Reaction allylsilane

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. 

Enediynes, silane-initiated cascade cyclization, 11, 400 Ene reactions, allylsilanes, 9, 313 Energetics, key elementary reactions, 1, 617 Enes, niobium complexes, 5, 87 Ene-yne metathesis... 

Diels-Alder reaction Allylsilanes and propargylsilanes condense with A-aryl-aldimines in the presence of Gads to provide 2,4-disubstituted tetrahydroquinolines and quinolines, respectively. 

Our studies have focused on the rigorous determination of the transition structure in Type 2 reactions (allylsilanes and allylstannanes) and the clarification of those factors which control the stereochemical outcome. To unambiguously define the spatial relationship between the allylmetal and the aldehyde we have made extensive use of model 1, Scheme 2 which permits a direct correla-... 

Nncleophilic Addition and Substitution Reactions. Allylsilane (3) has been shown to undergo conjugate addition to enones in the presence of tetra- -butylammonium fluoride (eq 6). The reaction demonstrates high regioselectivity, as no products arising from 1,2-addition to the enone or attack at the /-position of the allylsilane were isolated. Stereoselective C-glycosidation can be effected by reaction of allylsilane (3) with D-mannopyranoside derivatives in the presence of boron trifluoride etherate (eq 7). The a-C-glycoside arises from axial addition to the pyranoside oxonium ion. 

Conventional synthetic schemes to produce 1,6-disubstituted products, e.g. reaction of a - with d -synthons, are largely unsuccessful. An exception is the following reaction, which provides a useful alternative when Michael type additions fail, e. g., at angular or other tertiary carbon atoms. In such cases the addition of allylsilanes catalyzed by titanium tetrachloride, the Sakurai reaction, is most appropriate (A. Hosomi, 1977). Isomerization of the double bond with bis(benzonitrile-N)dichloropalladium gives the y-double bond in excellent yield. Subsequent ozonolysis provides a pathway to 1,4-dicarbonyl compounds. Thus 1,6-, 1,5- and 1,4-difunctional compounds are accessible by this reaction. 

The reaction of phenylmethylenecyclopropane with trimethylsilyl cyanide catalyzed by PdCl2 affords the allylsilane 81 in 71% yield[63]. 

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. 

Oshima and coworkers reported the preparation of allylsilanes such as 148 by ringopening reactions of three-membered rings such as aziridines 147 with silylalu-minium reagents as shown in Scheme 2.38 [58]. 

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. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

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. 

T.3.3.3.5.2. Reactions of Allylsilanes with Carbonyl Compounds Introduction... 

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. 

Ring formation by intramolecular reaction between an allylsilane and an aldehyde or ketone is well precedented54, 55. 

The diastereoselectivity of these reactions was initially explained in terms of acyclic, an-tiperiplanar transition states23, although a synclinal process has since been proposed on the basis of the stereoselectivity of Lewis acid induced intramolecular allylsilane- aldehyde cycliza-tions56. 

The reaction between a-alkoxyaldehydes and allylsilanes is highly stereoselective in favor of chelation-controlled products if tin(IV) chloride is used as the Lewis acid, whereas boron trifluoride gives modest stereoselectivity in favor of the nonchelation-controlled product58. 

Chelation control has been invoked to explain the stereoselectivity of the reaction between 2-methoxycyclohexanone and trimethyl(2-propenyl)silane promoted by titanium(IV) chloride59, and has been observed for addition of allylsilanes to a-amidoaldehydes60, although the stereoselectivity is very dependent upon the amount of Lewis acid used and the structure of the substrate, particularly when the Lewis acid can bind to more than one site61. 

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. 

Chelation control of the intramolecular reaction between an allylsilane and an aldehyde or ketone has been carefully investigated. Excellent stereoselectivity was found for cyclization of B-oxo esters using titanium(IV) chloride as the Lewis acid, less good selectivity for cyclization of /l-diketones70. 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

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... 

Allyltrialkoxy- or -tris(dialkylamino)titanium reagents are not capable of chelation-controlled addition reactions with oxy- or amino-substituted carbonyl compounds due to their low Lewis acidity87. To attain chelation control, the application of allylsilanes (Section 1.3.3.3.5.2.2.) and allylstannanes (Section I.3.3.3.6.I.3.2.) in the presence of bidentate Lewis acids like titanium(IV) chloride, tin(lV) chloride or magnesium bromide are the better options. 

Very recently a number of intermolecular a-amidoalkylation reactions related to the formation of C-C bonds with simple diastereoselection have been reported only activated 7t-nucle-ophiles, such as allylsilanes, enamines, enol ethers, etc. are used83 - 88. 

This section describes Michael-analogous processes in which, mostly under electrophilic conditions, ally - or alkynylsilanes undergo addition to enones or dienones (Sakurai reactions). The intramolecular addition of allylsilanes is an extremely useful reaction especially for the construction of carbocyclic ring systems, which occurs in a diastereoselective manner, in many cases with complete asymmetric induction. 

Almost 15 years ago Sakurai and Hosomi, in pioneering work, showed that intermolecular addition of an allylsilane to a,j6-unsaturated ketones in the presence of titanium(IV) chloride as the Lewis acid gave the desired 1,4-addition products1 4. In the case of 4,4a,5,6,7,8-hexahy-dro-2(3//)-naphthalenone, reaction was shown to proceed by 1,4-addition with exclusive production of the ris-fused product in high chemical yield. 

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. 

In the Sakurai reaction, the product-determining step should be the nucleophilic addition of the allylsilane to the Lewis acid coordinated enone28. 

The starting materials for annulative cyclization are cycloalkenones that contain the allylsilane side chain in the 4-position. Such starting materials can easily be prepared from vinylogous esters40. Furthermore, reactions of 3-alkoxy-2-cyclohexenones with functionalized iodides in the presence of lithium diisopropylamide provides an excellent route to such precursors41 34 35. 

Apparently the Lewis acid induced and the fluoride-induced reactions of allylsilanes with enones follow different pathways. The perpendicular approach of the nucleophilic allylsilane... 

The 1,6-intramolecular addition of unsymmetrical allylsilanes to conjugated dienones proceeded to give a trans relation between the angular substituents and the vinyl groups (complete 1,3-asymmetric induction). Ethylaluminum dichloride has been most successfully used in all reactions described35-53. 

Tricyclic compounds can be obtained directly by annulation on to cyclic allylsilanes, using either ethylaluminum dichloride or titanium(IV) chloride as Lewis acids57. The stereochemical outcome of this particular cyclization is controlled by the relative configuration of the cyclic allylsilane. The reaction follows the usual anti" SE2 process for reactions of allylsilanes with electrophiles. Thus, the reaction was stereospecific, which makes it very useful for stereocon-trolled syntheses of complex ring systems57. 

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