Vinylsilanes couplings

In Scheme 10, HMG-CoA reductase inhibitor 92 was synthesized via a Suzuki coupling approach. Hiyama s group also carried out a Hiyama coupling to make the same compound (93TL8263). Vinylsilane 119 was prepared by platinum-catalyzed reaction from terminal alkyne 89. 

Palladium-catalyzed cross-coupling between vinylsilane 119 and 3-iodoqui-noline 91 then assembled 92 in 83% yield. 

The cross metathesis of vinylsilanes is catalyzed by the first-generation ruthenium catalyst 9. This transformation has been extensively investigated from both preparative and mechanistic points of view by Marciniec et al. [86]. Interestingly, the same vinylsilanes obtained from cross metathesis may also result from a ruthenium-hydride-catalyzed silylative coupling and there might be some interference of metathesis and nonmetathesis mechanisms [87]. 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

Besides the acetylene—acetylene coupling reactions shown above, acetylene—allene [40] and allene—allene coupling reactions [40] are also feasible (Eqs. 9.18 and 9.19). These reactions provide convenient methods for the synthesis of stereodefined olefinic skeletons. As an addendum, the coupling of vinylsilane with acetylenes was effected by 1 to give stereodefined homoallylsilanes, as shown in Eq. 9.20 [41]. 

The enantiomeric purity of vinylsilane (S)-3a and (R)-3c are determined to be >95% ee by 1H-NMR (400 MHz) on the derived mandelate ester, obtained by a DCC-promoted coupling to (R)-O-acetylmandelic acid, and absolute stereochemical assignment is accomplished by 1H NMR analysis of the derived (R)-O-acetyl-mandelate esters. For details of this procedure see the published method of Trost.3... 

CYCLOHEXEN-l-ONE. Importantly, only 1.6 equivalents of Ag20 are required for efficient coupling. The final preparation in this series illustrates the hydrosilation of racemic 3-butyn-2-ol catalyzed by a phosphine based platinum(O) catalyst. The resultant racemic (E)-vinylsilane is then resolved with a commercially available lipase and subjected to a Johnson ortho ester Claisen rearrangement to afford [3R- AND 3S-]-(4E)-METHYL... 

Solvent Effects and NMR Coupling Constants Table 20. Solvent dependence of2JHmH in vinylsilanes (concentration <15 mole %)... 

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

Scheme 35 Intramolecular cathodic coupling of vinylsilanes with ketones yields 55%.

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

The relatively low mass balances of the reactions with vinylsilane terminating groups raised questions about the overall utility of vinylsilane groups with the anodic olefin-coupling reaction. Were the low mass balances due to an... 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

During the past two decades, within the series of our studies, we have developed a silylative coupling reaction of olefins with vinylsubstituted siHcon compounds which takes place in the presence of transition-metal complexes (e.g. mthenium and rhodium) that initially contain or generate M—H and M—Si bonds (for reviews, see Refs [5] and [6]). The reaction involves activation of the =C—H bond of olefins and cleavage of the =C—Si bond of vinylsilane. The reaction, which is catalyzed by complexes of the type [ M( x-OSiMe3)(cod) 2] (where M = Rh, Ir) occurs according to Equation 14.12 [71, 72). 

However, in the presence of iridium siloxide and bulky substituents at the silicon in vinylsilanes instead of silylative coupling, the codimerization (hydrovi-nylation) of styrene and vinylsilanes occurs according to Equation 14.13 ... 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

Burke et al. [84] synthetised nagilactone F (55) by a polyenic cyclization initiated with acetal and concluded with vinylsilane, giving an overall yield of 6%. The key steps in this synthesis were the coupling of substrates 166 and 167 with control of the absolute and relative stereochemistry, the cationic biscyclization to form the intermediate tricyclic trans-anti-trans 169 and the formation of the D ring by regio-selective intramolecular remote functionalization. 

The chiral Mo-alkylidene complex derived from AROM of a cyclic olefin may also participate in an intermolecular cross metathesis reaction. As depicted in Scheme 16, treatment of meso-72a with a solution of 5 mol % 4a and 2 equivalents of styrene leads to the formation of optically pure 73 in 57% isolated yield and >98% trans olefin selectivity [26]. The Mo-catalyzed AROM/CM reaction can be carried out in the presence of vinylsilanes the derived optically pure 74 (Scheme 16) may subsequently be subjected to Pd-catalyzed cross-coupling reactions, allowing access to a wider range of optically pure cyclopentanes. 

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