Lewis acid-catalyzed allylsilane addition

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. 

In Lewis acid catalyzed carbonyl additions of allylsilanes and -stannanes, syn diastereoselectiv-ity predominates, irrespective of the double-bond configuration, indicating that open-chain transition states are involved. 

Cycloheptane annelation.1 A new route to cycloheptanes is based on a Lewis acid-catalyzed, intramolecular addition of an allylsilane group to a 3-vinylcycloal-kenone (equation I). This annelation has been applied to a synthesis of the ses-... 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

Significantly better results in addition of non-stabilized nucleophiles have come from hydrogenolysis reactions using formate as a hydride donor as shown in Scheme 8E.46. The racemic cyclic acetate and prochiral linear carbonates were reduced in good enantioselectivities by monophosphine ligands (/ )-MOP (16) and (Zf)-MOP-phen (17), respectively [195]. The chirality of the allylsilane can be efficiently transferred to the carbinol center of the homoallylic alcohol by the subsequent Lewis acid catalyzed carbonyl addition reaction 1196], The analogous... 

Lewis acid-catalyzed stereoselective addition of crotylsilanes to chiral 74 has been studied in detail111,112. The presence of the chiral auxiliary at C2 (e.g. p-tolylsulfinyl or menthoxy carbonyl group) induces the diastereofacially selective addition of cyclopentenones with crotylsilanes. Thus, ( )-crotylsilane favors the erythro product, whilst (Z)-isomer favors the threo product. High enantioselectivity is observed in both reactions (equation 48). In a similar manner, conjugated addition of allylsilane to 75 proceeds with high efficiency (equation 49)113. Interestingly, the yield and enantiomeric excess of the product is dependent on the amount of TiCL used and the best selectivity... 

The Lewis acid-catalyzed reactions of allylsilanes with simple alkenes and alkynes introduce both allyl and silyl groups into the unsaturated bonds (Equations (53) and (54)).209 210b These allylsilylations are 7-regiospecific with respect to allylsilanes. The /raor-addition of allylsilanes occurs in the reaction with alkynes. There are two possible mechanisms for the Lewis acid-promoted allylsilylations, that is, trimethylsilyl cation- and Lewis acid-mediated pathways. 

In this synthesis of 1,5-dicaibonyl compounds, 3-butenyl halide is behaving as a madced 3-oxobutyl reagent, and can be used as an equivalent of metiiyl vinyl ketone. These reactirais offer new anellation methods. Also 1,4-addition of the allyl group to enones, followed by oxidatitm. offers a conveiuent synthetic method for 1,5-diketone preparation. Lewis acid promoted Nfichael addition of allylsilane (48) to a,p-unsaturated ketones, followed by the palladium-catalyzed oxidation, affods 1,5-diketones (Scheme 17). 3... 

A variety of aldehydes can function as heteroallenophiles in this [3 + 2] annelation (Figure 13 and Table 9). Reactions of the C(3)-substituted allenylsilanes (66) gave predominantly the ds-substituted dihydrofurans (71). The cis stereochemistry was anticipated, based on the well-documented stereochemical course of Lewis acid catalyzed additions of 3-substituted allylsilanes to aldehydes. ... 

The Lewis acid-catalyzed aUylsilylation of alkynes and alkenes with allylsilanes was introduced by Jung [511] and Yamamoto [512]. Jung and co-workers found fhat AICI3 catalyzes the aUylsilylation of simple alkenes and phenyl-substituted alkynes wifh allylsilanes (Scheme 10.191) [513]. The allylation occurs regiospecifically at the y-position of the allylsilanes. When cycloalkenes are used as substrates, trans adducts are obtained exclusively. Jung et al. initially reported [513 a] that the aUylsilylation of alkynes proceeded in a cis-addition mode they later noted, however, fhat fhe stereochemical assignment of fhe aUylsilylation products was incorrect and the products were not cis but trans adducts [511]. It has been proposed that the mechanism of fhe AlCls-catalyzed aUylsilylation involves addition of a silyl cation or its equivalent to fhe carbon-carbon multiple bond and subsequent aUylation of the resulting -silylcarbenium ion [511]. 

