Hydrosilylation, intramolecular

Davis has described an approach to related 1,3-diol synthons [49] (Eq. 20). Silylation of the -hydroxy ester 129 with diisopropylchlorosilane, followed by fluoride ion-catalyzed intramolecular hydrosilylation generated a 1 1 diastere-omeric mixture of acetals 131. These acetals were shown to undergo diastereo-selective nucleophilic additions vide infra). 

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. 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by 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 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... 

Table 10 Impact of the chelating phosphine on levels of enantioselectivity in rhodium-catalyzed intramolecular hydrosilylation with [Rh(P-P)(acetone)2]+.

The formation of a cyclic siloxane obtained by intramolecular hydrosilylation has been used to control the stereochemistry of a cross-coupling reaction (Equation (18)) 79... 

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

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). 

This intramolecular hydrosilylation can be extended to a-hydroxy enol ethers (2-alkoxy-l-alkene-2-ols) to provide access to 2,3-sy -l,2,3-triols.2 In this case a neutral catalyst, Pt(0)-vinylsiloxane,3 is preferred over H2PtCl6. 

Intramolecular hydrosilylation of the higher homologues 15 and 20 under similar conditions gave also excellent yields of cyclized products [5]. The homo-allyloxysilanes 15 afforded a mixture of six- and five-membered ring products in a ratio of 4.5 1 for R = Me and 2.5 1 for R = Ph in favour of the larger ring (Reaction 6.4). The EPR spectra obtained by the reaction of t-BuO radical... 

Intramolecular hydrosilylation of alkenyloxysilyl radicals has also been investigated using silylated cyclohexadienes as the starting substrates [6]. Scheme 6.5 shows the reaction of 21 in hexane at 80-85 °C and in the presence of di-tert-hvAy hyponitrite as radical initiator. The crude reaction mixture was treated with an excess of PhLi to provide alcohol 22 in moderate yields. Intermediates 23 and 24 are the expected species involved in the 5-endo-trig cyclization. 

Analogously silylated cyclohexadiene 25 having propargyl alcohol as a pendant was used in the radical intramolecular hydrosilylation followed by ionic ring opening to provide alcohol 26 in 55 % yield (Scheme 6.6) [6]. The cyclization of silyl radical 27 to radical 28 represents an example of a 5-endo-dig process. 

Alkylidenesilacyclopentanes 81, formed by intramolecular hydrosilylation of homopropargyl alcohols, are proved to... 

At this point, there were two problems in selective alkene functionalization to be addressed. Although all attempts at oxidation of the cyclopentene failed, intramolecular hydrosilylation proceeded smoothly, to give 13. On exposure of the derived cyclic carbonate to HgfOjCCF,), the cyclononene then underwent allylic oxidation, to give 14. 

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-... 

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]

The allyloxyhydrosilanes are in turn good substrates for intramolecular hydrosilylation, yielding heterocycles containing both Si and O atoms.162 Two groups have reported attempts toward a one-pot synthesis of the cyclic compounds, without isolation of the intermediate allyloxyhydrosi-lane.163 Catalyst choice is crucial, as both the alcoholysis and the intramolecular hydrosilylation must be efficiently catalyzed. The overall reaction is shown in Eq. (62). 

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... 

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]. 

Rh-diphosphine complex, [Rh(Chiraphos, 106)](C104)2, was used as a catalyst for the intramolecular hydrosilylations of homoallylic silane 107 and silyl allyl ether 108 in acetone at 25°C to give the corresponding 1,4-diol 109 (60% ee, R) in 84% yield and 1,3-diol 110 (with 56% ee, R) in 96% yield, respectively [64] (Scheme 2.7). The Rh-Chiraphos catalyzed reaction of l-(3-phenylpropen-2-yloxy)silacyclohexane (111a) gave diol 112 with 74% ee (R) in 61%... 

The mechanism of the intramolecular hydrosilylation catalyzed by Rh and Pt complexes was investigated by using deuterated silanes, which indicated the operation of both the traditional Chalk-Harrod hydrometallation and silylmetallation pathways accompanied by rapid P-hydride elimination [65]. This intramolecular reaction was applied to the syntheses of natural products [66],... 

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