Cross-coupling reactions alkenylsilanes

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... 

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... 

Transition Metal Promoted Coupling Reactions. In the presence of a Ni catalyst, the reagent undergoes cross-coupling reactions with aryl bromides (eq 4). Alkenyl iodides are stere-ospecifically converted to alkenylsilanes in the presence of a Pd catalyst (eq 5). 

The importance of the torquoselective olefination is illustrated in Fig. 34 for the particular case in which a multisubstituted alkenylsilane is converted to various kinds of multisubstituted olefins. The silyl-substituted allyl alcohol 93 is allylated to give the 1,4-diene 94, and the iodoalkene 95, prepared by desilyliodination of 93, is subjected to palladium-catalyzed cross-coupling reactions (the Heck reaction, Stille coupling) to afford the dienes 96 and 97 without ElZ isomerization. 

Transition metal-catalyzed silicon-based cross-coupling reaction has emerged as a versatile carbon-carbon bond-forming process with high stereocontrol and excellent functional group tolerance [35], For example, (a-benzoyloxy)alkenylsilanes 105, prepared as a pure -isomer by 0-acylation of a lithium enolate derived from the corresponding acylsilane, reacts with carboxylic acid anhydrides in the presence of [RhCl(CO)2]2, giving rise to a-acyloxy ketones 106, which are then converted into 1,2-diketones by acidic workup (Scheme 5.27) [36]. 

Scheme 530 Intramolecular cross-coupling reaction of alkenylsilanes leading to medium-sized rings.

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... 

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. 

Whereas the vinyl groups of Da are accessible for functionalization by hydroboration or hydrosilylation, they are inert to functionalization by cross-metathesis. Alternatively, formal metathesis products can be obtained by the ruthenium-catalyzed silylative coupling reaction. This method involves the combination of a vinyl silane and an olefin in the presence of a ruthenium catalyst, to provide an alkenylsilane (see eq 7). The application of this reaction to Da provides substitution at each of the four vinyl groups, resulting in a cyclic tetraalkenyltetramethylcyclote-trasiloxane. The silylative coupling reaction of both Da and has been demonstrated with styrenes and enol ethers. ... 

---