Hydrosilylations bis

The chemistry of metal-palladium complexes has received particular attention in relation to the mechanism of Pd-catalyzed reactions such as hydrosilylation, bis-silylation, or hydrostannation. 

One may envisage polymerizations analogous to the thiol-enc process using other bis- or multi transfer agents (e.g. radical-induced hydrosilylation between bis-silanes and dienes). However, none has been described or achieved significance. 

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). 

Stoichiometric reaction of 5 with phenylsilane produced the iron(O) bis(silane) c-complex 18, which was confirmed by the single-crystal X-ray analysis as well as SQUID (Superconducting QUantum Interference Device) magnetometry (Scheme 19). Complex 18 as a precatalyst showed high activity for the hydrosilylation of 1-hexene. 

Beller and coworkers reported hydrosilylation reactions of organic carbonyl compounds such as ketones and aldehydes catalyzed by Fe(OAc)2 with phosphorus ligands (Scheme 21). In case of aldehydes as starting materials, the Fe(OAc)2/PCy3 with polymethylhydrosiloxane (PMHS) as an H-Si compound produced the corresponding primary alcohols in good to excellent yields under mild conditions [67]. Use of other phosphorus ligands, for instance, PPhs, bis(diphenylphosphino) methane (dppm), and bis(diphenylphosphino)ethane (dppe) decreased the catalytic activity. It should be noted that frans-cinnamaldehyde was converted into the desired alcohol exclusively and 1,4-reduction products were not observed. 

The comparison of a bis(imino)pyridine iron complex and a pyridine bis (oxazoline) iron complex in hydrosilylation reactions is shown in Scheme 24 [73]. Both iron complexes showed efficient activity at 23°C and low to modest enantioselectivites. However, the steric hindered acetophenone derivatives such as 2, 4, 6 -trimethylacetophenone and 4 -ferf-butyl-2, 6 -dimethylacetophenone reacted sluggishly. The yields and enantioselectivities increased slightly when a combination of iron catalyst and B(CeF5)3 as an additive was used. 

Scheme 24 The comparison of a bis(imino)pyridine iron complex and a pyridine bis(oxazoline) iron complex for hydrosilylation of ketones...

Concerning enantioselective processes, Fujihara and Tamura have proved that palladium NPs containing (S)-BINAP (2,2 -bis(diphenylphosphino)-l,l -binaphthyl) as chiral stabiliser, catalyse the hydrosilylation of styrene with trichlorosilane, obtaining (S)-l-phenylethanol as the major isomer (ee = 75%) [42]. In contrast, the palladium complex [Pd(BINAP)(C3H5)]Cl is inactive for the same reaction [43]. 

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]. 

Axially chiral spirosilane 61 was efficiently prepared by double intramolecular hydrosilylation of bis (alkenyl) dihydrosilane 60. By use of SILOP ligand, a C2 symmetric spirosilane which is almost enantiomerically pure was obtained with high di-astereoselectivity (Scheme 3-24) [65]. SILOP ligand is much more stereoselective for this asymmetric hydrosilylation than DlOP (5) though they have similar structure. 

Scheme 10.47 Rh-catalysed hydrosilylation of acetophenone with bis(thiazolines) ligands.

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

Phosphinoalkylsilanes as chelate ligands with transition metals have been studied, principally to provide a better understanding of metal-catalyzed industrial reactions such as hydrosilylation.51 Although many examples of bis-chelate metal complexes possessing a cA-arrangement of the phosphinoalkylsilyl ligands in a typical square-planar M(II)(M = Pd, Pt) environment have been synthesized,52... 

Well-defined complicated macromolecular structures require complex synthetic procedures/techniques and characterization methods. Recently, several approaches leading to hyperbranched structures have been developed and will be the focus of this section. The preparation of hyperbranched poly(siloxysilane) has been reported [198] and is based on methylvinyl-bis(dimethyl siloxysilane), an A2B type monomer, and a progressive hydrosi-lylation reaction with platinum catalysts. An appropriate hydrosilylation reaction on the peripheral - SiH groups led to the introduction of polymeric chain (PIB, PEO) or functional groups (epoxy, - NH2) [199]. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Within group 8, a bis-dinitrogen complex of an iron(O) tridentate pyridinediimine structure has also recently been shown to catalyze the hydrosilylation of alkyne.60 This discovery is a new example of the utility of low-valent iron in catalysis.61... 

The final cyclization manifold has been realized with a different ruthenium catalyst (Scheme 22). The cationic [Cp Ru(MeCN)3]PF6 induces exclusive endo-dig cyclization of both homopropargylic and bis-homopropargylic alcohols.29 73 The clean reaction to form a seven-membered ring is noteworthy for several reasons intramolecular exo-dig cyclization with bis-homopropargylic alcohols is not well established, the platinum-catalyzed case has been reported to be problematic,80 and the selectivity for seven-membered ring formation over the exo-dig cyclization to form a six-membered ring is likely not thermodynamic. The endo-dig cyclization manifold was thus significant evidence that a re-examination of alkyne hydrosilylation mechanisms is necessary (see Section 10.17.2). 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

The palladium-catalyzed asymmetric hydrosilylation of styrenes has been applied to the catalytic asymmetric synthesis of l-aryl-l,2-diols from arylacetylenes (Scheme 6).46 Thus, ( )-l-aryl-2-(trichlorosilyl)ethenes, which are readily generated by platinum-catalyzed hydrosilylation of arylacetylenes, were treated with trichlorosilane and the palladium catalyst coordinated with MOP ligand 12f to give 1 -aryl-1,2-bis(silyl)ethanes, oxidation of which produced the enantiomerically enriched (95-98% ee) 1,2-diols. 

A chiral bis(oxazolinyl)phenylrhodium complex was found to catalyze the asymmetric hydrosilylation of styrenes with hydro(alkoxy)silanes such as HSiMe(OEt)2 (Scheme 7).47 Although the regioselectivity in forming branched product 27 is modest, the enantiomeric purity of the branched product 27 is excellent for styrene and its derivatives substituted on the phenyl group. The hydrosilylation products were readily converted into the corresponding benzylic alcohols 29 (up to 95% ee) by the Tamao oxidation. 

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