Styrenes hydrosilylation

Poyatos et al. investigated the activity of the N -chelating oxazole-NHC-containing Pt complex 85 with various substrates. From phenylacetylene, the p-( )-isomer and the a-isomer were the only two reaetion produets (60 40). Styrene hydrosilylation led to the formation of the linear produet with 85%... 

Poyatos et al. investigated the activity of the N-C-chelating oxazole-NHC containing Pt complex 142 with various substrates (Figure 13.16). From phenylacetylene, the p-( )-isomer and the a-isomer were the only two reaction products (60 40 respectively). Styrene hydrosilylation led to the formation of the linear product with 85% selectivity (70% conv.). The cationic Pt" complex, 143, of multidentate NHC, was found to be an efficient catalyst precursor for hydrosilylation of alkynes, and only a- and p-( ) regioisomers were produced (Figure 13.16). The water-soluble Pt complex 144, bearing sulfonated NHC, was an active and recyclable catalyst for the hydrosilylation of phenylacetylene and other alkynes at room temperature in water. ... 

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]. 

Palladium-catalyzed hydrosilylation of styrene derivatives usually proceeds with high regioselectivity to produce benzylic silanes, 1-aryl-1-silyle thanes, because of the... 

In all of these cases, paUadium-catalyzed hydrosilylation proceeds via hydropalla-dation followed by reductive elimination of alkyl- and silyl group from the palladium. In the reaction of o-aUylstyrene (24) with trichlorosilane, which gives hydrosilylation products on the styrene double bond 25 and cycUzed product 26, the hy-dropalladation process is supported by the absence of side products which would result from the intermediate of the silylpaUadation process (Scheme 3-11) [37]. 

In addition, various chiral (p-A -sulfonylaminoalkyl)phosphine ligands were earlier employed by Achiwa et al. for the asymmetric palladium-catalysed hydrosilylations of cyclopentadiene and styrene, affording the corresponding... 

Scheme 10.52 Pd-catalysed hydrosilylation of styrene with S/P-heterodonor ligands.

A summary of the ligand effects of the palladium-catalyzed enantioselective hydrosilylation of styrene is given in Table 9. 

Table 9 Palladium-catalyzed asymmetric hydrosilylation of styrene.

However, on a lightly cross-linked hydroxyethylmethacrylate/styrene polymer that swells in polar solvents (22, 365), or on a silica-gel support (366), catalyst performance matches that of the soluble one for the precursor amino acid substrates. A rhodium-DIOP analog has also been supported on a polymer containing pendent optically active alcohol sites [incidentally, formed via hydrosilylation and hydrolysis of a ketonic polymer component using an in situ rhodium(I)-DIOP catalyst]. The supported catalyst in alcohol again matched that of the soluble catalyst for... 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

Use of ferrocenylmonophosphine (fU-(A)-PPFA 5a for the same reaction improved the enantioselectivity.24,25,26 Here, the hydrosilylation product was oxidized into ( y)-l-phenylethanol 3 with 52% ee (entry 3). The ferrocenylmonophosphine 6 supported on Merrifield polystyrene resin has been also used for the hydrosilylation of styrene, though the enantioselectivity was lower (15% ee) (entry 4).27 Several chiral (/ -/V-sulfonylaminoalkyl)phosphines 7 were prepared from (A)-valinol and used for the asymmetric hydrosilylation of styrene.28 For styrene, phosphine 7a which contains methanesulfonyl group was most effective giving (asymmetric hydrosilylation (entries 6-9).29,29a... 

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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