Silylation 3-silyl elimination

Triorganovinylsilanes can be used as silylating reagents. In this case, the vinyl moiety functions as a hydrogen acceptor. The Ru3(CO)i2-catalyzed reaction of 3-acetylthiophene with trimethylvinylsilane affords 3-acetyl-2-(tri-methylsilyl)thiophene in 64% yield, as shown in Scheme 15.148 This reaction involves a /3-silyl elimination, yielding a metal species. 

Rhodium-catalyzed, silane-initiated cascade cyclization of 1,6,11-triynes 83 was proposed to occur via a silane-initiated cascade carbocyclization to form the silylated bicyclic triene (Z,Z)-In. / -Migratory insertion of the silylated G=G bond into the Rh-G bond of (Z,Z)-In followed by / -hydride elimination from frans-lln could then form 84a. Alternatively, cisitrans isomerization of (Z,Z)-In followed by / -migratory insertion of the silylated G=G bond into the Rh-G bond of resulting isomer ( ,Z)-In could form cis-Wn. Subsequent / -silyl elimination from m-IIn would form unsilylated tricycle 84b (Scheme 21). 

Allylsilanes 12 react with a-ketoesters 174 to give [3 + 2] cycloadducts 175 in 50-85% yields through 1,2-silyl migration (equation 73)59,228. Use of an allylic trimethylsilane produces an allyl alcohol by-product through a competing silyl elimination process. It can be circumvented by use of a bulkier silicon-containing reagent, such as the... 

Facile 6-elimination of the silyl group is also utilized in the intramolecular anodic olefin coupling reactions [159-161]. For example, the intramolecular anodic coupling of enol ether with allylsilane group has been reported [Eq. (44)]. This reaction seems to be quite useful for the construction of functionalized cyclic compounds because it leads to the regioselective formation of olefinic product via a facile 6-silyl elimination. 

As shown in the mechanism depicted in (Scheme 18), the Pd-catalyzed silyl-carbocyclization of 1,6-dienes involves a reversible insertion of an olefin moiety into the [Pd]-Si bond (E-II to E-III). However, the coordination of the second olefin moiety to the Pd metal (forming E-IV) would fill the coordination site required for the j8-silyl-elimination and would therefore render the C—Si bond formation irreversible, which leads to the irreversible carbometalation to jdeld E-V. Accordingly, when the chiral Pd(pyridine-oxazoline) complex is used as the catalyst, the enantioselectivity should be determined at the first olefin insertion step forming -silylalkyl-[Pd] complex E-III. 

These authors also proposed an ailternative mechanism for the active catalyst regeneration, and this involved a hydrorhodation of the triethylvinylsilane to give RhCH2CH2SlEt3, which undergoes a p-silyl elimination to form another active catalytic species Rh-SlEtj. 

The intramolecular reaction of bis(vinlysilyl) ethers of a 1,2-diol catalyzed by ruthenium gives 1,1-disilylethenes, which undergo the Heck reaction to give substituted 1,1-disilylethenes (Scheme 3-65). The ruthenium-catalyzed reaction is proposed to proceed via hydrometalation toward a vinyl moiety, y silyl elimination to give a Si-Ru species and free ethene, intramolecular silylruthenation followed by y hydride elimination. 

The substitution carboicyclization in D accompanied by reductive elimination should form G. Insertion of CO in D forms -[Rh](H) intermediate E. Reductive elimination of E results in the isolation of aldehyde 2.318. The carboicyclization of E gives tricyclic intermediate F, bearing silicon and [Rh] in the syn-position. Substitution (3-silyl-elimination... 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

A trialkylsilyl group can be introduced into aryl or alkenyl groups using hexaalkyidisilanes. The Si—Si bond is cleaved with a Pd catalyst, and trans-metallation and reductive elimination afford the silylated products. In this way, 1,2-bis-silylethylene 761 is prepared from 1,2-dichloroethylene (760)[625,626], The facile reaction of (Me3Si)2 to give 762 proceeds at room temperature in the presence of fluoride anion[627]. Alkenyl- and arylsilanes are prepared by the reaction of (Me3Si)3Al (763)[628],... 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

Allylsilanes and allylstannanes are also reactive toward electrophiles and usually undergo a concerted elimination of the silyl substituent. Several examples are shown below. 

Selective fluonnation in polar solvents has proved commercially successful in the synthesis of 5 fluorouracil and its pyrimidine relatives, an extensive subject that will be discussed in another section Selective fluonnation of enolates [47], enols [48], and silyl enol ethers [49] resulted in preparation of a/phn-fluoro ketones, fieto-diketones, heta-ketoesters, and aldehydes The reactions of fluorine with these functionalities is most probably an addition to the ene followed by elimination of fluonde ion or hydrogen fluoride rather than a simple substitution In a similar vein, selective fluonnation of pyridmes to give 2-fluoropyridines was shown to proceed through pyridine difluondes [50]... 

---