Biphenyls, chiral forms

As shown in Scheme 8-11, nucleophilic entry from the a-face (24a) may be hindered by the sterically bulky substituent R2 on the oxazoline moiety therefore entry from the / -face 24/ predominates. Free rotation of the magnesium methoxy bromide may be responsible for the sense of the axial chirality formed in the biaryl product. If the azaenolate intermediate 25 is re-aromatized with a 2 -methoxy substituent complexed to Mg, (iS )-biphenyl product is obtained. Upon re-aromatization of azaenolate 25B, (R)-product is obtained. 

Often symmetry operations cannot be used in a simple way to classify chiral forms because, e.g., the molecule consists of a number of conformations. Therefore, independent of the symmetry considerations, a chemical approach to describe chiral molecules has been introduced by the use of structural elements such as chiral centers, chiral axis, and chiral planes. Examples for a chiral center are the asymmetric carbon atom, i.e., a carbon atom with four different substituents or the asymmetric nitrogen atom where a free electron pair can be one of the four different substituents. A chiral axis exists with a biphenyl (Figure 3.2) and chiral planes are found with cyclo-phane structures [17]. Chiral elements were introduced originally to classify the absolute configuration of molecules within the R, S nomenclature [16]. In cases where the molecules are chiral as a whole, so-called inherent dissymmetric molecules, special names have often been introduced atropiso-mers, i.e., molecules with hindered rotation about a helical molecules [18], calixarenes, cyclophanes [17], dendrimers [19], and others [20]. 

Example 14.4 4,5-Dimethylphenanthrene. Using the C NMR spectrum measured by Stothers et al. [275], we deduce A/// = 36.8 kcal/mol assuming planarity. Closely neighboring methyl groups that are separated by five bonds in the molecular skeleton, however, result in chiral nonplanar conformations [276]. Modeling, where appropriate, the CC bonds on those described for biphenyl and m-stHbene, one predicts A/// = 47.8 for the nonplanar form, in acceptable accord with the reported value [248], 46.26 + 1.46 kcal/mol, a result that is self-explanatory. 

Mechanism of Separation. There are several requirements for chiral recognition. (/) Formation of an inclusion complex between the solute and the cydodextrin cavity is needed (4,10). This has been demonstrated by performing a normal-phase separation, eg, using hexane—isopropanol mobile phase, on a J3-CD column. The enantiomeric solute is then restricted to the outside surface of the cydodextrin cavity because the hydrophobic solvent occupies the interior of the cydodextrin. (2) The inclusion complex formed should provide a rdatively "tight fit" between the hydrophobic species and the cydodextrin cavity. This is evident by the fact that J3-CD exhibits better enantioselectivity for molecules the size of biphenyl or naphthalene than it does for smaller molecules. Smaller compounds are not as rigidly held and appear to be able to move in such a manner that they experience the same average environment. (5) The chiral center, or a substituent attached to the chiral center, must be near to and interact with the mouth of the cydodextrin cavity. When these three requirements are fulfilled the possibility of chiral recognition is favorable. 

Among the different types of compounds whose complexation properties have been studied are various amides linear oxoamide 9 [22], fumaramide 10 [23,24] and methanetricarboxamide 11 [25], biphenyl derivatives 12 [26], and derivatives of tartaric acid 13-16, that can also be prepared in an optically active form [27], The above-mentioned chiral hosts have been found to form inclusion complexes with chiral guests 17 and 18. Molecular recognition between chiral hosts and... 

The heterocyclic hemi-aminal 44 was formed as a model for the synthesis of the marine alkaloids zoanthamine and zoanthenol and is derived from the cyclic imine formed by the reduction of an appropriated substituted azide <07H(72)213>. A pyrrolidine-fused azepine has been isolated from the venom of the ant Myrmicaria melanogaster and assigned as 45 based upon GC and FT-IR comparison with synthetic material <07JNP160>. The axially chiral doubly bridged biphenyl azepine 46 has been synthesised and used with oxone as an epoxidation catalyst in a biphasic system <070BC501>. 

Warner, N.A. Martin, J.W. Wong, C.S., Chiral polychlorinated biphenyls are biotransformed enantioselectively by mammalian c3tiochrome P-450 isozymes to form hydroxylated metabolites Environ. Sci Technol. 2009, 43, 114—121. 

Many molecules are chiral, even in the absence of stereogenic centers that is, molecules containing adjacent 77-systems, which cannot adopt a coplanar conformation because of rotational restrictions due to steric hindrance, can exist in two mirror forms (atropiso-mers). This is the case for some dienes or olefins, for some nonplanar amides, and for the biphenyl or binaphthyl types of compounds. 

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