Allylic Trialkylsilanes

The addition of an allylsilane to an electrophile was first documented in 1948 by Sommer et al. [6]. These workers predicted that the allylsilane would react with an electrophile to generate a silicon-stabilized cationic intermediate. In 1956, Galas and co-workers demonstrated that allylsilanes undergo an allylic shift in the protiodesilylation of a cyclohexenylsilane to afford a methylidenecyclohexane [7], The first report of the reaction of allylsilanes with carbonyl compounds (1974) is also due to Galas [8]. These authors used activated substrates such as perfluoro-acetone and chloroacetone and AIGI3, GaGl3 or InGl3 as Lewis acids to promote 

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In parallel with the developments involving chiral allylation reagents, there have been a number of exciting reports of catalytic enantioselective processes [24, 30, 32, 119]. To date, these have most efficiently been achieved for chiral Lewis acid catalysts combined with type 2 allyl trialkylsilanes and trialkyl-stannanes. In addition, there have also been recent impressive reports of the use of Lewis basic catalysts with aUyl trihalosilanes. 

Allylation of Aldehydes with Dienes Promoted by Trialkylsilanes. 177... 

Intermolecular allylation of aldehydes with 1 -trialkylsilyl-1,3-dienes 22 in the presence of a stoichiometric amount of triethylsilane and a catalytic amount of Ni(cod)2 and PPI13 shows novel regio- and stereoselectivity (Scheme 6) [20-22], When a toluene solution of a 1-silyl-1,3-diene and an aldehyde is refluxed in the presence of trialkylsilane under the catalysis of Ni(cod)2 and PPh3, ( )-allylsilane (E)-23 is obtained exclusively. On the other hand, when the reaction is carried out in THF upon heating at 50 °C as... 

A dinuclear Rh(II) complex, Rh2(pfb)4 (pfb = perfluorobutanoate), effectively catalyzes the hydrosilylation of 1-alkynes to give (Z)-l-silyl-l-alkenes as the major product when a trialkylsilane is added to the mixture of a 1-alkyne and the catalyst in CH2CI2111. However, the reverse addition, i.e. addition of the 1-alkyne to the mixture of the hydrosilane and the catalyst, affords an allylic silane in place of the silylalkenes in high yield (vide infra)111. 

The comparison of propene, allyltrimethylsilane, and isobutene indicates, that introduction of a trimethyl silyl group in /3-position of the developing carbenium center activates more than a methyl group in a-position. Both series of triphenyl element compounds (left and right column Scheme 44) show the reactivity pattern Si < Ge < Sn, but variation of the substituents at silicon and tin was found to largely affect the reactivity of the double bond. While in the allyl series (right column), the trialkylsilanes and -stannanes are 2 to 3 orders of magnitude more reactive than the... 

Protodesilylation of allyl- or aryl-substituted trialkylsilanes with an acid partner is a cleaner method, producing propene or an aromatic hydrocarbon as a volatile inert byproduct. Although some methods of isolation have been reported, they can usually be prepared in situ in aprotic media and employed without further purification. Methods of preparation of representative organosilicon Lewis acids, la, trimethylsilyl bis(trifluoro-methanesulfone)imide (7) [9], trimethylsilyl perchlorate (6a) [10], iodotrimethylsilane, and trimethylsilyl tetrakis(trifluoromethanesulfonyl)borate [11] are described below. In principle, these analogs are also accessible in a similar manner (Table 1) [2d]. 

Crotyl silanes offer the possibility of diastereoselectivity in reactions with aldehydes in the same way as the corresponding boranes. The mechanism is completely different because crotyl trialkylsilanes react via an open transition state as the silicon is not Lewis acidic enough to bind the carbonyl oxygen of the electrophile. Instead, the aldehyde has to be activated by an additional Lewis acid or by conversion into a reactive oxonium ion by one of the methods described above. The stereoelectronic demands of the allylic silane system contribute to the success of this transformation. Addition takes place in an Se2 sense so that the electrophile is attached to the remote carbon on the opposite side of the n system to that originally occupied by silicon and the newly formed double bond is trans to minimize allylic strain. 

Some vinyl silanes were found to act feasibly as weakly interacting comonomers. Functional copolymers with ethylene and vinyl-Si(CH3)3, aUyl-Si(CH3)3,3-butenyl-Si(CH3)3, 4-pentenyl-Si(CH3)3, 5-hexenyl-Si(CH3)3 or 7-octenyl-Si(CH3)2Ph as comonomers were polymerized [23-25]. The short trialkylsilane monomers suffered from the electroific influence of sihcon, which led to poor polymerization performance. In these cases, the chain end of the S3mthesized polyethylene-co-allyl-Si (CH3)3 consisted of reactive allyhc silane groups and therefore the functionality and reactivity of these copolymers was higher than the weakly interacting trimethylsUane moiety can provide. Also it was found that the phenylene group in... 

Montgomery and co-workers examined this three-component reaction from a mechanistic point of view (Scheme 10.8). Using a combination of cross-over experiments between EtsSiD and PrsSiH, they demonstrated that catalyst formulations involving PBus and IMes proceed largely by fundamentally different pathways. As a lack of cross-over with IMes was observed, it was concluded that the RsSi and H units in the product were mainly from a single molecule of hydrosilane. Intermediates I or II appeared the most probable in the catalytic cycle. Also, since PBus gave cross-over between EtsSiD and PrsSiH, it was postulated that the trialkylsilane would react at the end of the reaction with a transient allyl-nickalate. 

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