Chiral centers, crowded

All these deviations have one similarity they occur in compounds having relatively crowded chiral centers bearing at least one highly anisotropic substituent. The differential effect of the CSA on the intramolecular anisotropic environment of a given nucleus (e.g., on aryl rotation) cannot presently be stated. 

Chirality is built into the catalysts by employing ligands with either chirality centers or axes (Section 7.9). Noyori s widely used BINAP has a chirality axis, and crowding prevents interconversion of enantiomers by restricting rotation around the bond connecting the naphthalene rings. The metal, usually ruthenium, is held in place by the two phosphorus atoms (yellow) in a chiral environment. The steric demands in the cavity occupied by the metal in Ru-BINAP cause reaction to occur preferentially at one face of the double bond. 

Burgess and Ohlmeyer [30] have reported that electronic effects are important in catalyzed hydroboration, e.g., allylic acetates are hydroborated with less syn selectivity than allylic trifluoroacetate is [25], and proposed the general model [A] (Fig 5.4) for catalyzed hydroboration of chiral allylic alcohols. The model predicts that the OCOCF, substituent (good a acceptors) will preferentially orientate anti to the approaching rhodium complex. The largest of the other two substituents on the chiral center will occupy the outside position, and the smallest will reside in the inside (crowded) site and thus, syn selectivity will result... 

There are many chiral molecules for which enantiomeric forms can be interconverted by a rotation about a single bond. The enantiomeric conformations of gauche butane provide an example, where rapid rotation interconverts the two under most conditions. If the rotation that interconverts a pair of such enantiomers is slow at ambient temperature, however, the two enantiomers can be separated and used. Recall from our first introduction of isomer terminology (Section 6.1) that stereoisomers that can be interconverted by rotation about single bonds, and for which the barrier to rotation about the bond is so large that the stereoisomers do not interconvert readily at room temperature and can be separated, are called atropisomers. One example is the binaphthol derivative shown in the margin. It is a more sterically crowded derivative of the biphenyl compound discussed previously as an example of a chiral molecule with no "chiral center". A second example is frans-cyclooctene, where the hydrocarbon chain must loop over either face of the double bond (Eq. 6.4). This creates a chiral structure, and the enantiomers interconvert by moving the loop to the other side of the double bond. 

With chiral cyclopentenes, which possess an almost planar structure, one face is selectively crowded by the largest substituent on the stereogenic center. For this reason, when cyclic enones such as 39-41 are irradiated in the presence of these chiral cyclopentenes, the cycloaddition occurs selectively from the less-hindered face, and usually only one stereoisomer can be isolated (Scheme 11) [37]. 

The efficiency of the synthesis could in fact be improved by a modification of the starting material, utilizing a more crowded chiral reagent with two centers of asymmetry ... 

Graf C-D, Malan C, Harms K, Knochel P. New homochiral ligands bearing nonstereogenic chirotopic centers. Lithiated N,N -dialkylureas as chiral bases and sterically crowded phosphines for asymmetric catalysis. J. Org. Chem. 1999 64 (15) 5581-5588. 

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