Chiral allenylsilanes

A new type of asymmetric hydrosilylation which produces axially chiral allenylsilanes has been reported by use of a palladium catalyst coordinated with the bisPPFOMe ligand 51b.64 The hydrosilylation of l-buten-3-ynes substituted with bulky groups such as tert-butyl at the acetylene terminus took place in a 1,4-fashion to give allenyl(trichloro)-silanes with high selectivity. The highest enantioselectivity (90% ee) was observed in the reaction of 5,5-dimethyl-T hexen-3-yne with trichlorosilane catalyzed by the bisPPFOMe-palladium complex (Scheme 13). 

The catalytic reaction giving allenes by the addition of a hydrosilane twice to 1,3-diynes65 has been applied to the asymmetric synthesis of axially chiral allenylsilanes although the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (23, 43 )-PPM is the best catalyst for the addition of phenyldimethyl-silane to diyne 52 giving allene 53 with 22% ee (Scheme 14).66 663... 

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

Scheme 4.6 Enantioselective total synthesis of the 5,11-methano-morphanthridine alkaloid (-)-coccinine via chiral allenylsilane 29.

A cuprate prepared in situ from tBuPh2SiLi and Cul has been found to react with alkynyl epoxides to afford allenylsilanes (Eq. 9.43) [50]. Enantioenriched alkynyl epoxides, which are readily prepared in high yield through Sharpless asymmetric epoxidation [51], afford chiral allenylsilanes with anti stereoselectivity. 

Epoxidations of chiral allenylsilanes are also highly stereoselective, especially if the silyl group is spatially demanding (Eq. 9.54) [60]. A bis-epoxide intermediate is formed which rearranges to an a,/8-unsaturated a -hydroxy ketone. Such products are of interest as branched carbohydrate analogues. 

Scheme 9.13 Proposed pathway for additions of chiral allenylsilanes to aldehydes.

Cycloadditions. The scope of the Evans asymmetric aziridination is broadened by the ready availability of nitrene precursor 4-02NCjH S02N=IPh. The oxazolidinone-Af-acetic acid derived from (+)-cw-2-amino-l,2-diphenylethanol participates in p-lactam formation with imines. The predominant products (>99 1) have the (3/f,45)-configuration. The intramolecular [2+2]photocycloaddition involving a chiral allenylsilane moiety is an excellent method for accessing optically active methylenecyclobutane derivatives. ... 

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 2001, a palladium-catalyzed asymmetric hydrosilylation of 4-substituted-but-l-en-3-ynes (146) was reported by Hayashi and co-workers [115]. It was found that a monodentate bulky chiral phosphine, (S)-(R)-bisPPFOMe, was effective for the asymmetric synthesis of the axially chiral allenes 147 and up to 90% ee was achieved (Scheme 3.75). The bulky substituent at the 4-position in 146 is essential for the selective formation of the allene 147 the reaction of nC6H13C=CCH=CH2 gave a complex mixture of hydrosilylation products which consisted of <20% of the allenylsilane. 

Jin and Weinreb reported the enantioselective total synthesis of 5,11-methano-morphanthridine Amaryllidaceae alkaloids via ethynylation of a chiral aldehyde followed by allenylsilane cyclization (Scheme 4.6) [10]. Addition of ethynylmagnesium bromide to 27 produced a 2 1 mixture of (S)- and (R)-propargyl alcohols 28. Both of these isomers were separately converted into the desired same acetate 28 by acetylation or Mitsunobu inversion reaction. After the reaction of 28 with a silyl cuprate, the resulting allene 29 was then converted into (-)-coccinine 31 via an allenylsilane cyclization. 

Tillack and co-workers developed a rhodium-catalyzed asymmetric hydrosilylation of butadiyne 258 to afford allenylsilane 260 (Scheme 4.67) [106]. Among more than 30 chiral phosphine ligands investigated, the highest enantioselectivity was observed when the catalyst was prepared from [Rh(COD)Cl]2 (1 mol%) and (S,S)-PPM 259 (2 mol%) to afford the optically active allene 260 with 27% ee. Other metals such as Ir, Pd, Pt or Ni were less effective for example, a nickel catalyst prepared from NiCl2 and (R,R)-DIOP 251 or (S,S)-PPM 259 gave the allene 260 with 7-11% ee. 

Irradiation of the enantiomerically enriched allenenone 42 afforded alkylididecy-clobutane 43 with high levels of chirality transfer. The silyl moiety of optically active allenylsilanes 44 and 47 functioned as a removable auxiliary to control the stereochemistry. Thus, the silyl-substituted photoadducts 45 and 48 underwent protode-silylation on treatment with TBAF to give the unsubstituted exo-mcth ylenccyclo-butanes 46 and 49, respectively [46]. 

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

Reactions of chiral allenes proceed with a preference for the formation of the syn diastereomer. The stereochemical outcome of these reactions can be rationalized by invoking an open transition state model for the addition reactions (Figure 12), which depicts an antiperiplanar orientation of the chiral allenylsi-lane to the aldehyde carbonyl. In this model, steric repulsion between the allenyl methyl and the aldehyde substituent is most likely responsible for the destabilization of transition state (B), which leads to the anti (minor) stereoisomer. This destabilizing interaction is minimized in transition state (A). Table 5 illustrates representative examples and summarizes the scope of the regiocontrolled synthesis of homopropargylic alcohols using allenylsilanes. 

Optically active allenyltrichlorosilanes can be prepared by the Pd-catalyzed asymmetric hydrosilylation of 1,3-enynes with I ISiCb, (Scheme 10.138) [389]. In this reaction use of bisPPFOMe (125) as the chiral ligand achieves high enantioselectivity. The allenylsilanes smoothly react with PhCHO in DMF to give optically active homopropargyl alcohols, and the axial chirality of allenylsilanes is completely transferred to the central chirality of the products by a syn-SE process. 

Suginome and Ito have developed a reliable method for the synthesis of highly enantioenriched allyl- and allenylsilanes. The synthetic process involves 1,3-chirality transfer from homochiral allyl and propargyl alcohols through Pd-catalyzed intramolecular bis-silylation and subsequent Peterson-type elimination (Scheme 10.142) [395]. This method provides an efficient route to enantioenriched allylsilanes bearing a hydroxyalkyl group, which are very valuable as synthetic intermediates for diastereo- and enantioselective synthesis of heterocycies and carbocycles [396]. Polymer-supported highly enantioenriched allylsilanes have been prepared from enantioenriched allyl alcohols and a polymer-supported disilanyl chloride [397]. 

The [3-1-2] cycloaddition of aldehydes with a-substituted allenylsilanes is a powerful means of access to dihydrofurans [450 b]. Quite recently Evans et al. reported the first example of the catalytic asymmetric version using a chiral scandium triflate complex (Scheme 10.172) [480]. 

The silylboronic ester 1 reacts with propargylic carbonate 33 in the presence of a rhodium catalyst to give allenylsilane 34 via Sn2 reaction (eq 32). A high level of chirality transfer is attained when the enantioenriched substrate is used. Allylic chlorides also undergo Sn2 reaction with 1 using a copper catalyst, giving branched allylic silane (eq 33). ... 

Allenylsilanes also undergo formal [3 + 2] cycloaddition reactions with enones, aldehydes, iminium species generated in situy and acid chlorides to give a variety of five-membered products (Scheme 3-111). The reaction is considered to proceed through stepwise mechanisms. The [3 + 2] cycloaddition reaction can be performed in an enatioselective manner by using the chiral scandium catalyst. 

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