Rh-Catalyzed Asymmetric Hydrosilylation

Similar asymmetric hydrosilylation reactions were also performed using Rh-(R,R)-f-Bu-MiniPHOS, and the enantioselectivities obtained (80-97% ee) [29] are comparable with those obtained by use of the most effective ligands [ 125]. 

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Scheme 29. Example of Rh-catalyzed asymmetric hydrosilylation reactions...

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 pyrolysis of 2-hydroxy-2,2-diphenylefhylstibane gives 1,1-diphenylethylene (Scheme 14.57) [127]. The optically active (S)-(-i-)- and (R)-(-)-2,2 -bis[di(p-tolyl) stibano]-l,T-binaphthyls (BINASb) are used as chiral ligands in the Rh-catalyzed asymmetric hydrosilylation of acetophenone [128]. 

Tillack and co-workers developed a rhodium-catalyzed asymmetric hydrosilylation of butadiyne 258 to afford allenylsilane 260 (Scheme 4.67) [106]. Among more than 30 chiral phosphine ligands investigated, the highest enantioselectivity was observed when the catalyst was prepared from [Rh(COD)Cl]2 (1 mol%) and (S,S)-PPM 259 (2 mol%) to afford the optically active allene 260 with 27% ee. Other metals such as Ir, Pd, Pt or Ni were less effective for example, a nickel catalyst prepared from NiCl2 and (R,R)-DIOP 251 or (S,S)-PPM 259 gave the allene 260 with 7-11% ee. 

Table 2 summarizes typical results of the Rh(I)-catalyzed asymmetric hydrosilylation of a variety of alkyl aryl ketones in the presence of a catalytic amount... 

It was described earlier that the diselenide 2 acted as chiral ligand for the Rh(I)-catalyzed asymmetric hydrosilylation of unfunctionalized alkyl aryl ketones with diphenylsilane in tetrahydrofuran [6]. When the reaction was carried out in methanol as solvent, it gave directly a chiral alcohol, not a hydro-silylated product [7] (Scheme 5). 

The chiral ferrocenylselenium reagents were found to act as the effective ligands for Rh(I)-catalyzed asymmetric hydrosilylation and transfer hydrogenation (see Sects. 2.1 and 2.2, respectively). Fukuzawa and Tsudzuki have found that the chiral ferrocenylselenium-based amino alcohols (DASF), prepared by treatment of the chiral diferrocenyl diselenide with NaBH4 in ethanol followed by the addition of epoxides (Scheme 11), efficiently catalyzed the diethylzinc addition to aldehydes to provide the corresponding secondary alcohols with up to 99% ee... 

The C2-symmetric biferrocene 16 has been reported by Ito and co-workers to behave as a frans-spanning chelating ligand. The combination of C2-symmetry and trans coordination geometry is conceptually new in the field and bears promises for future development. TRAP (the abbreviation for this kind of compounds) affords high enantioselectivities in the Rh-catalyzed asymmetric Michael reaction of 2-cyanopropionates [17], the hydrosilylation of simple ketones [18], the hydrogenation of )3,/9-disubstituted A-acetylaminoacrylic acid... 

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

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). 

Asymmetric hydrosilylation.1 Hydrosilylation of the 3,4-dihydro-(2//)-pyr-roles (1) with (C6H5)2SiH2 catalyzed by [Rh(COD)Cl]2 complexed with ( + )- or ( - )DIOP can proceed with as much as 64% ee. (S)-Enantiomers are formed preferably with ( + )-DIOP (R)-enantiomers are favored with ( - )-DIOP. Example ... 

A curious reversal of configuration is observed when Ir-catalyst is used instead of Rh-catalyst for the same chiral ligand209,213. For example, the asymmetric hydrosilylation followed by desilylation of acetophenone catalyzed by 207/[Rh(COD)Cl]2 gives (R)-1-phenylethanol with 91% ee, while the same reaction catalyzed by 207/[Ir(COD)Cl]2 yields (S )-alcohol with 96% ee (Scheme 21). The rationale for this remarkable reversal in the direction of asymmetric induction has not been given clearly, but either a change... 

As variations of Rh-catalyzed cycloisomerization Widenhoefer and coworkers have developed asymmetric 1,6-enyne cyclization/hydrosilylation reactions by using the Rh(I)/biphemp system excellent de and ee were obtained [42]. Brum-mond et al. also discovered a rhodium(I)-catalyzed allenic Alder-ene reaction that... 

The asymmetric hydrosilylation of thiochroman-4-one catalyzed by a Rh norbornadiene (nbd) complex with a mixed P and S ligand occurs with both high enantioselectivity (92%) and yield (91%) (Equation 87) <2003JA3534>. Cr complexes with amino acids effect the reduction but with only low enantioselectivity <1999TL1373, 2004TA1735>. 

Table 1. Asymmetric hydrosilylation of acetophenone catalyzed by variety of Rh(I)-chiral ligands...

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]. 

Table 2. Asymmetric hydrosilylation of alkyl aryl ketones catalyzed by Rh(I)-(2)...

Kinetic resolution is observed in the reaction of ( )-2-dimethylaminopropio-phenone with one half equivalent of HjSiPhj catalyzed by [(— )DIOP]-Rh(S)Cl. The recovered amino ketone is found to be (R)-enantiomer with 23% optical purity, while asymmetrically hydrosilylated products, after hydrolysis, are (lS,2S)-pseudomethyl-ephedrine and (l/ ,2S)-methylephedrine with 27% and 20% e.e., respectively ... 

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