Acetylenes hydrosilylation

To date, numerous radical-induced hydrosilylations of terminal olefins or acetylenes have been reported for the H-terminated Si(l 11) surfaces. These reactions are mainly performed by using thermal conditions, UV irradiation, or electrochemistry. More recently, a very mild method was developed for the attachment of high-quality organic monolayers on crystalline silicon surfaces. 

Hydrosilylation by Ziegler-type catalyst systems [e.g., Ni(acac)2/AlEt3] has been examined for the reaction of 1-octene with EtjSiH in benzene 178). Complications include competing isomerization and reduction to metal however, 1,3-dienes or terminal acetylenes are readily hydrosilylated withRC i CH, the major product is CH2 CR. CRiCHSiXj. 

Acetylenic acrylates have been used to reduce side reactions in the preparation of acrylic sil(ox)anes by hydrosilylation [13,14], Allylic acrylates are known to result in addition products with both types of double bonds. Elimination of propene under loss of the allylic group is a major concern, because this path yields acryloxy silicone compounds with SiOC linkages of low hydrolytic stability. 

This disadvantage can be ruled out by spacers between the allylic and the acrylic group [15], but the selectivity in favor of the allylic group is not improved. An acetylenic triple bond instead helps to clarify the situation. 2-Propynoxyethyl acrylate, available in 90 % yield from ethoxylated propargylic alcohol by esterification, is hydrosilylated very smoothly only at the triple bond, leaving the acrylic side virtually untouched (Eq. 5). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

A moderate pressure (>5 atm.) of CO in the reaction system leads to the selective formation of 29, while alkynes undergo rhodium-catalyzed hydrosilylation with a hydrosilane to afford vinylsilanes in the absence of CO. The presence of the rhodium complex is crucial for the smooth progression of siiyiformyiation, regardless of the presence of mononuclear or polynuclear complexes. This generalization is supported by the studies of many others [15]. The most important feature of this reaction is the excellent regioselectivity, which favors the formylation of the internal sp-carbon of the acetylenic bond of terminal... 

RhH(Cp )(Binap)](SbF5), a presumed intermediate in the hydrosilylation of phenyl acetylene [40], shows the hydride resonance at d -10.39. Hydride resonances in Ru(ii) phosphine complexes are often found in the same region [41-43]. 

Hydrosilylation is formally the addition of the units of R3Si and H (R may be alkyl, alkoxy, or halide) to an unsaturated group such as an olefin, acetylene, or ketone, e.g.,... 

Early reports stated that the course of reaction is strongly dependent on the reaction conditions (L e. the employed catalyst)20-28. Benkeser20-22 and his co-wor-kers intensively investigated hydrosilylation of monosubstituted acetylenes 1 [R = i-prop- (la) and t-But- (lb)]. 

Internal acetylenes, Ni-mediated reactions, 10, 546 Internal alkenes, ethylene co-polymers, 4, 1145 Internal alkynes in alder-ene reaction, 10, 567 intermolecular hydrosilylation with ruthenium, 10, 802 with yttrium, 10, 801 silylboration, 9, 163 silylformylation, 11, 483... 

Similar to olefins, acetylenes undergo facile addition reactions. Transition metal-catalyzed hydrosilylation is such a reaction that has been utilized for the synthesis of silicon-containing hyperbranched polymers. 

As noted in Section 14.19.2.1.1, the Lewis acid-catalyzed hydrosilylation of unactivated acetylenes provided a general method for the synthesis of alkenylsilanes. Hence, intramolecular /ra .r-hydrosilylation of TMS-substituted alkyne 30 bearing a benzene ring spacer led to the formation of rfb-cycliz.ation product 31 (Equation 10) <2000JOC8919>. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

Reaction of H2C—CHSiMeCh (obtained from the catalyzed hydrosilylation of acetylene with MeSiCbH) with an ephedrine-derived lithium dialkylamide produces Ae chiral vinylsilane (1). Addition of BuU to this vinylsilane followed by treatment with MgBr2 gives the corresponding Grignard reagent... 

Alternatives to the methyl chloride direct process have been reviewed (31). Processes to make phenyl and ethyl silicones have employed direct-process chemistry. Phenyl chloride has been used in place of methyl chloride to make phenylchlorosHanes (15). In addition, phenylchlorosilanes are produced by the reaction of benzene, HSiCl, and BCl (17,31). EthylsiUcones have been made primarily in the CIS, where the direct process is carried out with ethyl chloride in place of methyl chloride (32). Vinyl chloride can also be used in the direct process to produce vinylchlorosilanes (31). Alternative methods for making vinylchlorosilanes include reaction of vinyl chloride with HSiCl or the platinum-catalyzed hydrosilylation of acetylene with HSiCl. ... 

Acetylene is converted into vinylsilane (23) under appropriate hydrosilylation conditions with a Pt, Rh, Ru or A1 catalyst. However, depending on the catalyst and conditions, the initial 1 1 product (23) undergoes further reaction to afford 1,2-disilyIethane (24 equation 9). Formation of 1,1-disilylethane is scarcely recorded. 

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