Optically active allylsilane

Optically active allylsilanes. Optically active allysilanes have now been obtained by coupling a-(trimethylsily1)benzylmagnesium bromide with vinyl bromides, effected with PdCl2 complexed with the ferrocenylphosphine (R)-(S)-PPFA (this volume, la). The (R)-isomer is formed preferentially, usually in high optical purity. 

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. 

An iinportanl advance in this synthesis of allylsilanes involved the use of optically active palladium ferrocenyl complexes as catalysts to provide optically active allylsilanes with good enantiomeric excesses being obtained for (/. (-allylsilanes and less good enantiomeric excesses for (Z)-allylsilanes26,27. 

Claisen and Carroll rearrangements of hydroxyalkenylsilanes provide an asymmetric synthesis of allylsilanes from optically active secondary alcohols39,40. 

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. 

The enantioselective addition of an allylsilane to an aldehyde catalyzed by chiral acyloxyborane (CAB) 13 is an excellent method for obtaining optically active homoallyl alcohols.Itsuno and Kumagai reported that the synthesis of a new optically active polymer with chirality on the mainchain is possible by applying this reaction to the asymmetric polymerization of bis(allylsilane) and dialdehyde (Scheme 12.11). ... 

A tandem Beckmann rearrangement/allylsilane was used to produce the optically active cephalotaxine framework 482 (equation 208). Asymmetry could be induced as a result of stereochemical effects. 

As shown in Scheme 78, with or without zinc chloride, benzylic Grignard reagents couple with vinyl bromide to form allylbenzene derivatives in high ee (188). This method has been used to prepare optically active allylsilanes in up to 95% ee. Vinylic bromides with E configurations lead to the E allylsilanes with high ee, and Z bromides lead to Z allylsilanes with lower ee. 

The asymmetric cross-coupling was successfully applied to the synthesis of optically active allylsilanes [27,28] (Scheme 8F.9). The reaction of a-(trimethylsilyl)benzylmagnesium bro-... 

Carbamate and amide groups have been found to be stable under these coupling conditions73. In the presence of TiCLt or SnCLt, chiral a-keto amides 36 react with allyl-silane to produce, after hydrolysis, optically active tertiary alcohols 37 with extremely high optical selectivity (equation 23)74. The addition reaction appears to occur from the Si face of the carbonyl group. In a similar manner, a high degree of stereoselectivity is obtained from the reactions of A-Boc-a-amino aldehydes 38 with 2-substituted allylsilanes (equation 24)75. 

As for oxygen heterocycles, several reports involving the diastereoselective synthesis of highly substituted tetrahydrofuran derivatives have appeared. SnCLj-mediated [3 + 2] cycloaddition of allylsilane with optically active a-ketoesters affords 211 with excellent diastereoselectivity (equation 173)314-317. 

The (F,)- and (Z)-(R)-allylsilanes (3b) react with aldehydes to give optically active (F)-homoallylic alcohols 5 or 6. In reactions with (E)-3b, essentially only the erythro-diustcreoisomer 5 is formed, regardless of the aldehyde used. In reactions with (Z)-3b, both vrythro and fAmi-diastereoisomers (6) are obtained, the ratio depending on the itructiircol the aldehyde but the configuration at C, is identical in both products and... 

The synthesis of optically active vinyl oxepans 234 [55], was reported by Ito et al. starting from the enantioenriched allylsilanes 240. This annelating agent reacted with various aldehydes in the presence of TMSOTf (2.0 equivalents), leading to the oxepans 234 in good yields and excellent stereoselectivity (Scheme 13.89). The condensation of benzaldehyde with 237 is the only case in which an erosion of the tram cis stereoselectivity is observed, though this ratio still remains an impressive 50 1 (Scheme 13.87, entry 3). 

Hydroboration of silylallenes with Ipc2BH, followed by addition to aldehydes, produced. sy/z-homoallylalcohols, which were converted into optically active cyclic allylsilanes via RGM (Equation (169)).644,645... 

Asymmetric hydrosilylation of 1,3-dienes provides convenient access to optically active a-chiral allylsilanes.107 1073 1071 The combination of 7r-allylpalladium chloride dimer with axially chiral monophosphine ligand 17 realizes high catalytic activity and enantioselectivity in the reaction of cyclic 1,3-dienes with HSiCl3.108,108a The allyltrichlorosilanes obtained react with aldehydes in a syn-Se mode to give homoallyl alcohols with high diastereo-and enantioselectivity (Scheme 9). 

