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Si-centered chirality

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... [Pg.411]

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... [Pg.411]

Chiral carbon atoms are common, but they are not the only possible centers of chirality. Other possible chiral tetravalent atoms are Si, Ge, Sn, N, S, and P, while potential trivalent chiral atoms, in which non-bonding electrons occupy the position of the fourth ligand, are N, P, As, Sb, S, Se, and Te. Furthermore, a center of chirality does not even have to be an atom, as shown in the structure represented in Figure 2-70b, where the center of chirality is at the center of the achiral skeleton of adamantane. [Pg.78]

A molecule is prochiral if can be converted from achiral to chiral in a single chemical step. A prochiral sp2-hybridized atom has two faces, described as either Re or Si. An sp3-hybridized atom is a prochirality center if, by changing one of its attached atoms, a chirality center results. The atom whose replacement leads to an R chirality center is pro-R, and the atom whose replacement leads to an S chirality center is pro-S. [Pg.322]

Assign R or S stereochemistry to the two chirality centers in isocitrate, and tell whether OH and H add to the Si face or the Re face of the double bond. [Pg.727]

The 29Si resonance is therefore a single narrow line. However for dialkylpolysilanes with two different alkyl groups on each silicon, (RR Si)n, each silicon atom is a chiral center and the resonance for a particular silicon will depend upon the relative stereochemistry of other nearby silicon atoms. For such polymers, a rather symmetrical cluster of peaks is observed (Figure 5). These results are consistent with atactic structures, having a statistical (Bernoullian) distribution of relative configurations.(32,33)... [Pg.14]

Thus, (5)-chiral auxiliary gives rise to combination of the trigonal centers of enolate and nitroalkene with Si/Si topicity.79... [Pg.91]

Two compounds are diastereomers when they contain more than one chiral center. If the number of dissymmetric centers is given by N, then the number of possible diastereomers is given by 2N. Of these 2 v diastereomers, each will be characterized by its mirror image, so that the number of enantiomers is given by 2NI2. Whereas the physical properties of enantiomers in an achiral environment are necessarily identical, the physical properties (including solubility) of diastereomers are normally different. The differences arise since there is no structural requirement that the crystal lattices of different diastereomers be the same. For instance, the solubility of an (SS )-diastereomer could differ substantially from that of the (/ S)-diastereomer. However, it should be remembered that the solubility of the (SS)-diastereomer must be exactly identical to that of the (I 7 )-diastereomer, since these compounds are enantiomers of each other. At the same time, the solubilities of the (SI )-diastereomer and the (I S)-diastereomer must also be identical. [Pg.380]

In a similar manner, butadienyl phenylacetate 71, an achiral diene, is expected to approach the chiral dienophile (R)-10 from its Re-prochiral face. The two faces of the chelate ring are differentiated by the small hydrogen and large benzyl groups attached to the chiral center of (R)-10 (Scheme 1-18) the ratio of the Si attack product to the Re attack product is 1 8.88... [Pg.55]

The results of the study of the last-mentioned reaction, DCA-PhCOCH3 —> 167, provided a surprise. X-Ray analyses of the structure of the clathrate before and after partial reaction, and of the final product, 167, showed that the prochiral Re face of the ketone, initially the face more distant from the to-be-attacked host, and not the close-lying Si face, is the one that adds to the steroid. A rationalization of this extraordinary sterochemical effect, which results in formation of a new chiral center with quantitative asymmetric induction, has been proposed (241). [Pg.201]

The synthesis of RR)-1% involves in the final (1 + 1) cyclization step (see Scheme 4) a rare example of an inversion of configuration at both chiral centers in the (SiS)-ditosylate precursor during its successive alkylations of the dianion derived from catechol. The (5/ ,15/J)-5,15-dimethylbenzo-15-crown-5 derivative (/ R)-78 is believed (123) to be optically pure. [Pg.242]

There is a fundamental difference between a chirality center and a pseudoasymmetric center and that is that reflection and permutation of ligands have the same effect for the chirality center, but not for the pseudoasymmetric center. This is because, on reflection of the latter, the chirality senses of both the center, seen when the bonds are numbered, and each of the enantiomorphic ligands are reversed. Another way of stating the difference is that Re/Si and rejsi descriptors specify absolute and relative configuration, respectively. Pseudoasymmetric is a most unfortunate term, and in order to avoid it, the classical terms chirality center and pseudoasymmetric center would perhaps best be replaced by more neutral terms, such as stereogenic centers of type 1 and type 2, respectively, in order to emphasize the aspect of stereogenicity. [Pg.8]

Moving two-dimensional enantiomorphs out of the plane into three-dimensional space allows them to become congruent, i.e., identical. However, some two-dimensional aspects remain. When the two faces of a particular figure are examined, one perceives that the faces, and the corresponding half-spaces10, of the three-dimensional space, are enantiomorphic, and their chirality sense can be specified by Re/Si descriptors. Furthermore, for figures with more than one stereogenic center the Ikjul and ZjE descriptions are preserved in three-dimensional space. [Pg.8]

Enantiotopie groups A bound to a center Xi.AABC) or a center X(ABCD) are classified by the descriptor. Re or Si, of the corresponding chirotopic half-space defined by the triangle ABC, in which the group to be specified resides. A relevant question here would be to ask what property the descriptors RejSi describe. Logic demands that it describes the sense of local chirality. [Pg.18]


See other pages where Si-centered chirality is mentioned: [Pg.106]    [Pg.11]    [Pg.9]    [Pg.106]    [Pg.11]    [Pg.9]    [Pg.411]    [Pg.418]    [Pg.1246]    [Pg.321]    [Pg.110]    [Pg.229]    [Pg.287]    [Pg.183]    [Pg.97]    [Pg.281]    [Pg.250]    [Pg.136]    [Pg.482]    [Pg.514]    [Pg.46]    [Pg.194]    [Pg.208]    [Pg.224]    [Pg.226]    [Pg.66]    [Pg.422]    [Pg.11]    [Pg.100]    [Pg.643]    [Pg.20]    [Pg.23]    [Pg.193]    [Pg.8]    [Pg.1109]    [Pg.123]    [Pg.357]    [Pg.92]   
See also in sourсe #XX -- [ Pg.9 ]

See also in sourсe #XX -- [ Pg.9 ]




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