Allylsilanes anion

On the other hand, if the allylsilane anion is first complexed with certain metals, a-regioselectivity then predominates, and a high degree of complementary diastereoselectjvity (19) can be attained with aldehydes as electrophiles. For example, boron, aluminium and titanium complexation all induce threo selectivity whereas the use of tin results in an erytbro... 

Reactions of Allylsilane Anions Generally, deprotonated silanes react in a y-regio-selective manner with electrophiles to give vinylsilanes. 

The allylsilane anion reacted with CO and underwent a similar rearrangement to provide a unique route to the dienolates (Eq. (5.27)) [31]. 

A different approach is to add substituents to or manipulate substituents on an existing allyl- or propar-gyl-silane system. Thus propargyltrimethylsilane itself can be alkylated at the terminus and the triple bond reduced with Brown s nickel catalyst or by hydroalumination-protonation, both of which give the (Z)-allylsilane. Alternatively, zirconium-catalyzed carboalumination of propargylsilanes introduces more substituents onto the allyl framework.Allylsilane anions show some selectivity for alkylation a to the silyl group, and the geometry of the double bcmd can be controlled by using either the kinetic or the thermodynamic allyllithium intermediate (Scheme 78). ... 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

Allylsilanes and allylboranes are allyl anion equivalents, which are stable enough to be included in subsequent allylation reactions of aldehydes under... 

Many newer methods for generating cyclohexane derivatives from carbohydrates still depend on the intramolecular attack of nucleophilic carbon species at electrophilic centers, and the range of options is now extensive. Thus, the nucleophiles may be carb-anions stabilized by carbonyl, phosphonate, nitro, or dithio groups, and they may bond to carbonyl carbon atoms, or to those that carry appropriate leaving groups or are contained in epoxide rings, or as jj-centers of a,p-unsaturated carbonyl systems. Otherwise, the nucleophilic activity at the 7-centers of allylsilanes or a-positions of vinyl silanes may be used to react with electrophilic carbon atoms. 

The anion of cyclopentadiene has also been demonstrated to add via ligand addition (inversion).105 385-386 Indenyl nucleophiles derived from the corresponding allylsilane have been classified as adding via ligand addition,385 but the sodium salt of the indenyl anion has curiously been suggested to add via addition directly to the metal.386... 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

Whereas an allylsilane can serve as an allyl anion synthon, the reaction of 1,3-bis(silyl)propene with electrophiles can afford the 1,3-disubstituted propene. Thus, treatment of a mixture of (If)- and (Z)-2-aryl-l,3-bis(trimethylsilyl)propenes 5 with 2 equivalents of NBS at —78 °C stereoselectively yields the corresponding (Z)-2-aryl-l,3-dibromopropene 6. When 1 equivalent of NBS is employed, the monobromo product 7 is obtained (equation 5). The reactions apparently proceed via the pattern of sequential displacement of allylsilane moieties40,41. 

The use of silylallyl anion in organic synthesis has been extensive24,29. The regio-and stereochemistry of these reactions can be controlled. For example, alkylation of the anion generated from the corresponding allylsilane with an electrophile E+ takes place selectively at the /-position due to steric hindrance (equation 197)352. 

Allylsilanes.1 A general route to allylsilanes involves addition of the anion of 1 lo aldehydes and ketones to form an adduct (2). Mesylation in situ, of the adducts followed by sodium amalgam reduction, results in allylsilanes (4) in 85-95% overall yield, based on the carbonyl compound. 

In 1982, Sakurai [7] described a catalytic version of this reaction (Scheme 13.4). The addition of small quantities of fluoride anions to the allylsilane 1 generates the pentacoordinated silicon species 10, probably in equilibrium with the starting materials 1 and 11. This activated species can react with the carbonyl derivative 6 to yield the alkoxide 12 which is trapped by fluorotrimethylsilane. This last step not only furnishes the silylated compounds 13 but also regenerates the fluoride catalyst 11. Acidic work-up then leads to the desired homoallylic alcohol 7. 

Allylsilanes are readily deprotonated as the anion generated is stabilized not only by conjugation with the adjacent double bond but also by the neighbouring silyl group. The anion may react with electrophiles through either its a-carbon atom or its y-carbon atom. The regiochemical and stereochemical outcome of these reactions depends on several factors of which the most important is probably the identity of the counterion (Equations Si6.9— 12). 

The alkylation of substituted 2-(phenylsulfonyl)tetrahydropyran anions, such as compound 375 (prepared from the starting material 374), with the iodide 376 bearing an allylsilane moiety, led to the formation of spirocyclic ethers 377 (Scheme 98)551. 

In organic syntheses allylsilanes and allylstannanes have been used extensively as allyl anion equivalents during the last two decades [187-190]. The regioselective attack of electrophiles, which finally yields products with allylic inversion (Scheme 43), has been explained by the hyperconju-gative stabilization of carbenium centers by the carbon-silicon or carbon-tin bond in the j3-position [191-196], which has initially been derived from solvolytic experiments [197-199]. 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

THF at - 20°. All other anions should be prepared separately, and 2 should be added to 1 at - 20°. By these procedures mono- and disubstituted olefins, vinyl sulfides, vinyl ethers, and allylsilanes are available in 35-80% yield, usually as cis-trans mixtures. Trisubstituted olefins are best prepared by similar routes from a -branched nitrile anions. 

As we mentioned before, a classical Grignard reaction is formally described by the coupling of a covalent (albeit polarized) electrophile with an anionic nucleophile. Reactions shown in Scheme 2.41 (opposite) exemplify the alternative approach involving an interaction between cationic intermediates generated from carbonyl compounds (or their derivatives) under the action of Lewis acids and a purely covalent nucleophile, an allylsilane such as I09a or 109b. Similar electrophiles used in reactions with covalent silyl enolates such as 110 result in the formation of the aldol-Iike products (the Mukaiyama reaction ). 

Enantioselective protonation of prochiral allyl anion derivatives is a very simple and attractive route for the preparation of optically active olefins. The acid-promoted hydrolysis of allyltins or allylsilanes is an interesting alternative the enantioselectivity of which has not yet been investigated. Allyltrialkyltin, a synthetic equivalent of allyl anion, is more reactive than the corresponding allylsilane and can be isolated. The... 

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