Hydrosilylation addition products

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Nickel complexes formed in situ by the reaction of NiCl.S-COD) with the iini-dazolium salts IMesHCl or IPrHCl in the presence KO Bu catalyse the hydrosilylation of internal or terminal alkynes with EtjSiH. Interestingly, Ni tri-butylphosphine complexes are inactive in this hydrosilylation reaction. The monosilylated addition products were obtained with slow addition rates of the alkyne in the reaction mixture and were formed with variable degree of stereoselectivity, depending on the type of the alkyne, the silane and the ligand on Ni [50],... 

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. 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

Hydrosilylation of alkyl-substituted 1,3-dienes 46g-46j in the presence of a ferrocenylmonophosphine-palladium catalyst also proceeded with high regioselectivity to give the corresponding 1,4-addition products with moderate enantioselectivity (entries 13-16).52 62 Enantioselectivity was improved by using ligands 37f and 37g (entries 17 and 18).57a... 

Dienes. Hydrosilylation of conjugated dienes often leads to a complex mixture of products composed of regio- and stereoisomers of 1 1 addition, 1 2 addition products, and oligomers. Dihydrosilanes may form silacycloalkanes as well. In contrast... 

Thicker sections can be cross-linked by hydrosilylation addition. This is the same chemistry used to produce fluorosilicone monomers with the vinyl functionality present on silicon. The catalyst reaction occurs between a vinyl group and silicone hydride.66 The advantage of this system is that it does not produce volatile by-products. On the other hand, the disadvantage is that it is available only as a two-part system.67 Recently, however, one-part, platinum-catalyzed products have been developed.66 The reaction is very rapid and at room temperature it is completed in 10 to 30 minutes. It is accelerated with increasing temperature and at 150°C (302°F) it is completed within a few seconds. This makes the compounds ideal for fast automated injection molding operations.68 One-part systems use the chemical complexing of the catalyst, which is activated at elevated temperatures, or its encapsulation into an impermeable shell, which is solid at room temperature and melts at elevated temperatures68... 

Peris and coworkers have also disclosed Ir and Rh complexes 33 and 34 which can catalyze the hydrosilylation of alkynes [77]. Again, poor selectivity was observed as mixtures of the -trans, [ -cis, and a addition products were obtained. Generally speaking, it was found that Rh catalysts were more reactive than the Ir catalyst and the dimetallic complexes were much more active than their monometallic counterparts. It is believed that the difference in reactivity between the dimetallic and monometallic complexes arises from the dimetallic species ability to oxidize to the corresponding M(III) species, thus preventing oxidative addition of the silane. 

Dienylsilanes 3 and 6 are the cis and trans addition products of hydrosilanes across the triple bond of 1, respectively. Allylsilane is the product formed by hydrogenation followed by hydrosilylation of 1. Alkenylsilane 5 is the 1,4-adduct of hydrosilane across the enyne skeleton of 1, andbis(silyl)butenyne 7 is the de-hydrogenative silylation product of 1 [32]. 

Conjugated dienes like 1,3-dienes give 1 1 adducts of various types usually 1,2- and 1,4-addition products, and the regio- and stereo-isomers of each type, but also 1 2 adducts, and dimerization and reduction products. The product distribution depends on the kinds of dienes, silanes and particularly on the kind of catalyst. Of these, chromium hexacarbonyl is specific for the formation of (Z)-crotylsilanes from butadienes. Similar (Z)-selective hydrosilylation is observed with HSiCb and [Pd(PPh3)4] catalyst. The reaction pattern of 1,3-butadiene is summarized in Scheme 8. 

Hydrosilylation of isoprene (57) has been well studied and the results are summarized in Table 2. Since 1,3-dienes are relatively reactive, metal complexes which do not catalyze hydrosilylation of common alkenes may be employed. Pd, Rh, Ni and Cr catalysts prefer 1,4-addition products with the (Z)-configuration. No catalyst is available which prefers 1,2-addition with high selectivity. 

Norbomadiene (72) gives a tricyclic product (73a) in addition to the normal endo- and cj o-hydrosilyl-ated products (73b) and (73c) (equation 53). The tricyclic product is preferred under radical conditions. ... 

The asymmetric hydrosilylation of a-methylstyrene with methyldi-chlorosilane has been catalyzed by (/ )-benzylmethylphenylphosphine complexes of platinum(II) 302) or nickel(II) 304) to give a 5 or 17.6% excess of one enantiomer in the addition product, 2-phenylpropyl-methyldichlorosilane. The corresponding palladium(II) complexes were, however, only slightly useful for asymmetric synthesis in hydrosilylation of olefins. Nevertheless, palladium(II) complexes of methyldiphenyl-phosphine or epimeric neomethyldiphenylphosphine, where the dissymmetry is remote from the phosphorus, are especially useful for the induction of asymmetry in the hydrosilylation of styrene and some cyclic conjugated dienes 199). A similar procedure has been used for... 

Subsequent studies have demonstrated that the relatively robust and readily accessible rare-earth trisamide La N(SiMe3)2 3 can also be successfully employed to catalyze anft -Markovnikov hydroboration of alkenes (28) [82], The 1,2-addition product in the hydroboration of vinyl arenes constitutes the opposite 2,1-addition products commonly favored in vinyl arene hydrosilylations (Sect. 3.1), although both reactions are believed to proceed via analogous mechanisms. This fact may also be interpreted in terms of uncatalyzed addition of the borane to the alkene being competitive with the catalyzed process in some cases [82,83],... 

Hydrosilations of dienes and alkynes have been studied, and in both cases single and double silylations are possible. For 1,3-dienes, both 1,2 and 1,4 addition products can be observed8,195 213 271-272. A Ziegler-type catalyst composed of nickel(II) pentadienoate and triethylaluminum is active in hydrosilylation of terminal alkynes, but this reaction is accompanied by oxidative dimerization of the alkyne (equation 101). The major product in most cases was the head-to-head dimer273. 

Noteworthy, the stereochemistry of the vinyl moiety in both products was found to be that resulting from a trans addition to the triple bond, in contrast to the cis addition products expected from a concerted single-metal insertion.[67] This suggests that insertion reactions leading to trans addition products are entirely feasible in dinuclear species, probably as a consequence of the concerted action of both metal centers. This observation may be relevant in the context of some catalytic processes, such as alkyne hydrosilylations, which rather frequently afford trans addition products.[68] This unusual selectivity might result from intermolecular hydride transfer steps, similar to those recognized during catalytic hydroformylations,[69] or from the formation of dinuclear active species under catalytic conditions.[70]... 

Representative alkene hydrosilylation reactions carried out with the MOP ligands are identified in the following schemes. The low catalyst loading in many cases is noteworthy. Norbornene (2.145), dihydrofuran (2.146), styrene (2.147) and cy-clopentadiene (2.148) are all converted into the hydrosilylated derivatives with good to excellent enantioselectivity. In most cases, the trichlorosilanes were deriva-tised into the corresponding alcohols, although the trichloroallylsilane (2.152) was reacted with benzaldehyde to give the addition product (2.156). 

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