Alkenylsilanes functionalized

Under a pressure (20 bar) of carbon monoxide, carbonylative silylcarbocyclization of enyne 92 was examined in the presence of a cationic rhodium-BINAP catalyst (Scheme 31).86 Although the enantioselectivity is low, the five-membered carbocycle functionalized with an alkenylsilane moiety and a formyl group was obtained with high selectivity. 

Electro-initiated oxygenation of alkenylsilanes is found to proceed in the presence of thiophenol to give the corresponding a-(phenylthio)carbonyl compounds 173 regiospecif-ically (equation 147)261. Hydroxy and carbonyl functionalities remain intact under these conditions. 

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. ... 

This method provides not only simple silyl-substituted alkenes but also functionalized alkenylsilanes. Some electrophiles react with the alkenyhnetal species without difficulty. For example, treatment of the intermediate derived from PtCl2(P Bu3)2-catalyzed silylmagnesation of 1-dodecyne wifh iodine, methyl iodide, and valeraldehyde gave fhe corresponding silylalkenes carrying fhe electrophilic partner in the E configuration (Scheme 3.76). All these electrophiles reacted at 25 °C within 1 h. 

Although mechanistically different, functionalized alkenylsilanes are prepared stereoselectively by the reaction of 1-alkynes with iodotrimethylsilane (123) and diethylzinc. At hrst oxidative addition of 123 to Pd(0) generates 125. Then insertion of 1-octyne to 125 affords the alkenylpalladium 126. Transmetallation with Et2Zn gives 127 and reductive elimination provides the alkenylsilane 124. The reaction can be regarded as a Heck-type reaction of alkyne with MesSi-I, followed by Negishi coupling [37]. 

A well-known, cross-coupling palladium catalyst [Pd2(dba)3(CHCl3)] in combination with tricyclohexylphosphine has also been found as a good catalyst for regioselective hydrosilylation of functionalized alkynes to yield substituted (E)-alkenylsilanes (62)... 

Ruthenium-based catalysts allow stereoselective hydrosilylation of alkynes giving /3-(Z)-alkenylsilanes or a-vinylsilanes as predominant products, respectively. [ Ru(p-cymene)Cl2 2], [RuCl(SiMe2Ph)(CO)(Pi-Pr3)2], and [RuHCl(CO)(Pi-Prs)2] have been recently demonstrated as active catalysts for the efficient (Z)-selective hydrosilylation of a wide range of functionalized alkynes (90-92), whereas Grubbs first-generation catalyst or cationic cyclopentadienyl ruthe-nium(II) complex [ Cp RuGVIeCNlalPFe] affords selectively a-vinylsilanes (93-95). 

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. ... 

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

Use of the hydroboration reaction for the regiospecific preparation of functionalized organosilanes from alkenylsilanes, some unusual regioselectivities in the... 

Alkenylsilanes having a (methoxy)acetoxy group at the allylic position derived from optically active allylic alcohols undergo the Claisen-Ireland rearrangement to give highly functionalized allylsilanes in a highly diastereoselective manner (Scheme 3-54).t 2J... 

The coupling of trimethylsilyldiazomethane with alkenyl halides and secondary amine is considered to involve carebenoid intermediate to results in the formation of alkenylsilanes having an allylic amine functional group by using palladium catalysts (Scheme 3-70). ... 

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]. 

---