The second major modification was developed by Mukaiyama and coworkers (4) and Hosomi and Sakurai (5). These workers found that weakly nucleophilic silyl enol ethers and allylsilanes add to a,/ -unsaturated ketones that have been precomplexed with a Lewis acid. This process formally mimics the protic-acid-catalyzed Michael addition but allows for regiocontrol over enol generation. 

Protic-acid-catalyzed Michael additions (59) are subject to most of the limitations of base-catalyzed Michael additions (regioselectivity and stereoselectivity of enol generation, polyaddition, etc.), and hence, the stereochemistry has been little studied (60). At low temperatures silyl and stannyl enol ethers,+ ketene acetals, and allyl species are unreactive to all but the most reactive activated olefins. However, it was discovered by Mukaiyama and co-workers that enol ethers and ketene acetals react with a,/f-unsaturated carbonyl compounds in the presence of certain Lewis acids (4,61,62). Sakurai, Hosomi, and co-workers found that allylsilanes behave similarly (5,63,64). 

Allylation adducts of ( )-Y-(dimethylphenylsilyl)allylboronate 47 selectively undergo addition reactions with aldehydes. Lewis acid catalyzed [3 + 2] cycloaddition reaction of these allylsilanes with various aldehydes, e.g., 55, provides tetrahydrofuran adducts with high diastereoselectivity. ... 

Hydrozirconation and Zirconium-catalyzed Carboalumina-tion. Hydrozirconation of 3-trimethylsilyl-l-propyne using Schwartz s reagent gives the alfcenylzirconocene that can be further reacted to provide 1,3-dienes after Lewis-acid-catalyzed addition to aldehydes (eq 35), vinyl sulfones from reaction with sulfonyl chlorides (eq 36), and allylsilanes upon transmetalla-tion with CuBr followed by 1,4-addition (eq 37)." ... 

The anions, generated in situ by desilylation of silylacetylenes, allylsilanes, propargylsilanes, a-silyloxetanones, bis(trimethylsilylmethyl) sulfides, and other silane derivatives,can undergo nucleophilic addition to ketones and aldehydes (eq 11). Al-(C,C-bis(trimethylsilyl)methyl) amido derivatives can add to aldehydes followed by Peterson alkenation to form acyl enamines. Treatment of 2-trimethylsilyl-l,3-dithianes can generate dithianyl anions, which are capable of carbocyclization via direct addition to carbonyl or Michael addition (eq 12). The fluoride-catalyzed Michael additions are more general than Lewis acid-catalyzed reactions and proceed well even for those compounds with enolizable protons and/or severe steric hindrance (eq 13). ... 

Scheme 5.25 Lewis acid-catalyzed asymmetric aldehyde addition and cydization of 08-stannylmethyl)allylsilanes.

The Lewis acid promoted addition of allylsilane (48) to nitroalkene (47) gave the unsaturated ketone (49) after hydrolysis of the nitro group. The palladium-catalyzed oxidation affords the 1,4-diketone (50 Scheme 13). °... 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

Difficulties have been fonnd in the intermolecular coupling reactions of allylsilanes with epoxides other than the simplest ethylene oxide . For example, reactions of allylsilane 99 with epichlorohydrins in the presence of TiCLj or EtAlCl2 give chlorohydrins in moderate to good yields. However, treatment of epichlorohydrin with 99 fnrnishes the expected allylated chlorohydrin product 100 (equation 78). In the intramolecnlar addition of the allylsilane moiety to 2,3-epoxyether moiety of 101, the ratio of 6- and 7-membered ring products is affected by the nature of the Lewis acid. When BF3 OEta is nsed, a mixture of 102 and 103 is obtained (equation 79). Interestingly, 102 is the exclusive product in the TiCU-catalyzed reaction . 

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