Optically active silanes

Ohaya, Y., and T. Ashida The Structures of a-Naphthylphenylmethyl-silanes, Optically Active Silicon Compounds, and their Absolute Configurations. Acta cristallogr. [Copenhagen] 20, 461—471 (1966). 

Trimethyl(l-phenyl-2-propenyl)silane of high enantiomeric excess has also been prepared by asymmetric cross coupling, and reacts with aldehydes to give optically active products in the presence of titanium(IV) chloride. The stereoselectivity of these reactions is consistent with the antiperiplanar process previously outlined75. 

Very few optically active cyanohydrins, derived from ketones, are described in the literature. High diastcrcosclectivity is observed for the substrate-controlled addition of hydrocyanic acid to 17-oxosteroids27 and for the addition of trimethyl(2-propenyl)silane to optically active acyl cyanides28. The enantioselective hydrolysis of racemic ketone cyanohydrin esters with yeast cells of Pichia miso occurs with only moderate chemical yields20. 

Figure 4.2 Comparison of UV absorption spectra of four optically active poly silanes in THF at 30°C poly methyl-(iS )-2-methylbutylsilane (1, a of 0.59), poly -hexyl-(5 )-4-methylpentyl-silane (2, a of 0.75), poly -hexyl-(5 )-3-methylpentylsilane (3, a of 0.92), and poly -hexyl-(5)-2-methyl-butylsilane (4, a of 1.25).

In this section, we comprehensively focused on the controlled synthesis, chiroptical characterization, and manipulation of optically active poly(dialkyl-silane)s. Although many artificial polymers adopting preferential screw sense... 

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... 

A series of optically active poly alkyl(phenyl)silane derivative copolymers with different chiral molar composition 44 and 45, are shown in Chart 4.7, along with homopolymers poly(methyl(phenyl)silane) (42) and poly(n-hexyl(m-tolyl)silane) (43). 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

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... 

The axially chiral (allenylmethyl) silanes 110 were also prepared in optically active form using chiral Pd catalysts [98]. For the asymmetric synthesis of 110, a Pd/(R)-segphos system was much better in terms of enantioselectivity than the Pd/(R)-binap catalyst. Under the optimized conditions, 110m and llOt were obtained in 79% ee (57% yield) and 87% ee (63% yield), respectively (Scheme 3.56). The enantio-merically enriched (allenylmethyl) silanes 110 served for Lewis acid-promoted SE reaction with tBuCH(OMe)2 to give conjugated dienes 111 with a newly formed chiral carbon center (Scheme 3.56). During the SE reaction, the allenic axial chirality was transferred to the carbon central chirality with up to 88% transfer efficiency. 

In the case of the optically active silane, only cymantrene undergoes oxidative addition -... 

Dimanganese decacarbonyl reacts with optically active silanes, but the complex cannot be crystallized ... 

Phosphine-containing compounds are prepared by reaction of [PPh3(CO) Mn]2 with silanes and this reaction allows the synthesis of an optically active complex ... 

The selectivity of ionic hydrogenation strongly depends on the stability of the carbocation intermediate formed. It is possible to carry out selective monodeutera-tion by deuterated silanes of trialkylsusbtituted alkenes at the tertiary carbon atom.219 The transformation of the optically active (—)-30 (63% optical purity) yielded the cyclized saturated product with the same optical purity as the starting diene 220... 

The stability of five-coordinate intermediates also makes possible the ready racemization of optically active silanes by catalytic amounts of base. The base can add readily to form a five-coordinate intermediate. The latter can undergo Berry pseudorotation with complete scrambling of substituents followed by loss of the base to yield the racemized silane. 

Reduction Reactions of Some Optically Active Silanes and Representative Enones... 

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. 

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