BINAP hydrosilylation

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]. 

BINAP complexes (7 in Fig. 7.7) are among the most efficient chiral catalysts for enantioselective hydrogenations, hydrosilylations, etc. Heterogeniza-tion of this complex is highly desired because of the high price of the complex. 

Intramolecular hydrosilylation of the fe-alkenyl silane yields the chiral spirosilane with high diastereoselectivity (Scheme 30). With 0.3-0.5 mol.% of catalyst consisting of [Rh(hexadiene)Cl]2 and a range of chelating phosphines P-P (P-P = (R)-BINAP (6), (R,R)-DIOP (5)), a maximum chemical yield of spirosilane of 82% was found with 83% enantiomeric excess. These values were improved considerably by the use of the new ligand... 

Very recently, Wiedenhoefer272 has devised the first asymmetric 1,6-enyne hydrosilylation/cyclization tandem process using a rhodium(l) catalyst with (R)-276 as chiral ligand where rhodium-BINAP complexes were not effective (Scheme 70). More developments on this reaction are covered in Chapter 11.13. 

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... 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

Figure 3.24. Scope of Rh/(S)-binap-catalyzed tandem hydrosilylation/asymmetric 1,4-...

RhH(Cp )(Binap)](SbF5), a presumed intermediate in the hydrosilylation of phenyl acetylene [40], shows the hydride resonance at d -10.39. Hydride resonances in Ru(ii) phosphine complexes are often found in the same region [41-43]. 

Suisse and co-workers have studied the asymmetric cyclization/silylformylation of enynes employing catalytic mixtures of a rhodium(i) carbonyl complex and a chiral, non-racemic phosphine ligand. Unfortunately, only modest enantioselectivities were realized.For example, reaction of diethyl allylpropargylmalonate with dimethylphenyl-silane (1.2 equiv.) catalyzed by a 1 1 mixture of Rh(acac)(GO)2 and (i )-BINAP in toluene at 70 °G for 15 h under GO (20 bar) led to 90% conversion to form a 15 1 mixture of cyclization/silylformylation product 67 and cyclization/ hydrosilylation product 68. Aldehyde 67 was formed with 27% ee (Equation (46)). 

Intramolecular hydrosilylation of bis(2-piopenyl)methoxysilane, a meso diene, in the presence of an Rh(I) catalyst containing DIOP or BINAP followed by hydrogen peroxide oxidation, produces the optically active 1,3-diol in up to 93% ee (Scheme 5) 16a). The intramo-... 

Benzamido-cinnamic acid, 20, 38, 353 Benzofuran polymerization, 181 Benzoin condensation, 326 Benzomorphans, 37 Benzycinchoninium bromide, 334 Benzycinchoninium chloride, 334, 338 Bifiinctional catalysts, 328 Bifiinctional ketones, enantioselectivity, 66 BINAP allylation, 194 allylic alcohols, 46 axial chirality, 18 complex catalysts, 47 cyclic substrates, 115, 117 double hydrogenation, 72 Heck reaction, 191 hydrogen incorporation, 51 hydrogen shift, 100 hydrogenation, 18, 28, 57, 309 hydrosilylation, 126 inclusion complexes, oxides, 97 ligands, 19, 105 molecular structure, 50, 115 mono- and bis-complexes, 106 NMR spectra, 105 olefin isomerization, 96... 

Intramolecular hydrosilylation of siloxy acetone 55 catalyzed by a cationic Rh complex with DuPHOS-i-Pr (56), [Rh(COD)(DuPHOS-i-Pr)]OTf, to give the corresponding cyclic silyl ether with 93% ee (5) [42]. The product was converted to 1,2-diol 57, which can also be prepared by asymmetric dihydroxylation of propene. In the same reaction, the use of BINAP 58 gave only... 

