Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Silanes allylic, reaction with electrophiles

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. [Pg.862]

Scheme 9 demonstrates the further synthetic application of the thus obtained N,0-acetals. Substitution of the alkoxy or acyloxy group by nucleophiles like enol ethers, enol esters, enamines, other electron-rich olefins, CH-acidic compounds, electron-rich aromatics, isocyanides, trimethylsilyl cyanide, organometallics, vinyl and allyl silanes, hydroxy functions, or trialkylphosphites either catalyzed by Lewis acids or proton acids leads to the product of the amidoalkylation reaction (path a). In the presence of stereocenters as control elements, diasteroselective amidoalkylation reactions can be performed as shown in a large number of examples. On the other side, as Nyberg showed for the first time [196], elimination with formation of enecarbamates [208] and enamides [196,208,209] followed by reaction with electrophiles or nucleophiles (path b) also is possible. [Pg.571]

The most useful of all allyl anion equivalents are the allyl silanes.20 This is because it is easy to make them regioselectively, because they do not undergo allylic rearrangement (silicon does not do a [1,3] shift) and because their reactions with electrophiles are very well controlled addition always occurring at the opposite end to the silicon atom. Symmetrical allyl silanes can be made from allyl-lithiums or Grignards by displacement of chloride from silicon. A useful variant is to mix the halide with a metal, e.g. sodium, and Me3SiCl in the same reaction, rather after the style of the silicon acyloin reaction,21 as in the synthesis of the acetal 80. [Pg.178]

Palladium-catalyzed hydrosilylation of 1,3-dienes is one of the important synthetic methods for allylic silanes, and considerable attention has been directed to the asymmetric synthesis of the latter by catalytic methods [9]. Optically active allyhc silanes have been used as chiral allylating reagents in S reactions with electrophiles, typically aldehydes [38,39]. In the presence of Pd catalysts the reaction with hydrosilanes containing electron-withdrawing atoms or substituents on sihcon usually proceeds in a 1,4-fashion giving allyHc silanes [40,41]. Asymmetric hydrosilylation of cyclopentadiene (29) forming optically active 3-silylcyclopentene (30) has been most extensively studied (Scheme 13). In the first report, hydrosilylation of cyclopentadiene (29) with methyldichlorosilane in the presence of 0.01 mol % of palladium-(l )-(S)-PPFA (15a) as a catalyst gave... [Pg.325]

The reaction with EtAlCb is a Lewis-acid-catalysed conjugate addition of the allyl silane on to the enone. Conjugate addition is preferred because the nucleophile (allyl silane) is tethered to the electrophile (enone) and the five-membered ring is easier to form than the alternative seven-membered ring. [Pg.447]

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. [Pg.1302]

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. [Pg.318]

The Houk conformation for reactions ofalkenes with electrophiles Allyl silanes... [Pg.400]

We shall see in this section that allyl silanes can direct the transfer of chirality through Houk conformations without the need to form an enolate. The typical reaction of an allyl silane with an electrophile (chapter 12) is at the remote atom of the alkene with loss of the silyl group. This transfers, but does not create, chirality. So the allyl silane 152 reacts with formaldehyde and a Lewis acid to give the homoallylic alcohol 153 with no loss of ee. The silyl group has gone, the alkene is transposed, and the sense of the SE2 reaction is anti. All this is explained by the Houk conformation 154 with the C-Si bond able to interact with the alkene to raise the energy of the p-orbital and direct both the regio- and the stereoselectivity.25... [Pg.698]

Chatgilialoglu and Curran synthesized a variety of allyl tris(trimethylsilyl)silanes bearing substituents at the 2-position (Scheme 26) [70], These allylsilanes underwent reaction with alkyl halides when heated with a radical initiator to give very good yields of allylated products. The reactions were relatively sensitive to electronic effects electrophilic radicals reacted well only with electron-rich allyl silanes and vice versa. One potential drawback of this methodology is that the reactions reported were all carried out at 80 °C or above, suggesting that relatively high temperatures are necessary for efficient reaction. [Pg.68]

Radical Allylation Reactions. Trimethyl 2-[(tributylstannyl) methyl]-2-propenyl silane reacts with organic halides under photochemical conditions (irradiation with a medium pressure mercury lamp through a pyrex filter) at temperatures below 20 °C to give various allylsilanes (eq 7). The ease of the reaction was found to be halide dependent with those affording more electrophilic radicals generally behaving most effectively. [Pg.732]

The allylic silane (114) is prepared from the corresponding lithiated derivative by its reaction with C02 and by the nickel-catalysed reaction of the lithium enolate of ethyl trimethylsilylacetate with 2-bromopropene. Electrophiles react with (114) in the presence of Lewis acids to give a -unsaturated esters (115), predominantly with the ( ") configuration (Scheme 69). [Pg.125]

This book chapter is limited to Lewis acid-mediated reactions, and does not discuss the important field of Lewis base-mediated allylations, nor does it describe the reactions of allylsilanes with other electrophiles such as epoxides, imines, and allyl-X (X = -Cl, -OR, -OAc). The SaJcurai reaction has been covered under different forms in reviews focusing on The Stereochemistry of the Sakurai reaction , Intramolecular Addition Reactions of Allylic and Propargylic Silanes ," Selective Reactions Using Allylic Metals , Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , and Modem Carbonyl Chemistry . ... [Pg.539]


See other pages where Silanes allylic, reaction with electrophiles is mentioned: [Pg.83]    [Pg.1655]    [Pg.238]    [Pg.494]    [Pg.83]    [Pg.95]    [Pg.102]    [Pg.41]    [Pg.95]    [Pg.102]    [Pg.220]    [Pg.1373]    [Pg.41]    [Pg.541]    [Pg.1373]    [Pg.247]    [Pg.2048]    [Pg.95]    [Pg.102]    [Pg.709]    [Pg.112]    [Pg.716]    [Pg.716]    [Pg.840]    [Pg.2047]    [Pg.368]    [Pg.393]    [Pg.321]    [Pg.716]    [Pg.269]   
See also in sourсe #XX -- [ Pg.397 ]

See also in sourсe #XX -- [ Pg.388 ]

See also in sourсe #XX -- [ Pg.397 ]




SEARCH



Allyl electrophiles

Allyl electrophiles allylation

Allyl silane

Allyl silanes

Allyl silanes reactions

Allylation electrophilic

Allylic electrophiles, allylations

Allylic silane

Allylic silanes

Electrophiles allylation

Electrophiles allylic

Electrophiles, reaction with allyl silane

Electrophiles, reaction with allyl silane

Reactions with electrophiles

Reactions with silanes

Silane, reaction

Silanes allyl, reactions with electrophilic intermediate

Silanes reactions

Silanes reactions with electrophiles

Silanes, allyl, reaction with

Silanization reaction

With Electrophiles

With silane

© 2024 chempedia.info