Chiral diastereomeric conformers

A complete stereocontrol is achieved by addition of the bulky silyl radical (Me3Si)3Si to a chiral and conformationally flexible electron-deficient olefin 163, as shown in equation 68213. Replacement of (Me3Si)3Si with a less sterically hindered (n-Bu)3Sn gives a mixture of syn and anti diastereomeric adducts 164 and 165 in a ratio of 7 3. The A values (kcalmol-1) of the tin, carbon and silicon species follow the order (n-Bu Sn (1.1) < Me (1.7) < Me3Si (2.5), and the bond length for C—Sn (2.2 A) is longer than that for C-Si (1.85 A)214 215. 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

These differences reflect the conformations of (+)- and meso-isomers as they sit at the air-water interface. What is much harder to elucidate is the effect of stereochemistry on intermolecular interactions. How does changing the stereochemistry at one chiral center affect interactions between diastereomers Ab initio molecular orbital calculations have been used to address the problem of separating stereochemically dependent inter- and intra-molecular interactions in diastereomeric compounds (Craig et al., 1971). For example, diastereomeric compounds such as 2,3-dicyanobutane exhibit significant energetic dependence on intramolecular configuration about their chiral centers. So far, however, little experimental attention has been focused on this problem. 

Spectral nonequivalence in diastereomeric solvates results from the population of conformations that place at least one set of chemically equivalent nuclei in different positions with respect to a CSA substituent capable of perturbing their magnetic environment. Having three points of interaction or reference is a general requirement for chiral recognition (26). The manner in which this requirement is met is readily understood in terms of complexes 6 and 7 through... 

Diastereomeric distributions (A A) in several tris-ligand Fe + complexes of chiral catecholamide and terephthalamide ligands have been established in solution by CD and NMR spectroscopies. The complex of (253), whose structure in the solid state was determined, exists wholly in the A conformation in aqueous solution. Weak polar interactions determine conformational preferences here. ... 

The reaction protocol was further developed by alterations to the chiral controlling element of the reaction (49). Use of the precursor 183 under the standard ylide generation and cycloaddition conditions gave a greatly improved diastereomeric excess of >95%, an endo/exo ratio 1 15 and an isolated yield of 62%, with A-phenylmaleimide as the dipolarophile. The improvement in the reaction was rationalized by both endo and exo attack of the dipolarophile to the same diastereomerically favored face of the conformationally restricted U-shaped ylide 184 (Scheme 3.52). 

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