Chiral binaphthyls

A large number of chiral crowns have been prepared by numerous groups. The reader is directed to the tables at the end of this chapter to obtain an overview of these structures. It would not be useful to try to recount the synthetic approaches used in the preparation of all of these compounds we have chosen rather to subdivide this mass of compounds into three principal groups. The groups are (1) Cram s chiral binaphthyl systems (2) chiral crowns based on the tartaric acid unit and (3) crowns incorporating sugar subunits. These are discussed in turn, below. 

Cram and his coworkers have pioneered the use of bis-binaphthyl crowns as chiral com-plexing agents for ammonium salts and amino acid salts. In these systems, the chiral binaphthyl unit provides a steric barrier within the macrocycle which allows discrimina-... 

A series of chiral binaphthyl ligands in combination with AlMe3 has been used for the cycloaddition reaction of enamide aldehydes with Danishefsky s diene for the enantioselective synthesis of a chiral amino dihydroxy molecule [15]. The cycloaddition reaction, which was found to proceed via a Mukaiyama aldol condensation followed by a cyclization, gives the cycloaddition product in up to 60% yield and 78% ee. 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Borner reported the synthesis of pyrophosphites 149 with chiral binaphthyl substituents [118]. The results showed that the Hg-binaphthyl unit was the best for the Rh-catalyzed hydrogenation of methyl (Z)-2-acetamidocinnamate (48% ee) and dimethyl itaconate (70% ee). 

Suga et al. (63) reported the asymmetric cyclopropanation of styrene using chiral binaphthyl-derived diimines. Diastereoselectivities and enantioselectivities in the cyclopropanation of styrene are moderate, Eq. 45. Improved selectivities are observed using 1,1-diphenylethene, which provides the men thy 1 ester cyclopropane in 96% de. 

Scheme 16 summarizes the results obtained by enantioselective radical reduction of a-bromoester by chiral binaphthyl-derived tin hydride. The reactions were generally performed at - 78 °C. An increase in the temperature resulted in the lowering of the selectivity. All reactions mediated by (S)-configured chiral tin hydride showed an (R)-selective preference in the product. The use of the opposite enantiomer of the chiral stannane resulted in a quantitative reversal of the selectivity (not shown). The selectivity remained modest on addition of magnesium Lewis acids. These reductions were also feasible when a catalytic amount of chiral tin hydride (1 mol %) was employed in combination with an excess of achiral hydride NaCNBH3, providing similar results. 

A variety of optically active 4,4-disubstituted allenecarboxylates 245 were provided by HWE reaction of intermediate disubstituted ketene acetates 244 with homochiral HWE reagents 246 developed by Tanaka and co-workers (Scheme 4.63) [99]. a,a-Di-substituted phenyl or 2,6-di-tert-butyl-4-methylphenyl (BHT) acetates 243 were used for the formation of 245 [100]. Addition of ZnCl2 to a solution of the lithiated phos-phonate may cause binding of the rigidly chelated phosphonate anion by Zn2+, where the axially chiral binaphthyl group dictates the orientation of the approach to the electrophile from the less hindered si phase of the reagent. Similarly, the aryl phosphorus methylphosphonium salt 248 was converted to a titanium ylide, which was condensed with aromatic aldehydes to provide allenes 249 with poor ee (Scheme 4.64) [101]. 

More recently macrocycles with rigid and flexible aromatic and chiral binaphthyl systems with interesting fluorescence properties have been synthesized via hydroaminomethylation (Scheme 16) [62,63]. 

Another way of constructing axially chiral binaphthyls under nickel catalysis involves an enantioselective desymmetrization strategy. Thus, dinaphthothiophene... 

In 2007, Maruoka et al. introduced chiral dicarboxylic acids consisting of two carboxylic acid functionalities and an axially chiral binaphthyl moiety. They applied this new class of chiral Brpnsted acid catalyst to the asymmetric alkylation of diazo compounds withA-Boc imines [91]. The preparation of the dicarboxylic acid catalysts bearing aryl groups at the 3,3 -positions of the binaphthyl scaffold follows a synthetic route, which has been developed earlier in the Maruoka laboratory [92]. 

