Asymmetric hydrosilylation reactions

The resulting copolymer 167 was engaged in different asymmetric reactions. Hydrosilylation of styrene derivatives 168 furnished the corresponding trichlorosilyl products in high yields (88—98%) whatever the polymer... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

Widenhoefer and co-workers have developed an effective protocol for the asymmetric cyclization/hydrosilylation of functionalized 1,6-enynes catalyzed by enantiomerically enriched cationic rhodium bis(phosphine) complexes. For example, treatment of dimethyl allyl(2-butynyl)malonate with triethylsilane (5 equiv.) and a catalytic 1 1 mixture of [Rh(GOD)2] SbF6 and (i )-BIPHEMP (5 mol%) at 70 °G for 90 min gave the silylated alkylidene cyclopentane 12 in 81% yield with 98% de and 92% ee (Table 4, entry 1). A number of tertiary silanes were effective for the rhodium-catalyzed asymmetric cyclization/hydrosilylation of dimethyl allyl(2-butynyl)malonate with yields ranging from 71% to 81% and with 77-92% ee (Table 4, entries 1-5). Although the scope of the protocol was limited, a small number of functionalized 1,6-enynes including A-allyl-A-(2-butynyl)-4-methylbenzenesulfonamide underwent reaction in moderate yield with >80% ee (Table 4, entries 6-8). 

Bercaw has investigated the application of the 6 2-symmetric, enantiomerically pure lanthanide metallocene derivative (i ,A)-BnBpYH 34 as a catalyst for the asymmetric cyclization/hydrosilylation of 1,5- and 1,6-dienes. Although 34 displayed high activity for the reaction of a number of dienes, asymmetric induction was low. In the best case, reaction of 3,3-dimethyl-1,5-hexadiene with phenylsilane catalyzed by 34 gave silylated cyclopentene 35 in 95% yield with 50% ee (Equation (25)). 

Cationic Rh(I) catalysts containing (/ ,/ )-i-Pr-DuPHOS promote asymmetric intramolecular hydrosilylation of certain a-siloxy ketones with high selectivity (Scheme 8) (25). Reaction of 4-dimethylsiloxy-2-butanone produces an (/ )-l,3-[Pg.74]

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 cyclization-hydrosilylation, examples, 10, 833 Asymmetric [3+2]-Cycloaddition reactions, via silver catalysts, 9, 566... 

The third part of this chapter reviews previously described catalytic asymmetric reactions that can be promoted by chiral lanthanoid complexes. Transformations such as Diels-Alder reactions, Mukaiyama aldol reactions, several types of reductions, Michael addition reactions, hydrosilylations, and hydroaminations proceed under asymmetric catalysis in the presence of chiral lanthanoid complexes. 

It should be pointed out that asymmetric reactions other than hydrogenation have been carried out with chiral phosphine complexes of rhodium (and a few other metals). For example, asymmetric hydrosilylations (addition of Si—H across C=C, C=0, and C=N bonds) have been catalyzed by such complexes... 

Kumada et al. have examined a number of chiral ferrocenylphosphines as ligands for asymmetric reactions catalyzed by transition metals. They are of interest because they contain a planar element of chirality as well as an asymmetric carbon atom. They were first used in combination with rhodium catalysts for asymmetric hydrosilylation of ketones with di- and trialkylsilanes in moderate optical yields (5-50%). High stereoselectivity was observed in the hydrogenation of a-acetamidoacrylic acids (equation 1) with rhodium catalysts and ferrocenylphosphines. ... 

Review R. E. Merrill, Asymmetric synthesis using chiral Phosphine ligands, Reaction Design Corp., Hillside, N.J., 1979. This review covers the literature to mid-1979 (234 references). It discusses mechanisms and applications to asymmetric hydrogenation, hydrosilylation, hydroformylation and alkylation. 

Compared to the rhodium-catalyzed asymmetric reactions, studies on the iridium-catalyzed ones have been limited until recently [8]. Hydrosilylation of acetophenone in the presence of a catalytic amount of lr(I) complex and the ligand 2 proceeded smoothly to give 1-phenylethanol in good yield, but with a much lower enantioselectivity (up to 23% ee) than the case of Rh(I) complex [6]. It is noteworthy that the alcohol of the opposite configuration was obtained when the metal is changed from Rh(I) to lr(I) [6] as has been also observed in similar reactions using chiral oxazolinylferrocenylphosphines as ligands [8]. 

Hydrosilylation of monosubstituted alkenes with palladium catalysts and trichlorosilane follows a course which favors branched products. By using a chiral phosphine ligand, asymmetric reaction is feasible. Initially, menthyldiphenylphosphine (MDPP, 93) and neomenthyldiphenylphosphine (NMDPP, 94) were employed with little success. Later, (/ )-/VA -dimethyl-l-[(S)-2-diphenylphosphinoferroce-nyl]ethylamine [(R)-(S)-PPFA] (95) and its enantiomer were prepared, and these have proved to be the... 

First attempts to achieve silyl enol ethers ) are known since the late 195O s when hydrosilylation-type procedures were applied to a,d-unsaturated ketones is—i is) based upon the observation by Duffaut and Galas ) that simple ketones are able to add trichlorosilane (2) under UV irradiation. These hydrosilylation reactions were widely expanded and intensively studied ). a,/3-unsaturated ketones react via 1,4-addition ) to silyl enol ethers ) affecting only the conjugated double bonds. It is worth mentioning that the employment of chiral catalysts induces an asymmetrical reaction " ) (Scheme 24). 

The asymmetric synthesis at a prochiral silicon center in the sense of catalytic asymmetric reactions has been effected in the hydrosilylation of ketones with dihydrosilanes using rhodium(I) complexes as catalysts137,155. 

Further examples of asymmetric reactions involving chiral monoterpenoids include the formation of chiral /S-keto-sulphoxides from (-)-menthyl carboxy-lates, " synthesis of diastereomeric tetrahedral molybdenum complexes,the enantioselective hydrosilylation of ketones,and the preparation of the chiral l-amino-2-phenylethylphosphonic acids. 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

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