Silicon allyl silanes

Vinyl and aryl silanes react with electrophiles at the same (ipso or a) atom occupied by silicon. Allyl silanes react at the end of the alkene furthest from silicon (y). In both case a (J-silyl cation is an intermediate. 

Trichloro- and trifluorosilanes introduce another dimension into the reactivity of allylic silanes. The silicon in these compounds is electrophilic and can expand to pentaco-ordinate and hexacoordinate structures. These reactions can occur through a cyclic or chelated TS. 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

While trifluoro and other halosilanes function by increased electrophilicity at silicon, nucleophilic reactivity of allylic silanes can be enhanced by formation of anionic adducts (silicates). Reaction of allylic silanes with aldehydes and ketones can... 

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

Reaction of allylic silanes with aldehydes and ketones can also be induced by fluoride ion, which is usually supplied by the THF-soluble salt tetrabutylammonium fluoride (TBAF). Fluoride adds at silicon to form a hypervalent anion with much enhanced nucleophilicity.73 An alternative reagent to TBAF is tetrabutylammonium triphenyldi-fluorosilicate.74... 

To explain the regioselectivity trend in the photo-oxygenation of allylic silanes, it was proposed that an interaction between the negatively charged oxygen of the perepoxide and the silicon atom controls the abstraction of allylic hydrogen atoms (Scheme 20). The formation of cis ene adducts [86] was attributed either to the intermediacy of a zwitterionic intermediate or to a nonconcerted pathway. A perepoxide intermediate was considered unlikely. 

Activation by silicon of a P-C-H bond to an intramolecular carbene insertion reaction is exemplified by the silicon-directed Bamford-Stevens reaction.68 For example, thermal decomposition of P-trimethylsilyl /V-aziridinyl imines 72 in toluene (Scheme 8) [with or without Rh2(OAc)4 catalyst] results in the formation of allylic silanes 73 as major or exclusive products by the preferential insertion of the carbene intermediate into the C-H bond P to the silicon substituent. 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

Dioxolane formation is not observed in allyl silanes as a result of silicons diminished ability in comparison to tin to stabilize a P positive charge. Nevertheless, the propensity of the carbon-silicon bond to donate electron density to electron deficient sites leads to population of a perpendicular conformation (Sch. 10) and ultimately to an unusually high yield of the sterically less stable Z-allylic hydroperoxide (e.g., 17). 

Another successful method for the highly diastereoselective silicon allylation reaction is the allyltrifluorosilane- cesium fluoride system discovered by Sakurai et al. in 1987s (Scheme 3.2e). After a mixture of aldehyde,the allylic trifluorosi-lane 4, and cesium fluoride in a ratio of 1 2 23 was stirred in THF, the reaction mixture was quenched with a solution of HC1 in MeOH to afford the products desired (5) in excellent yield and exceptionally high diastereoselectivity. In addition, the reaction is highly regioselective in that the carbon-carbon bond formation occurs exclusively at the ycarbon of allylic silanes. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

Allyl silanes react with electrophiles with even greater regioselectivity than that of vinyl silanes. The cation 3 to the silyl group is again formed but there are two important differences. Most obviously, the electrophile attacks at the other end of the allylic system and there is no rotation necessary as the C-Si bond is already in a position to overlap efficiently with the intermediate cation. Electrophilic attack occurs on the face of the alkene anti to the silyl group. The process is terminated by loss of silicon in the usual way to regenerate an alkene. 

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

A closely related reactive oxonium ion can be prepared by Lewis-acid-catalysed breakdown of the corresponding acetal. Alternatively, especially if the acetal is at least partly a silyl acetal, the same oxonium ion can be produced in situ using yet more silicon in the form of TMSOTf as the Lewis acid catalyst. All these intermediate oxonium ions act as powerful electrophiles towards allyl silanes producing homoallylic alcohols or ethers. 

The regio control that results from using an allyl silane to direct the final elimination is illustrated by this example of an intramolecular reaction on to an acetal promoted by tin tetrachloride. The same reaction can be run in the absence of silicon but the intermediate cation can then lose a range of protons to produce five different products ... 

Next, reaction with a silicon-substituted Grignard reagent in the presence of Ni(II) gives an allyl silane. What kind of reaction is this, what was the role of phosphorus, and why was a metal other than sodium added (You know nothing specific about Ni as yet but you should see the comparison with another metal. Consult Chapter 23 if you need help.)... 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

Isomerization of methyl-substituted allylsilanes. Substrates of this type when heated with fluoride ion in THF rearrange to allyl silanes in which the silicon atom is bonded to the less-substituted carbon atom of the allylic group. 

The corresponding inter-and intra-raolecular additions of allyl-silanes to iminium salts give rise after desilylation to synthetically useful biradical intermediates. Evidence for the existence of two mechanisms for such reactions, differing only in the timing of carbon-silicon bond cleavage and carbon-carbon bond formation, has been described. Analogous electron transfer... 

Basic silicon chemistry the Peterson reaction and allyl silanes as nucleophiles. 

Now a Lewis-acid-catalysed reaction of the allyl silane via a p-silyl cation, gives the lactone. The This silicon chemistry was double bond in these exo-methylene lactones easily moves into the ring in acid or base so the mild devised by A. Haider, Synthesis, conditions for allyl silane reactions are ideal. 19S5, 271. 

The first reaction is a Lewis-acid-catalysed reaction between the amine and the aldehyde ar.i addition of the allyl silane to the resulting iminium salt. Addition occurs at the other end of th allylic system and on the opposite face to the silicon. The most obvious way to put the reagent, together looks excellent until we realize, from the Newman projection, that the two Hs in tht product would then be anti whereas they are actually syn. 

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