Allylic compounds Allylsilanes

Essentially all allylsilanes (M = SiR3, Section D.l.3.3.3.5.) with the exception of fluorosil-iconates11 and most of the trialkyl(allyl)stannancs (Section D.l. 3.3.3.6.), which have only very weak Lewis acidic properties, require a strong Lewis acid to trigger the reaction with a carbonyl compound by the preceding formation of an x-oxycarbenium ion, which attacks the allylic compound in an ionic open-chain pathway. These Lewis acid catalyzed carbonyl additions offer new possibilities for the control of the simple and induced diastereoselectivity12. 

In 1982, Ohga and Mariano first reported the photoallylation of iminium salts. The photoreaction of iminium salts with allyl-trimethylsilane in acetonitrile affords allylated compounds via the addition of an allyl radical to an a-aminomethyl radical. Allyl radicals are produced via the cleavage of C-Si bonds of allylsilane radical cations and the a-aminomethyl radicals are generated by one-electron reduction of iminium salts as shown in Scheme 4. 

Previously the author reported that an allylic nucleophilic species is generated readily from trimethylallylsilane by the catalysis with fluoride ions. The reaction provides a versatile method of introducing an allyl group into carbonyl compounds.However, the exclusive regiospecificity of allylation, which is characteristic of the Lewis acid-catalyzed allylation with allylsilanes, disappeared. 

CM with Allylic Compounds Allylic compounds such as allyl silanes, allyl phosphonates, allyl halides, and primary allylic alcohols are categorized as type I olefins and are used in CM reactions with various olefins for the synthesis of natural products. The CM reaction of allyl silanes has been reported to be combined with intramolecular Sakurai-type reaction for the construction of cyclic compounds.In the concise synthesis of the indolizidine alkaloid by Marsden and McElhinney, " allyltrimehylsilane (4 equiv) was employed as a CM partner of the type I olefin 80 giving rise to the desired allylsilane 81 in 73% yield as a 3 1 mixture of E- and Z-isomers (Scheme 24.21). After the... 

Lewis acid catalyzed carbonyl addition of allylsilanes (Section D.l.3.3.3.5.) and allylstannanes (Section D.l.3.3.3.6.) usually proceed with clean allylic inversion (Section D.l.3.3.1.2.). Since these compounds are prepared by several routes and are also stable enough to be purified, each regioisomer can be approached. 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

The majority of catalytic enantioselective allylation reactions involve the chiral Lewis-acid-catalysed additions of allylsilanes or allylstannanes to carbonyl compounds. Monothiobinaphthol has been used by Woodward et al. as a chiral promoter in the enantioselective catalytic allylation of aryl ketones with impure Sn(allyl)4, prepared from allyl chloride, air-oxidised magnesium and SnCl4. Therefore, the allylation of arylketones in these conditions was achieved very efficiently, since the corresponding allylic alcohols were formed in... 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

Allylation of aldehydes or ketones using allylsilanes, known as the Hosomi-Sakurai reaction, is a useful method for obtaining homoallylic alcohols. TiIV compounds have been successfully applied to this reaction (Scheme 21) 80 Besides aldehydes and ketones, acylsilanes, 0,0-acetals, and A-,(7-acetals can be employed.81-83 1,4-Addition of an allyl group to an a,/ -unsaturated ketone has been also reported.84... 

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

Allylic phosphorus compounds Tris(tetrabutylammonium) pyrophosphate, 338 Allylsilanes t-Butyllithium, 58 Chlorotrimethylsilane-Lithium, 81 Lithium bis(dimethylphenylsilyl)-cuprate, 161 Methyllithium, 188 Trichlorosilane-t-Amines, 322 Allylic sulfur compounds (Phenylsulfonyl)allene, 247 Sodium benzenesulfinate, 289... 

Allyl transfer does not take place with 76 under nucleophilic conditions39. By contrast, the reactions of allylsilanes with carbonyl compounds are activated by fluoride ion353. 

A major advance in this area is represented by the fluoride ion-promoted addition of allylsilanes to thioketones423 (equation 116) giving the compounds arising from a direct thiophilic addition, namely the corresponding allyl sulfides. 

Allylsilanes. A general synthesis of allylsilanes involves alkylation at the imposition of an allylic alcohol substituted at the 7-position by a trimethylsilyl group or at the a-position by a trimethylsilylmethyl group with an organolithium compound mediated by this phosphonium iodide (l).2... 

