Hydrosilylation platinum complexes

Hirayaina et al. [22] have shown that polyhydrogenmethylsiloxane can bind to aluminum, copper and steel surfaces via activation with a suitable platinum complex. The attached polymer promotes the adhesion of silicone materials whose cure involves SiH/SiVinyl hydrosilylation. Presumably, it would also be effective with other unsaturated organic polymer phases. 

Scheme 10. Addition reaction by hydrosilylation catalyzed by a platinum complex.

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

The robustness and excellent turnover numbers of platinum complexes with terminal alkynes have made it the catalyst of choice for the synthesis of polymers and other macromolecular architechtures. Alkyne hydrosilylation with platinum has also served as a key element in the synthesis of dendrimers. Sequential reaction of an alkyne with HSiMeCl2 and lithiated phenylacetylene afforded the branching unit of a dendrimer synthesis which has been used to afford a large variety of structures at high generation.44,4411 441 ... 

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. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

The addition of Si—H bonds and the reactions of silyls with platinum complexes is of significance because of the early discovery of chloroplatinic acid as a hydrosilylation catalyst.53 This section focuses on the formation of hydrides from silanes. 

Although homogeneously catalyzed reactions of platinum complexes are mostly concerned with hydrogenation, hydroformylation, isomerization and hydrosilylation reactions, the complexes trans-PtHX(PPh3)2 (X = C1, Br, I) have been used used as catalysts for the oxidative chlorination of n-pentane. H2PtCl6 and K2PtCl are used as oxidants.201... 

A new functionalized polysiloxane bearing crown ether moieties in the main chain as well as its platinum complex are prepared from 3,19-dihydroxy-l-thia-5,8,ll,14,17-pentaoxacycloicosane through ether formation with ro-chloroundecene, followed by hydrosilylation with triethoxysilane, copolymerization with octamethylcyclotetrasiloxane and reaction with potassium chloroplatinate279. 

Formerly, Levis [97] has found that hydrosilylation proceeds with high efficiency on platinum complexes, reduced to the colloid form. Preliminary results show that polyaddition of 1,5-dihyd-rohexaethylcyclotetrasiloxane to 1,5-divinylhexaethylcyclotetrasiloxane on reduced CPDP in the form of colloid platinum decreases polydispersion degree of carbosiloxane polymer from 8.21 to 10.3. Taking into account encouraging results of the work [96], a sequence of cyclolinear carbo-siloxane polymers were synthesized [93] in the presence of three different forms of platinum cata-lysts, introduced into the reaction mixture after preliminary formation. 

T-Olefin platinum(O) complexes are important starting materials for oxidative addition see Oxidative Addition) or catalysts. Karstedt s catalysts, which are the most active ones for hydrosilylation, have been structurally characterized and found to show the structure of Pt2(M y M y )3 (9), wherein = divinyltetramethyldisiloxane." A styrene analogue Pt°(styrene)3 provides a convenient route to get an r-alkyne platinum complex by displacement (Scheme 27). DFT calculations indicate that aUcyne in the... 

Although the platinum-catalyzed reactions are not as practical as palladium used for the same purpose, several types of unique discoveries still are noteworthy. Besides electrophihc C-H activation, the platinum complexes are efficient catalysts for hydrosilation see Hydrosilation Catalysis) that is important to silicon polymer industry. Platinum carbene catalysts are prepared from Karstedt catalyst (9) and imidazolium salts. The trials of model hydrosilylation reactions show very high yields of regioselective products (<1% impurity) with remarkable TON (see Turnover) (<30ppm catalyst load) (Scheme 81). ... 

Of the new Pt catalysts reported since 1990 platinum complexes with new ligands and activators are noteworthy. Cyclodextrin complexes of platinum (as host-guest complexes) have been employed as hydrosilylation catalysts active at elevated temperature after releasing the guest compound [40]. Some other organic compounds have recently been used as activators (ligands) of Pt complexes, e. g., unsaturated secondary and tertiary alcohols and silylated unsaturated alcohols [41], alkadiynes, cyclooctadiene [42], and vinylnorbomene as well as quinones and methylnaphthoquinones [43]. 

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 silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

Asymmetric hydrosilylation of a-methylstyrene with HSiMeCl2 catalyzed by [(-t- )BMPP]2NiCl2 (BMPP = benzylmethylphenylphosphine) gives the corresponding optically active /3-adduct, 1, in 31% yield (20.9% enantiomeric excess (e.e.), R). When a chiral platinum complex, [(-I- )BMPP-PtCl2]2, is employed, the reaction gives 1 (5.2%... 

U sing polymers was one of the very first methods in order to heterogenize the catalyst in a homogeneous catalytic reaction. Thus, thanks to these supports, the catalyst acquires the property of insolubility while maintaining its catalytic performance [39-42]. Some authors synthesized phosphonated resins, such as polystyrene, and used them as a ligand in several rhodium and platinum complexes. Thus, hydrogenation [43, 44], hydrosilylation [45], and hydroformylation of olefins were catalyzed. 

The hydrosilylation of 1-hexene (28) with triethoxysilane (29) in the presence of a poly-y-mercaptopropylsiloxane platinum complex leads to hexyltriethoxysilane (30) (equation 16), whereas the hydrosilylation of acetylene (31) can afford both the mono- and disilylated product, i.e. vinyltriethoxysilane (32) and l,2-bis(triethoxysilyl)ethane (33) respectively (equation 17)34. 

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. 

The mechanism of catalytic hydrosilylation is not well understood. Study of these reactions is hampered by their complexity induction periods are often involved, reaction conditions such as the nature of the catalyst and reacting groups are critical factors, and side-reactions, such as alkene rearrangements, are common. A widely accepted mechanism for homogeneous catalysis by platinum complexes is based on the work of Chalk and Harrod (Scheme 10)5,8,262. With Speier s catalyst, it appears that initially, and probably during the induction period, silane reduces the platinum to a Pt(0) or Pt(II) complex that is the active catalytic species265. 

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