Allylation allylsilane

Type I (fast homodimerization) Terminal olefins, allylsilanes" Terminal olefins, allylsilanes," 1° allylic alcohols, ethers, and esters, " allyl boronate esters, allyl halides, alkyl-substituted allenes Terminal olefms, allyl boronate esters, 1° allylic alcohols, ethers, and esters,styrenes (no large ortho substit.), " " allyl allylsilanes, allyl sulfides, allyl phosphonates, " allyl phosphine oxides, protected allylamines ... 

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

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

CONJUGATE ALLYLATION OF a. -UNSATURATED KETONES WITH ALLYLSILANES 4-PHENYL-6-HEPTEH-2-0NE (6-Hepten-2-one, 4-pheny1-)... 

CONJUGATE ALLYLATION OF a,g-ENONES WITH ALLYLSILANES PROMOTED BY TITANIUM TETRACHLORIDE Conditions... 

Allylation of perfluoroalkyl halides with allylsilanes is catalyzed by iron or ruthenium carbonyl complexes [77S] (equation 119) Alkenyl-, allyl-, and alkynyl-stannanes react with perfluoroalkyl iodides 111 the presence ot a palladium complex to give alkenes and alkynes bearing perfluoroalkyl groups [139] (equation 120)... 

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. 

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. 

A large number of publications appeared on these aspects5, but most of these studies did not address stereochemical questions. In most cases, a given synthetic problem can be better solved by other allylmetals. Grignard reagents have some importance as intermediates for the preparation of allylboronates (Section D.1.3.3.3.3.2.1.), allylsilanes (Section D.1.3.3.3.5.2.L), allyl-stannanes (Section D. 1.3.3.3.6.2.1.1.), or allyltitanium derivatives (Section D.I.3.3.3.8.2.). 

Symmetric allylsilanes and unsymmetric allylsilanes, in which the silyl substituent is at the less substituted end of the allyl fragment, are available from allyl halides and trimethylsilylmetal reagents13. 2-Chloro-l-cyclohexenes react with inversion of configuration and a 1,3-shift, with better results in the presence of coppcr(l) iodide14. 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

Allylsilanes in which the silyl group is at the more substituted end of the allyl system have been prepared by a reaction sequence involving the conjugate addition of silylcuprates to a, jS-unsat-urated esters followed by reduction and dehydration via selenoxide elimination38. 

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. 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

Efficient asymmetric transfer is also observed for 1,2-benzenediol induced reactions of a-sub-stituted (Z)- and ( ,)-allyl(trialkoxy)silanes prepared by hydrosilation of conjugated dienes, although in this case the electrophile attacks the C-C double bond of the allylsilane syn with... 

Lewis acid induced alkylation of 4-alkoxy-3,5-dialkyl-2-oxazolidinones with allylsilanes gives the 4-allyl derivatives with complete irons stereoselectivity114,115. Cleavage of the oxazolidi-none ring with aqueous sodium hydroxide in ethanol leads to vicinal twP -aminoalkanols. 

Some other examples of tram allylations with allylsilanes are the bicyclic lactam 21123 and the imidazolidinone derivative 23124. 

Sakurai reactions proceed regiospecifically with a large variety of electrophiles due to the so-called /1-effect5-9. However, allylsilanes are also known as masked allyl carbanions, which may be activated by the presence of fluoride ion10-12. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

In the synthesis of spirocyclic systems via an intramolecular Sakurai reaction, allylsilanes with an allyl moiety attached to the 3-position of the 2-cyclohexenone are required as starting materials. 

Unlike many other metal-allyl systems, allylsilanes are rcgio-stable at inormal temperatures, with 1,3-sigmatropic shifts occurring at a significant Irate only at temperatures in excess of 300°C. 5-Trimethylsilylcyclo-Ipentadiene provides an exception to this generalization, but it can still be lhandled quite readily. 

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

Allyl acetates can similarly be transformed into allylsilanes by treatment with bis(silyl)cuprates (5). 

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... 

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... 

Substituted (5R,6A,)-6-(dimethyl(phenyl)silyl)-2-phenyldihydropyrazolo[l,2- ][l,2,4]triazole-l,3(2//,5//)-dione 716, synthesized via the [3+2] annulation of a-substituted allylic silanes 715 with PTAD, were oxidized to the corresponding hydroxy substituted urazoles 717. This work shows that allylsilanes with a single substituent at the allylic carbon undergo exclusive stereoselective [3+2] annulation (Scheme 114) <2007TL6671>. 

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. 

In the second approach55 an allylsilane was employed as carbon nucleophile in the side chain. Allylsilanes have been frequently used as masked allyl carbanions, usually in reactions with a keto function57. Palladium-catalyzed reaction of allylsilane 57 with LiCl under similar conditions as used for the other intramolecular 1,4-oxidations afforded 58 (equation 22). Interestingly, the carbochlorination over the diene was highly 1,4-syn... 

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