Hydrosilylation of olefins

A variety of oxidative tether cleavage protocols have also been evaluated which allow regioselective monoprotection of the resulting 1,3-diol products, facilitating differentia- 

An enantioselective version of the hydrosilylation reaction would greatly extend its synthetic utility. The reaction mechanism of this catalytic asymmetric process has been investigated in great detail and shown to be extremely complicated [132]. Nevertheless, 

Addition of the Si-H bond of silanes to olefins (Eqn. 6.7) is a reaction of considerable importance in silicone technology, and several catalysts have been developed which are based on soluble platinum metal compounds. Speier s catalyst (alcoholic H2PtQ6) is perhaps the best known of these. 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum rhodium ruthenium = iridium osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267] 

This work did much to raise awareness about the possibility that colloidal metals might be present and active as catalysts in homogeneous catalytic reactions under reducing conditions. Thus it is becoming more common for the criteria described above to be applied in determining the homogeneity or heterogeneity of catalytic processes. 

Colloidal copper, prepared by borohydride reduction of copper(II) salts in the presence of protective polymers and with particle sizes of 5.0-15.0 nm (depending on preparation details), is an active catalyst for the hydration of unsaturated nitriles to their corresponding unsaturated amides with 100% selectivity. [53, 268] The copper particle size was unaffected by the catalytic process. The catidyst performance was optimized in a detailed study of the effects of polymer molecular weight, polymer/metal ratio, and the chemical constitution of the polymer. [268, 269] 

A variety of inorganic redox reactions are catalyzed by platinum metal, [270] in several of which platinum sols have been observed to be active. Gold sols have also been shown to be active redox catalysts in, for example, the reaction between ferricyanide and thiosulfate ions (Eqn. 6.8). [271, 272] 

---

The proposed mechanism for the hydrosilylation of olefins catalyzed by chloroplatinic acid is outlined in Fig. 6. Catalysis by square-planar or trigonal bipyramidal rf complexes can be similarly described (54, 55, 105). 

Very recently, the yttrium hydride [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -di-methylbiphenyl]YH(THF) 2 (36), conveniently generated in situ from [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -dimethylbiphenyl]YMe(THF)2 (35) demonstrated its high catalytic activity in olefin hydrosilylation. This system represents the first use of a d° metal complex with non-Cp ligands for the catalytic hydrosilylation of olefins. Hydrosilylation of norbornene with PhSiHs gave the corresponding product (37) of 90% ee (Scheme 3-15) [43]. 

Figure 5. General schemes for the Chalk-Harrod (right) and the modified-Chalk-Harrod (left) mechanisms for the metal catalyzed hydrosilylation of olefins.

The reaction of thiyl radicals with silicon hydrides (Reaction 3.18) is the key step of the so called polarity-reversal catalysis in the radical-chain reduction of alkyl halides as well as in the hydrosilylation of olefins using silane-thiol couple (see Sections 4.5 and 5.1) [33]. The reaction is strongly endothermic and reversible (Reaction —3.18). 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

The silica-supported heterogeneous rhodium catalysts 1-5 were then tested in the conversion of organosilicon compounds, that is in the hydrosilylation of olefins with HSi(OEt)3 [36c,d, 37], heptamethyltrisiloxane and polyhydrosiloxane ]36a, 36c-e, 38]. 

Similar catalytic activity of rhodium-immobilized complexes was observed in the hydrosilylation of olefins with polyhydrosiloxane (Table 7.5). 

It is likely that more silicon-carbon bonds are produced by the hydrosilylation of olefins than by any other method except the direct process. This deceptively simple addition of an Si-H bond to a C-C multiple bond can be promoted by a variety of means, but transition metal catalysis is by far the most significant. Two relatively old catalysts, H2PtCl6 ( Speier s catalyst ) and Pt2(Me2ViSiOSiMe2Vi)3 ( Karstedt s catalyst ), remain the most effective, and the remarkable rates and turnover numbers observed in these systems are among the most impressive in all of organometallic chemistry. The bulk of the literature on hydrosilylation falls outside the scope of this review, and readers are directed to the comprehensive work on hydrosilylation edited by Marciniec.93... 

The well-established catalytic hydrosilylation of olefin substrates is in some instances accompanied by a dehydrogenative silylation reaction, yield-... 

ASYMMETRIC HYDROSILYLATION OF OLEFINS WITH TRANSITION-METAL CATALYSTS 127... 

The manganese complex, Ph3SiMn(CO)5 has been used to promote hydrosilylation of olefins both thermally and photochemically. With HD4 and pentene, and 0.1 mol% of Ph3SiMn(CO)5, thermal activation gave a 20% yield of D4-D4 but none from photochemical activation.109 A few complexes of iron, Fe(CO)5,110a Fe(CO)2 P(OPh)3]2H(SiMe2Ph),110b and Fe(dppe)2(CH2=CH2)ul have been reported to couple tertiary silanes but few details are available. 

Reaction (9) was discovered by Chalk and Harrod 59, 60 and later investigated by MacDiarmid2S 26>11 118>. The authors showed that it proceeds in two steps. The primary reaction product is HCo(CO)4 which is thought to be responsible for the catalytic activity of Co2(CO)8 in the hydrosilylation of olefins ... 

B C6 H4(S i IVIc2C H2C H2C6F13)-/) 4]. was employed in hydrosilylation of olefins with a perfluorinated rhodium catalyst.1971... 

In recent years, we have been investigating easy and economical functionalization of widely nsed carbon based polymers snch as polybntadienes. The preliminary results of these studies have led our group to discover a highly selective and mild synthetic route to silyl-functionalization of 1,2-polybutadienes (PBD) via Pt-nanocluster catalyzed hydrosilylation of olefin bonds. Unlike other catalytic systems, our system was found to be equally effective with all varieties of functional silanes such as halo-, alkyl-, aryl- and alkoxy- silanes affording high yields and selectivities. In addition, all the hydrosilylation reactions were found to be very clean with the ease of product separation and purifications (Scheme 2). 

Typical Procedure of Hydrosilylation of Olefins with PMHS... 

Hou, Z.M., Zhang, Y, Tardif, O. etal. (2001) (Pentamethylcyclopentadienyl)samarium(II) alkyl complex with the neutral CsMesK ligand a precursor to the first dihydrido lanthanide(III) complex and a precatalyst for hydrosilylation of olefins. Journal of the American Chemical Society, 123, 9216. 

Volume E 21 D.4.3. Hydrosilylation of Olefinic Double Bonds Followed by Oxidation... 

---