Pyridine, axially chiral

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). 

The first example of the use of a chiral-core dendrimer as enantioselective receptor in molecular recognition processes was reported by Diederich (see Fig. 4.70). This dendrimer, designated as a dendrocleff, acts as enantioselective receptor for monosaccharides. It bears a central axially chiral 9,9 -spirobi[9H-fluorene] unit which is linked via two 2,6-bis(carbonylamino)pyridine spacer groups, each in 2,2 position, with triethylene glycol monomethyl ether dendrons [14b]. Both... 

The organic acylation catalysts currently known are tertiary amines, N-heteroar-omatic compounds (for example pyridine derivatives), or phosphines they can be of central, planar, and axial chirality. Finally, small peptides carrying N-methylhis-tidine as the catalytically active subunit have also been employed they also will be discussed in this chapter. 

Pyridine-type N-oxides (Fig. 7.2) represent another, no less-successful class of catalysts for the allylation reaction. Thus, Nakajima first demonstrated that the axially chiral biquinoline N,N -bisoxide 17 can indeed catalyze the allylation... 

The same group had earlier described the use of bisoxazoline ligands for nickel catalyzed fluorination reactions of 1,3-dicarbonyl compounds, [39] which was also observed for an elaborate 2-oxazolinyl pyridine ligand bearing an axially chiral methylenylamine in 6-position [40]. 

At first we tried to address the problem with Moreau s method of kinetic resolution which has been applied with great success to compounds of central chirality (ref. 18). To our knowledge this technique has not yet been used for compounds with axial chirality. To be on safe ground, we had to calibrate the effects first with compounds of similar structure and known absolute configuration. We selected 2,2 -dihydroxy-1,1 -binaphthyl (31) as an example and reacted it in the usual way with an excess of racemic 2-phenyl butanoic anhydride in the presence of pyridine. The optical excess in the remaining anhydride was determined by means of the gas chromatographic variant of Brooks and Gilbert (ref. 19) (Scheme 6). 

In contrast to the Rh-catalyzed asymmetric intramolecular direct C—H bond functionalization reactions described above, their asymmetric inter-molecular variants have been rarely explored. In 2000, Murai and co-workers reported a Rh-catalyzed intermolecular asymmetric C—H activation/olefin coupling reaction of achiral biaryl pyridine (132) or isoquinoline derivatives to deliver axially chiral biaryls (133) (Scheme 5.46a). Although both the efficiency (up to 37% yield) and the enantioselectivity (up to 49% ee) of the reaction were only moderate, this protocol provided an alternative method for the synthesis of optically active biaryl compounds. To some extent, this reaction was similar to a formal dynamic kinetic resolution. The two atropisomers of the biaryl starting materials could interconvert with each other freely due to a low inversion energy barrier. A properly chosen chiral catalyst could react preferentially with one atropisomer. The increased steric bulkiness of the final alkylated products can prevent the epimerization and the biaryl compounds possessing a stable axial chirality are established. However, due to the relatively low efficiency of the catalyst, the yields of the desired products are generally low and the starting materials can be recovered (Scheme 5.46b). 

Certain heterocycles, e.g. pyridines or quinolines, bearing of an electron-withdrawing group such as oxazoline, undergo the Michael-type nucleophilic 1,4-addition accompanied with loss of aromaticity to give the new C-C bond. Thus formed dihydropyridine or benzodihydropyridine can be oxidatively aromatized with conservation of chirality, primary induced by an influence of chiral oxazoline moiety. In this manner, Meyers and coworkers [27] described the Michael-type addition of 1-naphthyllithium (609) to the oxazoline 610 at low temperature to form 611 in 90% yield. The latter was oxidatively aromatized to the naphthylquinoline 612 in 87% yield with 88 12 ratio of two diastereomers. Diastereoselectivity in this reaction remained on the same level as obtained by the nucleophilic addition of 609 to 610 indicating the virtually complete conservation of chirahty, from sp -type in the compound 611 to the axially chiral compound 612, Scheme 11. 

