Chiral conformationally rigid

Interestingly, the hydroformylation results obtained with ligands 2b and 2d, which have conformationally flexible axially chiral biphenyl moieties, are similar to those obtained with ligand 2m, which have conformationally rigid binaphthyl moieties. This indicates that diphosphite ligands that contain the conformationally flexible axially chiral biphenyl moieties predominantly exist as single atropoisomer in the [RhH(CO)2 (diphosphite)] complexes... 

In most reported cases, the covalently bound chiral auxiliary has been attached to the dienophile via an acyl linkage, but there are also many examples known in which the auxiliary has been attached to the diene via an acyl, alkyl or heteroatom linkage, the first example of the latter being Trost s diene147. Lewis acids are often added to the reaction mixtures when the chiral auxiliary attached to the dienophile contains an additional Lewis basic site. This is not only to enhance the reaction rate, but especially to enhance the diastereofacial selectivity by complexing to the dienophile in a bidentate fashion. This makes the dienophile more conformationally rigid. 

In the presence of conformationally rigid methyl cyclohexanecarboxy-lates (7.9-7.12), PLE displayed a marked preference for a carboxy group in an equatorial position. In fact, the rate of PLE-catalyzed hydrolysis decreased significantly in the series 7.9 >7.10 >7.11 >7.12 [24], Another element of complexity exists in this series of compounds since 7.10 and 7.12 are chiral, the structures shown being those of the preferred enantiomeric substrates. With these two chiral esters, however, the substrate enantiospecificity of PLE was found to be very modest. 

The central point of Evans s methodology is the induction of a 7t-enantiotopic facial differentiation through a conformationally rigid highly ordered transition state. Since the dialkylboron enolates of AT-acyl-2-oxazolidinones exhibit excellent syn-diastereoselectivity syn.anti >97 3) when reacted with a variety of aldehydes, Evans [14] studied the aldol condensation with the chiral equivalents 32 and 38. which are synthesised from fS)-valine (35) and the hydrochloride of (15, 2R)-norephedrine (36) (Scheme 9.11), respectively, and presently are commercially available. 

As already reported in Sects. IV-C and V, there are a few examples of chiral polymers not containing classical asyrrunetric carbon atoms or odier elements of localized chirality (substituted biphenyls, etc.). In such cases chirality is related to the presence of rigid helical conformations and optical activity derives from an excess of chain segments turned in one or the other sense. We may distinguish polymers in which conformational rigidity (and hence the possibility... 

The 3-acyl-2(3F/)-oxazolones function as good dienophiles in cycloaddition reactions with cyclic 2,4-dienes such as cyclopentadienes and anthracenes. Thus, the thermal reaction of 3-acetyl-2(37/)-oxazolone with cyclopentadiene and the hexachloro and hexamethyl derivatives gives endo-cycloadducts exclusively. In particular, the chiral cycloadducts 221 and 223 derived from the diastereoselective Diels-Alder reactions of 3-(2-exo-alkoxy-l-apocamphanecarbonyl)-2(3/7)-oxazo-lones with hexamethylcyclopentadiene and 9,10-dimethylanthracene, respectively, are highly useful as chiral 2-oxazolidinone auxiliaries. The conformationally rigid roofed structures play a crucial role in affording excellent chiral induction (Fig. 5.54). 

An analogous study has been carried out on ylide formation in cyclic sulfonium salts. Using the conformationally rigid systems in Table 12, relative kinetic acidities for Ho and Heq were determined. The results show that it is possible to remove one proton selectively, and thereby transfer chirality at sulfur to the neighbouring carbon atom (78JA200). As rigidity increases the differential kinetic acidity also increases but this is due not to an increase in overall acidity, rather to a decrease in lability of the axial proton in these... 

Although the conformationally rigid, N-spiro structure created by two chiral binaphthyl subunits represents a characteristic feature of 1 and related catalyst 9, Maruoka and coworkers have generally used their (S,S)- and (R,R)-isomers. Surprisingly, however, when the diastereomeric (R,S)-lc was used for asymmetric benzyla-tion of 2, the reaction was found to proceed very slowly, such that even after 60 h the... 

A new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

Figure 2.13 Conformationally rigid chiral molecules whose VCD has been studied.

Solvent effects on the optical rotation of several conformationally rigid chiral organic molecules (fenchone, camphor, a- and /3-pinene, camphorquinone, verbenone and methy-... 

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