Coupling of alkenylsilanes

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

Yoshida and co-workers have reported on the use of alkenyldimethyl(2-pyridil)silanes as versatile platforms for olefin synthesis.The combination of Mizoroki-Heck-type coupling and Hiyama cross-coupling provided a diverse range of stereodefmed polysubstituted olefins. 

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The following mechanism is suggested for the cross-coupling of alkenylsilanes. Nucleophilic attack of a fluoride ion to the silicon atom of alkenylsilanes is first assumed to afford a pentacoordinate silicate and enhance both nucleophilicity of the silicon-substituted carbon and Lewis acidity of silicon to assist transmetalation effectively through a four-centered transition state (Scheme 2). Lewis acidity on silicon is critical as evidenced by the fact that hexacoordinate pentafluorosilicates that are fully coordinated and thus should have sufficient nucleophilicity are much less effective for the cross-coupling reaction (Eq. 2, vide supra). 

TBAF-Promoted Palladium-Catalyzed Cross-Coupling of Alkenylsilanes with Aryl or Alkenyl Halides. ((1E)-1-Heptenylbenzene (E)-14)... 

Pd-Catalyzed Fluoride-Free Cross Coupling of Alkenylsilanes and Germanes... 

Scheme 3-178. Cross-coupling of alkenylsilanes with acetic anhydrides.

The cross-coupling of aryl or alkenylsilanes with aryl chlorides bearing electron-withdrawing substituents has been reported to require trialkylphosphine (P Pr3) ligands.407... 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

To facilitate the coupling reaction of alkenylsilanes, benzenediazonium salts could be employed in place of aryl halides [4], However, the coupling reaction is sometimes accompanied by the formation of regioisomers [Eq.(4)J. This fact is ascribed to a mechanism that involves carbopalladation of an arylpalladium complex to alkenylsilanes followed by elimination of the silyl group and the palladium metal, as is the case in Heck-type reactions. 

Scheme 10-1 Mechanism of palladium-catalyzed coupling reaction of alkenylsilanes.

Silicon/boron exchange. For the sake of increasing reactivity in coupling an alkenylsilane is converted to a boronate by reaction with BCI3 and then with catechol. Such a product can be used in Suzuki coupling. 

The coupling products are obtained by one-pot hydrosilylation/cross-coupling of alkynes. Pt-catalyzed hydrosilylation of alkyne with tetramethyldisiloxane (52) generates the alkenylsilane 53, which is coupled with 4-iodoanisole (2) using ligandless Pd catalyst and TBAF at room temperature to afford 46 in 84 % yield [103]. 

Later, they extended their methodology to alkenylsilanes and alkenyl iodides. In 2001 Kang and his team reported on the carbonylative coupling of alkynyl-silanes with triarylantimony diacetates [85]. Alkynones were synthesized in good yields in the presence of a Pd catalyst and Cul at 50 °C (Scheme 4.46). 

Transmetallation of alkenylsilanes takes place with retention of the double bond configuration, as in other cross-coupling reactions [295]. Owing to the lower transmetallation rate, competing 1,2-insertion of the alkene in the intermediate organopalladium complex (as a Heck type reaction) may take place, which affects the regioselectivity of the Hiyama reaction in some cases (Scheme 1.38) [296]. This... 

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