The axial chirality has not been observed in the pyridine or the pyridinium salt as well as in the dihydropyridine derivatives with a carbamoyl side chain composed of a primary amine. This is also true... 

The Rh(I)/dppf complex-catalysed 2-i-2-i-2-cycloaddition of oximes and diynes formed substituted pyridines in moderate to good yields (88%), under mild conditions. A one-pot procedure has been developed using aldehydes.The Ni-catalysed 2 -I-2 -I-2-cycloaddition of isocyanates (117) with 1,3-dienes (118) in MeCN produced 6-substituted dihydropyrimidine 2,4-diones (119). A key intermediate in this reaction is a five-membered azanickelacyclic species (Scheme 35). " The enantioselective cationic Rh(I)-catalysed 2-i-2-i-2-cycloaddition of diynes and isocyanates formed axially chiral pyridones with high ees (82%). The unique source of chirality is provided by a system containing [Rh(cod)Cl]2, l,4-bis(diphenylphosphino)butane, and the silver phosphate salt Ag(5)-TRIP. " " The Rh-catalysed 2-I-2-I-2-cycloaddition of diynes to sulfonimines in DCE, at r.t. to 80 °C, yielded 1,2-dihydropyridines in good yields (54-86%) and enantioselectivity (61-96%... 

The enantioselective desymmetrization of substituted malononitriles via cationic rhodium(I)/axially chiral biaryl bisphosphine-catalyzed [2 + 2 + 2] cycloaddition with a 1,6-diyne proceeded at room temperature to give enantio-enriched bicycUc pyridines in good yields with moderate ee values (Scheme 4.66) [55],... 

Axially chiral pyridines as well as axially chiral pyridones could be synthesized via rhodium-catalyzed enantioselective [2 + 2 + 2] cycloaddition. The reaction of 1,6-diyne 68 with ethyl cyanoformate (69) in the presence of the cationic rhodium(I)/Segphos catalyst furnished axially chiral arylpyridine 70 as a single regioi-somer with excellent ee value (Scheme 9.25) [19],... 

In the axially chiral biaryl synthesis via the double [2 - - 2 - - 2] cycloaddition shown in Scheme 9.18, the use of ethyl cyanoformate (69) instead of monoynes furnished the corresponding C2-symmetric axially chiral pyridines 71 with excellent enantios-electivity, although the yield of 71 was low, due to the formation of regioisomeric by-products (Scheme 9.26) [19],... 

Axially chiral bis-arylthiourea-based organocatalyst (6) has been developed and subjected to Friedel-Crafts type addition of indole and N-methyl pyridine to nitroalkenes (Scheme 2.31) [82]. 

Although these enantioselective photoreactions are limited to amide or salt derived from achiral acid and chiral amine, one enantioselective photoisomerization reaction of cobaloxime coordinated with chiral axial ligands such as 1-methylpropylamine, l-(l-naphthyl)ethylamine, and 2-phenylglycinol has been reported. For example, finely powdered (2-cyanoethyl)cobaloxime (60), suspended in liquid paraffin and spread onto a Petri dish, was irradiated to give (S)-(-)-61 of about 80% ee after displacement of the chiral auxiliary of the complex with pyridine [32],... 

Several ruthenium complexes bearing chiral Schiff s base ligands have been published. RuL(PPh3)(H20)2], complex C (Fig. 11), with PhIO produced (S)-styrene oxide in 80% ee [61]. Chiral Schiff s base complex D was examined using molecular oxygen with aldehyde, with or without 2,6-dichloropyridine N-oxide as an axial ligand. Styrene oxide was produced in up to 24% ee[62]. A chiral bis(oxazolinyl)pyridine ruthenium complex E with iodosylbenzene diacetate PhI(OAc)2 produced (lS,2S)-fra s-stilbene oxide in 74% ee [63]. Similarly, chiral ruthenium bis(bipyridine) sulfoxide complex F [64] was effective in combination with PhI(OAc)2 as an oxidant and resulted in in 33% ee for (R,R) trans-stilbene oxide and 94% ee for (R,R) trans-/i-Me-styrene (after 75 h at 25 °C). 

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