Racemic modifications may be resolved. There are very few examples of this approach having been employed successfully. The racemic cylic ether (RS)-36, which contains two CH2OCH2CO2H arms attached to the 3 and 3 positions on the axially chiral binaphthyl units, has been resolved (48-50, 93, 94) to optical purity in both its enantiomers by liquid-liquid chromatography using a chiral stationary phase of either (R)- or (S)-valine adsorbed on diatomaceous eaitii. Very recently, the optical resolution of crown ethers (/ S)-37 and (/ 5)-38, incorporating the elements of planar chirality in the form of a rron -doubly bridged ethylene unit, has been achieved (95) by HPLC on (+)-poly(triphenyl-methyl methacrylate). 

Connon and co-workers synthesized a small library of novel axially chiral binaphthyl-derived bis(thio)ureas 152-165 and elucidated the influence of the steric and electronic characteristics of both the chiral backbone and the achiral N-aryl(alkyl) substituents on catalyst efficiency and stereodifferentiation in the FC type additions of indole and N-methylindole to nitroalkenes (Figure 6.50) [315]. 

The effect of additives on Shibasaki s lanthanide-BINOL catalysts has been investigated by Inanaga and coworkers. From a variety of additives, triphenylphosphine oxide turned out to be the best one improving, for example, the obtained ee for the chalcone epoxide from 73% to 96% (Table 16) . The explanation for the positive effect of the additive was the disruption of the oligomeric structure of the catalyst by coordination of the phosphine oxide. As a consequence, epoxidation takes place in the coordination sphere of the ytterbium where the reaction site might become closer to the chiral binaphthyl ring due to the phosphine oxide ligand with suitable steric buUdness. In contrast to the Shibasaki... 

Pu reported the synthesis of axially chiral-conjugated polymer 82 bearing a chiral binaphthyl moiety in the main chain by the cross-coupling polymerization of chiral bifunctional boronic acid 80 with dibromide 81 (Equation (39)). The polymer is soluble in common organic solvents, such as THE, benzene, toluene, pyridine, chlorobenzene, dichloromethane, chloroform, and 1,2-dichloroethane. The polymer composed of racemic 80 was also synthesized, and the difference of characteristics was examined. Optically active polymer 82 was shown to enhance fluorescence quantum yield up to = 0.8 compared with the racemic 82 ( = 0.5). Morphologies of the optically active and racemic polymers were also compared with a systematic atomic-force microscopy (AEM). 

Callot and co-workers established in 1982 that iodorhodium(III) porphyrin complexes could be used as cyclopropanation catalysts with diazo esters and alkenes with c/.s-disubstituted alkenes these catalysts provide preferential production of cis(syn) disubsdtutcd cyclopropancs (syn/anti up to 3.3 with 1,4-cyclohexadiene) [72], More recently, chiral porphyrins have been designed and prepared by Kodadek and co-workers [73], and their iodorhodium(lll) complexes have been examined for asymmetric induction in catalytic cyclopropanation reactions [74,751. The intent here has been to affix chiral attachments onto the four porphyrin positions that are occupied in tetraphenylporphyrin by a phenyl group. Iodorhodium(III) catalysts with chiral binaphthyl (27, called chiral wall porphyrin [74]) and the structurally analogous chiral pyrenyl-naphthyl (28,... 

Figure 6B.9. Role of the chiral binaphthyl unit of 15 in its asymmetric induction.

Chiral induction was observed in the cyclopolymerization of optically active dimethacrylate monomer 42 [88], Free-radical polymerization of 42 proceeds via a cycliza-tion mechanism, and the resulting polymer can be converted to PMMA. The PMMA exhibits optical activity ([ct]405 -4.3°) and the tacticity of the polymer (mm/mr/rr =12/49 / 39) is different from that of free-radical polymerization products of MMA. Free-radical polymerization of vinyl ethers with a chiral binaphthyl structure also involved chiral induction [91,92]. Optically active PMMA was also synthesized through the polymerization of methacrylic acid complexed with chitosan and conversion of the resulting polymer into methyl ester [93,94]. 

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... 

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