Allylmetal compounds can be prepared by the Pd-catalysed allylation of dimetal compounds. The phenylallylboronate 195 is prepared by the reaction of 2-phenylallyl acetate with bis(pinacolate)diboron (194) catalysed by ligandless Pd in DMSO [95], A good synthetic route to the allylsilane 197 is the reaction of Me3SiSiMe3 with geranyl trifluoroacetate (196) catalysed by ligandless Pd via transmetallation of the n-allylpalladium intermediate at room temperature in DMF [96],... 

Hwu et al. have examined the dependence of the metal oxidant on the mode of reactivity in silicon-controlled allylation of 1,3-dioxo compounds [95JOC856]. The use of manganese(III) acetate furnished the dihydrofuran product 22 only. On the other hand, use of cerium(IV) nitrate resulted in the formation of both acyclic (23) as well as the cyclized compound, with the product distribution dependent on the nature of the allylsilane. Facile synthesis of dihydrofurans by the cerium(IV) mediated oxidative addition of 1,3-dicarbonyl compounds to cyclic and acyclic alkenes has also been reported [95JCS(P1)187]. 

The resulting 5-methylene-2-oxa-l-silacyclohexanes are insufficiently Lewis acidic to react with a second equivalent of the carbonyl compound. However, the incipient allylsilane does react with dimethyl acetals in decent yields in the presence of external Lewis acids including BF3-Et20 or AICI3. Based on these results, double allylation of dicarbonyl compounds with 3-methylene-l,l-diphenyl-l-silacyclobutane was examined, leading to the formation of 3-methylene-oxabicyclo[3.2.1]octanes. This transformation proceeded in one pot and in the presence of BF3-Et20 (Scheme 42). 

The effects of the cr—JT interaction on the ground-state properties of allyltrimethylmetal compounds are paralleled by the effect on reactivity towards electrophilic reagents. Mayr demonstrated that allyltrialkylsilanes, allyltrialkyl-germanes, and trialkylstannanes react with diphenylcarbenium ions at rates 105,5.6 x 105, and 109, respectively, relative to propene.158 The reaction rates were also found to be sensitive to the inductive effects of the other substituents attached to the metal. A theoretical evaluation of the factors determining the regiochemistry and stereochemistry of electrophilic addition to allylsilanes and other allyl systems is reported by Hehre et al.159 They predict a preference for electrophilic attack anti with respect to the silane substituent, a prediction that is supported by many experimental studies.82,160... 

Not surprisingly, two trimethylsilylmethyl groups further increase reactivity of cyclopropanes. Therefore 262 opens to 263 even at —78 °C yielding a new functionalized allylsilane as the product. An attempt to transform 262 into the corresponding P-ketosulfone 264 leads to the unusual cleaved compound 265, which carries an allyl and vinylsilane unit as well as the P-ketosulfone moiety. It is evident that this mode of cyclopropane opening is initiated under the basic reaction conditions by deprotonating the intermediate 264 in the side chain a to one of the trimethylsilyl groups 113). 

Keywords Allylation, Allylsilanes, Allylstannanes, Carbonyl compounds, Chiral Lewis acid catalysts, Chiral Lewis base catalysts... 

Catalytic asymmetric allylations of aldehydes or ketones are roughly classified into two methods, namely, those using chiral Lewis acid catalysts and those using chiral Lewis base catalysts. The former method uses less reactive allylsilanes or allylstannanes as the allyl source. The latter method requires allyltrichlorosi-lane or more reactive allylmetals. Both processes are applicable to the reactions with substituted allylmetal compounds or propargylation. 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

Indium-mediated allylation of trialkyl(difluoroacetyl)silane 70 in aqueous media gives homoallylic alcohol 71 exclusively (Scheme 60). Both water and THF are essential for the allylation reaction. It is worth noting that homoallylic alcohol 71 is formed exclusively under these reaction conditions. On the contrary, enol silyl ether 72 is a major product of the fluorinated acylsilanes reaction with other organometallic compounds than indium via a Brook rearrangement and defluorination. Indium-mediated allylsilylation of carbonyl compounds provides a facile route to 2-(hydroxyethyl)allylsilanes. The allene homologs are similarly prepared (Scheme 61).244,244a... 

Allylsilanes are known as interesting and useful reagents for organic synthesis.85-88 Hurd first reported the direct reaction of elemental silicon with allyl chloride in the presence of copper catalyst to give a mixture of allylchlorosilanes and polymeric materials.15 Among the volatile compounds, allyltrichlorosilane, formed by the 1 1 1 reaction of allyl chloride, hydrogen chloride and elemental silicon, was observed as the major product instead of the normally expected diallyldichlorosilane. Since diallyldichlorosilane thermally polymerizes at temperatures above 130°C, the product mixture polymerized during the reaction or upon distillation. 

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