Allylsilanes reactions with electrophilic carbon

The second proposal by Speckamp and co-woikers used an allylsilane as an excellent nucleophile for an A -acyliminium cyclization reaction (Esch et al. 1987) (Scheme 7.13). The 3-effect of the silicon atom is a powerM determinant of the regiochemistry of allylsilane reactions with electrophiles, so the new carbon-carbon bond is formed at the vinyl caibon distal to silicon, that is, at the y-position (Hiemstra et al. 1984,1985). 

In the presence of a Lewis acid (such as Et2AlCl), allylsilanes react with electrophiles in a regiospecific manner. The intermediate (3-carbocation is stabilized by (a-Tc)-conjugation with the C-Si bond. The most important feature of this reaction is that the electrophile reacts with the terminus (y-carbon) of the allyl system, and the n-system is relocated adjacent to its original position. Even substituted allylic silanes can be acylated at the more hindered site. Because of this predictability and their high nucleophilicity, allylsilanes are valuable in many synthetic transformations. 

A different set of mechanistic possibilities comes into play when nucleophilic catalysis is used in place of the electrophilic catalysis induced by Lewis acids. The most common form of this reaction is fluoride ion catalysis of allylsilane reactions, where the carbon nucleophile is probably a hypervalent allylsilane (12), with the fluoride ion coordinated to the silicon, making the allylsilane unit nucleophilic enough to react with aldehydes without acid catalysis. ... 

One group of alkenes which react efficiently with electrophilic carbon species are the allylsilanes. The carbon-silicon bond is broken as the reaction proceeds, thus providing a low-energy product.Among the electrophilic carbon species... 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

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). 

Like their carbon counterparts, silylcuprates and stannylcuprates readily participate in SN2 -substitution reaction with allylic electrophiles.43,51 For example, <37z//-diastereoselective allylic substitution of the vinyl oxiranes 147 with the lower-order cyanocuprate (PhMe2Si)Cu(CN)Li cleanly affords the allylsilanes 148 (Scheme 38).106 Products of this type can be converted into the corresponding diols with retention of configuration by Tamao-Fleming oxidation 53,53a,53b,107... 

On the other hand, the organosilyl group is capable of stabilizing the carbocation (3 to silicon. This effect is believed to be attributable to the electron-donating property of the a-bond between silicon and a-carbon atoms ((a-p)7i conjugation) (Scheme IB). The reactions of allylsilanes or vinylsilanes with electrophiles are facilitated with this stabilization of intermediate carbocations (eq (10)). 

A general mechanism of this reaction involves globally a regiospecifictranspositionoftheallylicmoiety (Scheme 12.12). The reaction is initiated by the nucleophilic addition of an allylsilane 39 over an electrophilic carbon, which is normally activated by a strong Lewis acid. Thus, a secondary carbocation intermediate (p-silyl cation, 46) stabilized by hyperconjugation is formed. Then it evolves into a compound with a new double bond by elimination of the silyl group. 

Silylcupration of allene with (PhMe2Si)2CuLi followed by treatment of the intermediate cuprate with H" ", carbon electrophiles, and CI2 affords the corresponding vinylsilanes (eq 9). However, reaction of the intermediate cuprate with I2 affords an allylsilane, from which other more highly substituted allylsilanes are readily obtainable (eq 9). In reaction with substituted allenes, the product ratio (vinylsilane vs. allylsilane) is dependent upon the degree of substitution. ... 

Silicon.— Allylsilanes are outstandingly useful carbon nucleophiles as a result of their ability to react with electrophiles in a controlled and specific manner to form new carbon-carbon bonds. Furthermore, metallation to give a-(trimethyl-silyl)allylic carbanions introduces the possibility of an additional level of control over the mode of attack (either a or y) exhibited by the allylsilane. It is not surprising, therefore, to find the chemistry of these species occupying a prominent position in this year s organosilicon literature, and in addition to the many applications reported some new routes to these compounds have been described. One such example is shown in Scheme 13, in which the starting silyl sulphone is readily prepared by oxidation of the product obtained from reaction of thio-phenol with vinyltrimethylsilane." ... 

Support-bound allylsilanes can be cleaved either by treatment with acids or by treatment with carbon electrophiles. The third reaction in Figure 2 is an example of a cleavage, in which the electrophile is an a-alkoxy carbocation generated from an acetal and TiCl4. 

In 1976 we reported that aldehydes and ketones are efficiently allylated with allyl-trimethylsilane in the presence of a substoichiometric amount of I iLf, [332]. Subsequently, BU4NF, a fluoride ion source, was found to be an effective catalyst of this allylation reaction [333]. After these initial reports of the Hosomi-Sakurai reaction, several allylsilanes, including highly functionalized compounds, were used for regio- and stereoselective allylation of a variety of carbon electrophiles [6, 13, 14, 334]. In the nineteen-eighties, some Lewis acids (TMSOTf [335], TMSI [336]... 

Chiral allylsilanes react antarafacially the incoming electrophile attacks the double bond on the surface opposite to the silyl group (Scheme 13) and the major piquets are syn, in this case even from the (Z)-allylsilane. The overall anti stereospecific reaction (anti to the extent of 90 10) in favor of product (32) having the substituents on the carbon chain syn to the extent of about 80 20 is still found with the (Z, )-silane (31) in an Se2 reaction (Scheme 14), even though the new stereocenters are five and six... 

In fluoride ion catalyzed reactions of allylsilanes, aldehydes are the only carbon electrophiles that work well protodesilylation is unavoidable with most other electrophiles, even in the most rigorously dried media. The most commonly used fluoride ion source, which only needs to be present in catalytic amounts, is TBAF in THF at room temperature or under reflux, benzyltrimethylammonium fluoride may be better, and cesium fluoride in DMF, potassium fluoride with 18-crown-6 in THF, and TASF in polar... 

The stabilization of a 3-carbonium ion is also involved in the reaction of an allylsilane with an electrophile (Eq. 14.16). An advantage of silicon over other metals in this context is that it does not undergo 1,3 shifts, and so the point of attachment of the electrophile can be reliably predicted (Eq. 14.17). - This 3 stabilization of the carbonium ion also has stereochemical implications Eq. 14.18 shows how the stereochemistry of a vinylsilicon reagent can be retained on protonation. A TMS group on carbon has been described as a... 

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