Platinum-siloxane complex

A patent (FR 2474891 see CA 96 124298c) assigned to B.A. Ashby, GE Company, claims a platinum-siloxane complex which catalyzes hydrosilation of vinylsiloxanes or SiOH groups in the preparation of silicone resins and rubbers. 

Hydrosilylation of the protected allyl-glycoside 1 with the carbosilane 2 (by means of Silopren , a platinum-siloxane complex from Bayer AG) led via Si-C bond formation to a glycosidic carbosilane dendrimer (Fig. 4.42) [82]. 

Although platinum(O) complexes are able to mediate hydrosilylation with various silanes, bis(trimethylsilyloxy)methylsilane (3) constitutes our favorite reagent. Indeed, it is cheap, stable, and mimics the linkage of siloxane polymers (Si-O-Si). On the other hand, it is small and displays a low reactivity aldn to that of dialkoxysilanes, which makes it a challenging substrate for highly selective hydrosilylations. Gratifyingly, the Tamao-Kumada oxidation of allmethyl silanes can be successfully performed under mild and neutral conditions [15a, 19,20]. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

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. 

The rate of reaction is then determined by the activity of the platinum complex with different vinyl siloxane ligands. 

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... 

The hydrosilylation of 1 -alkenes can be carried out with catalysts of subgroup VI11. Platinum compounds, e.g., the Speier catalyst (H2PtClg H2O) and the Karstedt solution, a complex compound of H2PtCl5 (H20)g and vinyl-substituted di-siloxanes, are well known and very active catalysts [18]. Several other catalytic systems, e.g., Pt(cod)2, leading to the formation of platinum colloids, have been examined [19]. More recently, hydrosilylation with the Speier catalyst has been examined both under single- and two-phase conditions. The hydrosilylation reaction was thereby optimized for the possibility of technical realization [20]. 

Addition-cured silicones Vinyl-terminated polydimethyl siloxanes DMS-V03-V22, and-V33 (Gelest) Tri- or tetra- functional potysihcon hydride (10-15 phr) Platinum-complex catalysts (2-50 ppm), PCO 72, 73, 74, and 75 (United Chemical Tech.)... 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

Formation of a siloxane network via hydrosilylation can also be initiated by a free-radical mechanism (300-302). A photochemical route makes use of photosensitizers such as peresters to generate radicals in the system. Unfor-timately, the reaction is quite sluggish. Several complexes of platinum such as (jj-cyclopentadienyl)trialkylplatinum(rV) compoimds have been found to be photoactive. The mixture of silicone polymer containing alkenyl functional groups with silicon hydride cross-linker materials and a catalytic amoimt of a cy-clopentadienylplatinum(IV) compound is stable in the dark. Under UV radiation, however, the platinum complex imdergoes rapid decomposition with release of platinum species that catalyze rapid hydrosilylation and network formation (303-308). Other UV-active hydrosilylation catalyst precursors include (acetylacetonate)Pt(CH3)3 (309), (acetylacetonate)2Pt (310-312), platinum tri-azene compounds (313,314), and other sytems (315,316). 

Karstedt, B.D. (1973) Platinum complexes of unsaturated siloxanes and platinum containing organopolysiloxanes. US Patent 3,775,452 filed Apr. 28, 1971 and issued Nov. 27. 

Materials. The ultraviolet initiator Darocur 1173 (2-hydroxy-2-methyl-l-phenyl-propan-1-one) was purchased from EM Science and was used as received. Dimethylacrylamide (DMA), methacryloyl chloride (MC), allyloxytrimethylsilane and (tris(triphenylphosphine)rhodium)chloride were purchased from Aldrich Chemical Co. DMA and MC were distilled under nitrogen prior to use. 1,3-Tetramethyldisiloxane, methacryloylpropyltrichlorosilane, and 1,3-tetramethyldisiloxane platinum complex (2 % platinum in xylenes) were purchased from Gelest, The fluorinated allylic ether, allyloxy octafluoropentane, was prepared by the phase transfer catalyzed reaction of allyl bromide with octafluoropentanol using tetrabutylammonium hydrogen sulfate, tetrahydrofuran and 50% (w/w) NaOH (11). The fluorinated side-chain methacrylate end-capped siloxane (FSi) was prepared according to a literature procedure. All other solvents and reagents were used as received. 

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