There thus exists a preference for anti (or antara) hydroxylation in these cyclohexenylstannanes, where electrophilic substitutions are known to proceed faitiifully with allylic rearrangement. A more likely padiway is shown in Scheme 4, which is supported by results with optically active allylsilanes, whidi require anti attack by MCPBA on the silane conformation maximizing C—Si tr nr interaction. 

A different usage of such oxazin-l,4-diones in synthesis is exemplified by the preparation of the enantiomerically pure acyloin (417). The S-proline derivative (413) was treated with an allylsilane in the presence of a Lewis acid to give a mixture of optically active tertiary homoallyl alcohols... 

Enantioselective protonation of prochiral allyl anion derivatives is a very simple and attractive route for the preparation of optically active olefins. The acid-promoted hydrolysis of allyltins or allylsilanes is an interesting alternative the enantioselectivity of which has not yet been investigated. Allyltrialkyltin, a synthetic equivalent of allyl anion, is more reactive than the corresponding allylsilane and can be isolated. The... 

Silver(I) compounds are often used as promoters for substitution reactions of aliphatic halides with carbon nucleophiles. A cyclic (8-bromo ether 29 can be reacted with allyltrimethylsilane (30) imder the influence of AgBp4, yielding a mixture of ally-lated products 31 and 32 (Sch. 7) [15]. Product 31 is formed by direct substitution of the bromine atom in ether 29 by an allyl group and isomeric ether 32 arises from the carboxonium ion which is generated by debromination and subsequent [l,2]-hydrogen shift. A synthesis of optically active 4-allylazetidinone 33 (Ft = phthalimido) has also been achieved by employing the silver-promoted substitution reaction of 4-chloro-azetidinone 34 with allylsilane 30 [16]... 

Intramolecular reaction of an allylsilane and an aldehyde was performed to prepare cyclic compounds as illustrated in Eq. (90) [233], which shows a high diastereoselec-tivity. The cyclization of optically active allylsilane proceeds stereoselectively in a manner consistent with the antiS l mechanism (Eq. 91) [234]. Development of a new reagent with a bis-allylsilane moiety effected tandem inter- and intramolecular cyclizations to give cyclic compounds as exemplified in Eqs (92) [235] and (93) [236-238]. In Eq. (92), the double addition product initially formed underwent a pinacol-type rearrangement under the influence of TiCU to give, eventually, the methyl cyclopentyl ketone. Further examples of intramolecular cyclization of allylsilanes are summarized in Table 8. 

C(3)-substituted allylsilanes (crotylsilanes) participate in chelation-controlled addition reactions with aldehydes to give syn addition products as the major stereoisomers. Generally, the ( )-crotylsilanes are highly selective in the syn sense (>9S 5). In contrast, the (Z)-crotylsilanes are much less selective (60-70 40-30 syn anti Scheme 42). Hayashi and Kumada have report a successful approach to optically active homoallyl alcohols using this strategy. They have reported that useful levels of asymmetric... 

Table 11 summarizes the important results of asymmetric induction in addition reactions of optically active allylsilanes (90) and (92). The origin of the ir-facial selectivity in these reactions can be traced to the anti selectivity that is commonly observed for 5e2 reactions of lylsilanes (Figure 14). The syn stereochemistry with respect to the double bond configuration (E or Z) is also very high even with aldehydes bearing sterically bulky substituent gro q)s, as illustrated by entry (/ )-(Z)-(92b). 

Table 11 Asymmetric Induction in Additions of Optically Active Allylsilanes (R)-( )-(90) and (R)-(Z)-(92) to...

L. Coppi, A. Mordini and M. Taddei, Tetrahedron Lett., 1987, 28,969 for the preparation of carbon-centered optically active allylsilanes, see L. Coppi, A. Ricci and M. TaMei, Tetrahedron Lett., 1987,28,965. 

Hayashi, T., Konishi, M., Kumada, M. Optically active allylsilanes. 2. High stereoselectivity in asymmetric reaction with aldehydes producing... 

Elimination of silyl groups with p-oxy groups, i.e., Peterson-type elimination, is a useful method for preparing stereodefined alkenes [101,102]. The synthetically useful allylsilanes are effectively synthesized in geometrically and enantio-merically pure forms through the Peterson-type elimination of organosilanes prepared by palladium-catalyzed bis-silylation (Eq. 57) [72, 73]. The intramolecular bis-silylation of optically active allylic alcohols in refluxing toluene af-... 

To achieve asymmetric allylation with allylsilanes, the control of absolute configuration by use of chiral auxiliaries was extensively studied in the nineteen-eighties. Recently, much effort has been directed toward the use of optically active a-chiral allylsilanes and catalytic asymmetric allylation using chiral Lewis adds and bases. 

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