Allenylsilane

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. 

Thermal conditions were effective in the stereoselective oxa-ene cycloisomerization of allenylsilanes, furnishing both substituted cyclopentanes and, as in Equation (88), substituted cyclohexanes. 

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... 

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. 

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

Scheme 4.37 Palladium-catalyzed bis-silylation of propargyl alcohol 139 to form an allenylsilane.

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. 

Scheme 4.68 Palladium-catalyzed asymmetric hydrosilylation of enynes 261 forming allenylsilanes 267.

Comparable treatment of the allenic stannane resulted in a similar outcome, producing a mixture of regioisomers from the in situ silylation and near exclusive formation of the allenylsilane from the sequential addition (Eq. 9.8). 

It is thought that a large fraction of the allenylsilane product is formed from the solvent-separated ion pair B. The expectedly high reactivity of this species would minimize steric effects associated with the silylation, thus explaining the lack of selectivity observed for Me3Si versus iPrMe2Si in the competition experiment. 

The preparation of allenylsilanes by silylation of allenic and propargylic lithium reagents has been noted previously in connection with structural and reactivity studies on these intermediates (Eqs. 9.7and 9.8). Some additional examples are depicted in Eqs. 9.37and 9.38 [44, 45]. 

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. 

A little-explored route to allenylsilanes utilizes tosylhydrazone derivatives of tri-methylsilyl alkynyl ketones [52]. Treatment with NaBH3CN in acidic medium leads to transient propargylic diazines, which rearrange with loss of nitrogen (Eq. 9.44). 

A number of additional methods involve the addition of alkynylsilanes to electrophiles with concomitant 1,3-isomerization to afford allenylsilanes geminally substituted with the electrophile moiety. The first of these methods employed a trimethyl-silyl-substituted propargylic silane as the alkynylsilane and various acetals as the electrophile precursors (Table 9.29) [53], The allenylsilanes are formed without contamination by alkynyl isomers. 

Table 9.29 Synthesis of allenylsilanes from propargylic 1,3-b/s-silanes.

It is also possible to employ trimethylsilyl-substituted propargylic trichlorosilanes in electrophilic substitution reactions leading to allenylsilanes (Eq. 9.45) [50]. The trichlorosilanes can be prepared by SN2 or SN2 displacement of allenic or propargylic bromides by a trichlorosilyl copper reagent. The overall process, starting from an enantioenriched propargylic bromide of unknown enantiopurity, afforded a racemic allenylsilane (Eq. 9.46)... 

A novel intramolecular bis-silylation of propargylic disilanyl ethers followed by a Peterson-type syn elimination provides access to a variety of allenylsilanes (Table 9.36) [57]. The elimination is initiated by treatment of the presumed four-membered siloxane intermediate with BuLi. This intermediate could not be isolated, but spectral data were in accord with the assigned structure. The cycloaddition and elimination steps were shown to take place stereospecifically (Eq. 9.48). 

Additions of electrophiles to allenylsilanes have been shown to proceed by an anti SE2 process (Eqs. 9.52 and 9.53) [59]. Reactions of enantiopure allenylsilanes yield propargylic adducts with <1% racemization. 

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. 

Allenylsilanes react with acetals to afford homopropargylic ethers (Table 9.37) [61]. These reactions are promoted by silyl and carbocation species. A variation of this conversion involves in situ formation of the acetal from an aldehyde and Me3SiOMe (Eq. 9.55). The success of this method indicates that conversion of the aldehyde to the acetal and its subsequent reaction with the silane must be faster than direct reaction of the aldehyde with the silane. 

Table 9.37 Addition of benzaldehyde dimethyl acetal to allenylsilanes.

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

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