difficulty in determining the kinetic order of water in aqueous-organic solvents arises from the observation that as the concentration of water is varied, the polarity of the solvent and the activity of the acid change [40], A plot of the logarithm of the rate constant vs. the logarithm of water concentration often does not yield a straight line. These... 

The accepted sequence for the base catalyzed reaction is given by the work of Schowen [8, 16]. He reported the a-p relationships for the methanolysis of a series of phenoxy substituted silanes. The base-catalyzed reaction is presented as having a penta-sustituted silicon intermediate. 

Rh-diphosphine complex, [Rh(Chiraphos, 106)](C104)2, was used as a catalyst for the intramolecular hydrosilylations of homoallylic silane 107 and silyl allyl ether 108 in acetone at 25°C to give the corresponding 1,4-diol 109 (60% ee, R) in 84% yield and 1,3-diol 110 (with 56% ee, R) in 96% yield, respectively [64] (Scheme 2.7). The Rh-Chiraphos catalyzed reaction of l-(3-phenylpropen-2-yloxy)silacyclohexane (111a) gave diol 112 with 74% ee (R) in 61%... 

Copper-catalyzed reactions of [(tosylimino)iodo]benzene with unsaturated compounds sometimes lead to tosylamidation. Examples include conversions of silyl enol ethers to a-tosylamido ketones [173], and tosylamidation of allylic silanes with loss of the silyl group [190] (Scheme 69). 

The [Mn(CO)3(P)2(CH2Cl2)][BArF] complex, 23, also catalyzes reaction of phenol with triethylsilane, presumably by a mechanism similar to that proposed for the Fe system (Scheme 9). The ratio of silane to the catalyst was about 24 1, and a slight deficiency of phenol was added at —78°C in an NMR tube reaction. XH NMR spectra recorded from... 

Ph3P)CuH]6 did not catalyze reaction of Et2NH with secondary and tertiary silanes.145 This is not surprising in view of the results of Liu and Harrod, who showed that CuCl, or CuH catalysts do not activate the second N—H of primary amines, or the third N—H bond of NH3, toward reaction with silanes.147148... 

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