Asymmetric Intramolecular Hydrosilylation

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

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. 

Intramolecular asymmetric hydrosilylation of allylic silyl ethers 150 bearing a silacyclohexyl group gives spirobicyclic oxasilacyclopentanes 151 with extremely high enantiopurity when [Rh(BINAP)]+C104 is used as the chiral catalyst (Scheme 70) (128,129). 

Until 1968, not a single nonenzymic catalytic asymmetric synthesis had been achieved with a yield above 50%. Now, barely 15 years later, no fewer than six types of reactions can be carried out with yields of 75-100% using amino acid catalysts, i.e., catalytic hydrogenation, intramolecular aldol cyclizations, cyanhydrin synthesis, alkylation of carbonyl compounds, hydrosilylation, and epoxidations. 

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]

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... 

Applications of the intramolecular hydrosilylation to catalytic asymmetric synthesis will be discussed in Section V (vide infra). 

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. 

As pointed out in the introduction, a particular feature of hydrosilylation reactions is that they require catalysis. Arguably the most valuable of enantioselective synthetic methods are those in which asymmetric induction occurs from small quantities of enantiomerically pure catalysts. It is natural, therefore, that considerable effort has been directed towards the catalytic enantioselective hydrosilylation-oxidation of C —C double bonds. Some degree of success has been met in the hydrosilylation of simple alkenes and 1,3-dienes, and in intramolecular hydrosilyla-tions. Also, as discussed at end of this section, a catalytic enantioselective disilylation (effectively the same as a hydrosilylation) has been developed for a,)3-unsaturated ketones. 

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