Alkenylsilanes coupling

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

So far only a few examples of the uses of organosilicon compounds in cross-couplings have been published. Noteworthy is the smooth reaction with a sterically hindered substrate (87).295 The synthesis of the alkaloid nitidine included a cross-coupling step using an alkenylsilane (88),296 while the syntheses of some antitumor agents involved the alkenylation of unprotected iodouracyls using alkenyl-silicon species.297... 

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

Alkenylfluorosilanes readily couple with alkenyl iodides in the presence of a palladium ) catalyst and TASF to form dienes of high stereospecificity (equation 160)274. Since the alkenylsilane preparation and coupling reaction are conducted under neutral conditions, without the involvement of strong reducing agents, this coupling reaction has wide applicability. 

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. 

Both amido and pinacol derivatives of B-Si compounds 125 and 126 added to terminal and internal alkynes in the presence of a palladium244-246 or platinum(O) catalyst247 by a mechanism involving an oxidative addition-insertion process (Equation (39)).248 On the other hand, phosphine-free nickel(O) catalyst resulted in the dimerization of alkynes giving a Z,Z-isomer of l-silyl-4-borylbutadiene derivatives.249 Since the palladium-catalyzed cross-coupling at the C-B bond is faster than the G-Si bond of 137, a silylboration-cross-coupling sequence provided a method for the synthesis of 1-alkenylsilanes.246... 

There are scattering examples that neutral organosilanes R-SiMe3 undergo a desilylative coupling reaction with aryl halides in the presence of a palladium catalyst. Hallberg and his co-workers disclosed that trimethyl(vinyl)silane reacts with an arylpalladium(II) complex to afford coupled products accompanied by aryl-substituted alkenylsilanes formed through the Heck type reaction... 

Eq. 1) [5]. On the other hand, Kikukawa and his co-workers found that the reaction of trimethyl(a- or -styryl)silane with arenediazonium tetrafluoroborates gave regioisomeric mixtures of coupling products [6-8]. The catalytic cycle of the reaction is considered to involve carbopalladation toward the C-C double bond of an alkenylsilane by an arylpalladium intermediate followed by tetraflu-oroborate-assisted elimination of the silyl group and regeneration of a palladi-um(0) species (Scheme 1). 

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). 

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... 

Under an atmospheric pressure of carbon monoxide, aryl- and alkenylsilanes undergo a carbonylative coupling reaction with aryl and alkenyl halides [44,45]. The optimized conditions for arylsilanes involve the use of N,M-dimethyl-2-imidazolidinone (DMI) as a solvent and KF as a fluoride ion source (Eq. 38), whereas alkenylsilanes prefer THF and TBAF (Eq. 39). 

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. 

Since a variety of 1-alkenylboron reagents including (E)- and (Z)-isomers are now available, their cross-coupling with 1-halo-l-alkenes affords various stereodefined alkadienes and trienes [64-66]. Many syntheses of alkadienes and trienes such as unsaturated fatty acid amides [76], alkenylsilanes [77], gem-difluoroalkenes [69, 78], cyclic alkenes [79], rran5-(C,o)-alIofamesene [80], trisporol B [81], and vinyl sulfides [82] are reported, by application of the reported Pd-catalyzed cross-coupling. The representative syntheses and reaction conditions ai e summarized in Table 2-2. 

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. 

Coupling reactions. Conjugated dienes and enynes are obtained from coupling reactions involving RMgCl/MnClj LiCl with 1-chlorodienes and 1-chloroenynes. With 1,1-dibromomethylsilanes the products are (Ej-alkenylsilanes. ... 

Coupling reactions involving aryl and alkenylsilanes in which the silicon atom bears an electron-withdrawing group are also valuable. Thus, 1,4-dienes arise from ally lie carbonates and alkenylfluorosilanes. Functionalized biaryls are prepared, and promotion of such reactions by NaOH has been noted. ... 

Although alkenylsilanes undergo palladium-catalyzed coupling reactions with aryl iodides (eq (19)) [16], the similar reactions of 5-coordinated silicates proceeds under milder conditions to give better yields (eq (20)) [17]. 

Alkenylsilanes. Addition of (MesS sSiH to 1-alkynes in the presence of a Rh(I) complex furnishes the ( )-l-silylalkenes, whereas the (Z)-isomers are obtained from formal group exchange reaction starting from (Z)-l-alkenyl sulfones. These alkenylsilanes can be used in Hiyama coupling after treatment with alkaline H2O2. 

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