Asymmetric intramolecular hydrosilylation of a-dimethylsiloxyketones (216), which are prepared from a-hydroxyketones 215, catalyzed by [(S ,S )-R-DuPHOS)Rh(COD)]+CF3 SO3-, (219) proceeds smoothly at 20-25 °C to give siladioxolanes 217. Desilylation of 217 affords 1,2-diols 218 with 65-93% ee in good yields (Scheme 22)231. The best result (93% ee) is obtained for the reaction of a-hydroxyacetone using (S, S)-i-Pr-DuPHOS-Rh+ as the catalyst. The same reactions using (S ,S )-Chiraphos and (S)-binap give 218 (R = Me) with 46 and 20% ee, respectively. 

Catalytic asymmetric intramolecular hydrosilylation of dialkyl- and diarylsilyl ethers of bis(2-propenyl)methanol (245) catalyzed by (R, R)-DIOP-Rh or (R)-binap-Rh complex, followed by Tamao oxidation, gives (2S, 3R)-2-methyl-4-pentene-l,3-diol (247) with 71-93% ee and excellent syn selectivity (syn/anti = 95/5- > 99/1) (equation 96)249. The enantioselectivity of this reaction depends on the bulkiness of the silyl moiety, i.e. the bulkier the substituent, the higher is the enantiopurity of the product, except for the case of 2-MeCgH4 R = Me, 80% ee (binap-Rh) R = Ph, 83% ee (DIOP-Rh) R = 2-McC.fiI I4, 4% ee (DIOP-Rh) R = 3-MeC6H4, 87% ee (DIOP-Rh) R = 3,5-Me2C6H3, 93% ee (DIOP-Rh). This methodology is successfully applied to the asymmetric synthesis of versatile poly oxygenated synthetic intermediate 249 (equation 97)249. 

Asymmetric, intramolecular hydrosilylation, catalysed by Rh(I) coordinated to chiral diphosphine complexes, such as chiraphos or BINAP, has been reported to give up to... 

Asymmetric intramolecular hydrosilylation.9 The intramolecular hydrosilyl-ation of allylic alcohols (14, 137) can be enantioselective when catalyzed by Rh(I) complexed with either (R)-BINAP or (R.R)-DIOP. The enantioselectivity is dependent on the groups attached to silicon, being higher with a phenyl than with a methyl group. Highest enantioselectivity (93% ee) was obtained with the di(3,5-xylyl)silyl ether, ROSiH[C6H3(CH3)2-3,5]2. 

The optically active (A)-(+)- and (i )-(—)-2,2 -bis[diarylstibano]-l,l -binaphthyls (BINASb) have been prepared and used as chiral auxiliaries in the Rh-catalyzed asymmetric hydrosilylation of ketones with diphenylsilane.49,49a When acetophenone is reduced using 0.25 mol% of [Rh(COD)Cl]2 as the catalyst and 0.5 mol% of (i )-BINASb (aryl = />-tolyl) as the ligand, (i )-l-phenylethanol is formed in 78% yield and in 32% ee (Equation (15)). When (i )-BINAP is used as the chiral ligand instead of (i )-BINASb, the yield and enantiomeric excess of (7 )-l-phenylethanol are 42% and 0.6%, respectively. 

The carbon-nitrogen double bonds of nitrones N1-N3 (Fig. 14) were catalytical-ly reduced with diphenylsilane in the presence of Ru2Cl4(Tol-BINAP, L24)2(NEt3) to give hydroxylamines in high % ees [56]. The hydroxylamine HI was obtained in 63% yield with 86% ee (S) and the hydroxylamine H3 was formed in 91% ee. It was also proposed that this process opened a new access to optically active amines from racemic amines, via nitrones and hydroxylamines. The iron complex [(Cp)2Fe2(HPMen2> L25)(CO)2] was reported to be a catalyst in the asymmetric hydrosilylation of ketones under irradiation, where acetophenone was reduced in up to 33% ee [57]. 

Copper-based asymmetric hydrosilylation of aryl/alkyl ketones can also be achieved in up to 92% ee using 1 mol% CuF in the presence of BINAP and PhSiHs. In contrast to the method developed by Lipshutz, this process does not require the use of anaerobic reaction conditions and is actually enhanced when performed under oxygen. 

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