Allyl alcohols intramolecular hydrosilylation

The rhodium-catalyzed intramolecular hydrosilylation of allylic alcohol derived silyl ethers has been described. Oxidative cleavage of the resulting cyclized hydrosilylation products affords a route to optically active diols (Scheme 28).129,130... 

Intramolecular hydrosilylation.1 Hydrosilylation of internal double bonds requires drastic conditions and results in concomitant isomerization to the terminal position. However, an intramolecular hydrosilylation is possible with allylic or homoallylic alcohols under mild conditions by reaction with 1 at 25° to give a hydrosilyl ether (a), which then forms a cyclic ether (2) in the presence of H2PtCl6-6H20 at 60°. Oxidative cleavage of the C—Si bond results in a 1,3-diol (3). 

In 1990, the enantioselective intramolecular hydrosilylation of allylic alcohols was successfully applied to the synthesis of chiral 1,3-diol [63] (Scheme 2.6). The reaction of 3-diarysiloxy-1,4-pentadiene (104) catalyzed by (-)-DIOP-[Rh(C2H4)Cl]2 (2 mol%), followed by Tamao oxidation, gave (2S,3R)- 1,3-diol 105 (symanti = 98 2) with 93% ee [63]. 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

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. 

Finally, the regio- and stereoselectivities of the intramolecular hydrosilylation of allyl alcohols, homoallyl alcohols, and allylamlnes are summarized in Scheme 1. While these results have been obtained with the dimethylsilyl (HMe2Si) group in the presence of a platinum catalyst, stereoselectivity, but not regioselectivity, can be controlled by other silyl groups such as the diphenyl (HPh2Si) or di(isopropyl)silyl [H(i-Pr)2Si] groups. 

Intramolecular hydrosilylation of allyl and homoallyl alcohols, with subsequent oxidative cleavage of the resultant C—Si bond, has provided a new approach to the regiocontrolled synthesis of 1,2-and/or... 

The intramolecular hydrosilylation of allylic and homoallylic alcohols and the subsequent oxidative cleavage of the initially formed... 

Curtis and Holmes provided an early display of the synthetic utility of the intramolecular hydrosilylation in their synthesis of the /ran -diol 335, an intermediate in their approach to obtusenyne 336 [130]. The allylic silyl ether 337 was prepared from the corresponding alcohol using an excess of 1,1,3,3-tetramethyldisilazane in the presence of a catalytic amount of ammonium chloride, and then used without purification. A wide variety of catalysts were investigated with the rhodium complex [Rh(acac)(norboma-diene)] providing the best results in terms of stereoselectivity transicis >95 5) (Scheme 10-109). Unfortunately, partial loss of the silyl group under the reaction conditions resulted in a moderate yield of the diol product which was not improved when the corresponding diisopropylsilyl ether was used. 

Scheme 10-111 Pt-catalyzed intramolecular hydrosilylation of allylic amines provides 1,2-amino alcohols. Reagents and conditions i) BuLi, Me2SiHCl, Et20 ii) [Pt [(CH2=CH)Me2Si]20 2], rt iii) EDTA 2Na, hexane, rt then 30% H2O2, KF, KHCO3, MeOH, THF, rt.

For the last two decades, the use of the platinum- or rhodium-catalyzed intramolecular hydrosilylation/Tamao-Fleming oxidation sequence has been well recognized as a powerful method for the stereoselective synthesis of various structurally diverse alcohols (1,3-diols, 2-alkoxy-l,3-diols, and 2-aminoalcohols) and ketone derivatives (/3-hydroxyketones, y-hydroxyketones, o, /3-dihydroxyketones, and a,y-dihydroxyketones) from simple and readily available starting materials such as substituted allyl or propargyl alcohols and their homologues (4,177). Selected applications are presented in equations (24-26). 

The synthetic potential of this strategy based on consecutive Pt- or Ru-catalyzed intramolecular hydrosilylation of propargyl and homopropargyl alcohols Pd-catalyzed cross-coupling reactions have been proved in the stereoselective synthesis of a wide variety of aryl-substituted (E)- and (Z)-allyl and homoallyl alcohols (Scheme 25) (178-180) ... 

Scheme 7.29 Syn and anti intramolecular hydrosilylation/cross-coupling to generate substituted allylic alcohols.

Bis(allyl)homoallyloxysilanes 56a and 56b are designed for a tandem intramolecular silylformylation-allylsUylation reaction, which has turned out to be an efficient approach to construct polyol and polyketide frameworks [21], For example, heating a solution of 56 in benzene at 60 °C in the presence of Rh(acac)(CO)2 under CO atmosphere followed by the Tamao oxidation gives syn,syn-triols 59 stereoselectively via oxasilacyclopentanes 57 and 58 (Scheme 5.14). Bis(ds-cro-tyl)silane 56b is readily prepared by double Pd-catalyzed 1,4-hydrosilylation of 1,3-butadiene with dichlorosilane followed by reduction with UAIH4 and alcoholysis with the corresponding homoallylic alcohol. 

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