Silanes chiral

Additionally, copolymers of 30-37 containing 20% enantiopure chiral silane units (see Chart 4.6) are optically active helical polymers which obey the sergeants-and-soldiers principle, as shown in Figure 4.18.29g Interestingly, from the observation of the CD sign in the phenyl region, the arrangement of phenyl... 

Reactions of chiral silanes with chiral aldehydes exhibit matching and mismatching characteristics (Eqs. 9.56 and 9.57) [48]. The additions proceed through an acyclic transition state, which favors syn adducts. The matched (M)/(R) pairing of Eq. 9.56 proceeds by way of a favorable Felkin-Anh arrangement to afford the syn,syn homopropargylic alcohol product. However, if the silanes possess an a-hydrogen, a vinylic chloride intermediate is formed, as shown in Scheme 9.13. Subsequent treat-... 

Inspired by the previous results, Leighton et al. reported the enantioselective [3 + 2] acylhydrazone-enol ether cycloaddition reaction by employing the same pseudoephedrine-based chiral silane. The pyrazohdine product was obtained in 61% yield with 6 1 dr and 77% ee in 24 h. The use of tert-butyl vinyl ether led to an improvement in both diastereoselectivity and enantioselectivity as shown in Scheme 34 [108]. 

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

Panek, J.S. Liu, P. (2000) Total synthesis of the actin-depoiymerizing agait (-)-mycalolide A application of chiral silane-sased bond construction methodology. J. Am. Chem. Soc., 122, 11090-7. 

A simple chiral silane [(13), 2 1 mixture of diastereoisomers] mediates the enan-tioselective reaction of electron-rich arenes and heteroarenes with the benzoylhydra-zone of isopropyl glyoxylate under optimized conditions, leading to a useful synthesis... 

A simple chiral silane Lewis acid has been used in the highly diastereo- and (g enantio-selective 3 + 2-cycloaddition of acylhydrazones with enol ethers.73 The 1,3-dipolar cycloaddition of isocyanides (CNCH2EWG) with electron-deficient alkynes ... 

Once again the mono-alkoxy product le was used as our starting point. This mono-alkoxy product was treated with several alcohols using the Mn(CO)sBr as catalyst in CH2CI2 at room temperature in air. The results are shown in Table 8 and Table 9. The success of this reaction was very encouraging to us because we were able to synthesize unsymmetrical bis-alkoxysilanes containing modifiable functional groups. This will allow us to explore stereospecific intramolecular reactions with chiral silanes. 

We have tried to synthesize chiral silanes using our method and the prochiral silanes methylphenylsilane and methylcyclohexylsilane with (+)-ethyl lactate and /.9-menthol. Both of these silanes gave racemic mono-alkoxy products with the alcohols used (Table 12). The... 

Once a chiral silane is synthesized, intramolecular reaction such as the Heck reaction and the radical cyclization reaction with transfer of chirality from the silicon center can be pursued. Other reactions of interest include the stereoselective hydrosilylation of aldehydes and ketones. Our method could be used to tether chiral silanes to solid support for uses in solid support... 

During the process of hydrosilation two prochiral sp" hybridized atoms are converted to potentially chiral see Chiral) sp atoms. This opens up the possibility for asymmetric catalysis. The use of prochiral silanes of the type RR SiH2 also allows for the generation of chiral silanes. 

Catalytic hydrosilylation of alkenes performed in the presence of a chiral catalyst results in the formation of chiral silanes. Initially platinium catalysts of the type L PtCl2, L = (/ )-benzyl-(methyl)phenylphosphine (BMPP) or (/ )-methyl(phenyl)propylphosphine and 1,1-disubstituted prostereogenic alkenes, such as a-methylstyrene, 2,3-dimethyl-l-butene and 2-methyl-l-butene, were used however, the stereoselectivity was low4,5. A slightly higher stereoselectivity is obtained when dichlorobis[(/ )-benzyl(methyl)phenylphosphine]nickel [Ni(BMPP)2Cl2] is used as the catalyst. Note that two chiral silanes are formed in this reaction, both of which are products of anti-Markovnikov addition. The major product is the expected dichlorosilane 3, while the byproduct is an anomalous chlorosilane 4 both products were separated by fractional distillation and the major product methylated to give the trimethylsilanes 56 7. 

Silylation of a carbanion obtained by enantiotopically differentiating deprotonation of the precursor carbamate in the presence of sparteine leads to the chiral silane 10 with >95 % ee by analogy with the reaction with other electrophiles29 (for analogous derivatives see Section D.8.). 

As part of our studies on a-metalated organosilanes, we use (aminomethyl)(lithiomethyl)silanes of the general type A [1]. Systems of types B and C containing defined stereochemical information could be synthesized starting from either chiral amines or from 5i-chiral silanes. A new structural motif (D) arises if the stereogenic center is adjacent to silicon (preferably as part of a rigid system). The goal of these studies was the synthesis of the enantiomerically pure, lithiated 2-silyl-substituted pyrrolidine E. 

Homopropargyl and allenyl alcohols. Chiral silanes are formed from propargylic mesylates. Subsequent reaction with aldehydes proceeds in a manner depending on the substituent on the sp-carbon of the mesylates. Starting from nonracemic mesylates the products are obtained with excellent ee. 

Chirality may also be introduced onto the soHd phase by immobihzing a chiral component of the final structure first, and then using it as a template for further C-C bond formation. Some alkylation reactions that proceed via defined cyclic transition states have been applied. One interesting example is the traceless attachment of a chiral silane onto a polymeric support (Scheme 179). The latter can serve as a template for Lewis acid induced diastereoselective allylations [441]. 

Cross-coupling of chiral silanes with aryltriflates under Pd(PPh3)4 catalysis in the presence of fluoride ions has been described. A good chirality transfer has been observed [1656]. 

In organosilicon chemistry the molecular structure and absolute configuration of only a few chiral silanes have been determined by the Bijvoet method. These... 

Esters of ( + )-menthol with aliphatic sulfonic acids have been used for the synthesis of chiral silanes (Section D.9.) the synthesis of the important synthetic intermediate [(—)-menthyl] toluenesulfinate is described in Section D.5. (7i)-5-[(3f )-Menthyloxy]-2(5//)-furanone (9) was developed as achiral dienophile (Section D. 1.6.1.1.1.1.2.), which is also commercially available. The synthesis starts with cheap 2-furancarbaldehyde which is photooxygenated to a hydroxyfura-none9. Subsequent treatment with (-)-menthol leads to a mixture of diastereomeric menth-yloxyfuranones from which the desired (J )-isomer crystallizes8. 

Use of a chiral complex in hydrosilylation of a ketone can provide a route to chiral silanes. This method with RhCl(DIOP) as catalyst was investigated by Corriu et al. 26,31), A prochiral silane such as (a-Np)-... 

The selectivity shown by allylsilanes towards electrophiles makes asymmetric synthesis via chiral silanes an attractive prospect. A series of optically active allylsilanes has been prepared... 

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