Biaryls chirality

In 1999 and 2002, Denmark and coworkers reported several 7-membered carbo-cyclic biaryl chiral ketones (22a-d) (Scheme 3.17) [6a, 22]. For these ketones the chirality is closer to the reacting carbonyl compared with 11-membered ketone 9, which could further enhance the stereodifferentiation for the epoxidation. It was found that the epoxidation rate was greatly accelerated by fluorine substitution at the aorbon... 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

Meyers has also reported the use of chiral oxazolines in asymmetric copper-catalyzed Ullmann coupling reactions. For example, treatment of bromooxazoline 50 with activated copper powder in refluxing DMF afforded binaphthyl oxazoline 51 as a 93 7 mixture of atropisomers diastereomerically pure material was obtained in 57% yield after a single recrystallization. Reductive cleavage of the oxazoline groups as described above afforded diol 52 in 88% yield. This methodology has also been applied to the synthesis of biaryl derivatives. 

Atroposelective cleavage of configurationally unstable lactone cycle in biaryl derivatives as effective route to chiral natural products and useful reagents 99S525. 

Chiral aluminum catalyst 2, prepared from Et2AlCl and a Vaulted biaryl ligand, is reported to be an effective Lewis acid catalyst of the Diels-AIder reaction between methacrolein and cyclopentadiene, affording the adduct in 97.7% ee [4] (Scheme 1.2). Although the Diels-AIder reaction with other a,/ -unsaturated aldehydes has not been described, that only 0.5 mol% loading is sufficient to promote the reaction is a great advantage of this catalyst. 

In conclusion, further work to increase the scope of this reaction, specifically to obtain higher functional group tolerance, is desirable. In addition, the development of a chiral catalyst that enables the production of enantiopure tetra-ort/io substituted biaryls would be of significant interest. 

Similar reaction pathways were recently shown to be available to the widely used chiral ligand l,l -binaphthol (BINOL) (138).87 Irradiation of BINOL in aqueous acetonitrile initiated ESIPT to the 4 -, 5 -, and 7 -ring carbons to give biaryl quinone... 

Burnham, K. S. Schuster, G. B. A search for chiral photochromic optical triggers for liquid crystals photoracemization of l,l -binaphthylpyran through a transient biaryl quinone methide intermediate. J. Am. Chem. Soc. 1998, 120, 12619-12625. 

The key synthetic challenges in the target molecule 1 were the chiral 2-arylpyrrolidine fragment, the densely functionalized benzimidazole ring, and the hindered biaryl ether linkage. 

Chan has discovered a completely atropdiasteroselective synthesis of a biaryl diphosphine by asymmetric intramolecular Ullmann coupling or Fe(m)-promoted oxidative coupling. A chiral atropisomeric biaryl bisphosphine ligand 2 was synthesized through this central-to-axial chirality transfer.38 Recently, a xylyl-biaryl bisphosphine ligand, Xyl-TetraPHEMP, was introduced by Moran, and is found to be effective for the Ru-catalyzed hydrogenation of aryl ketone.39... 

The first catalytic 1,4-addition of diethylzinc to 2-cyclopentenone with over 90% ee was described by Pfaltz and Escher, who used phosphite 54 with biaryl groups at the 3,3 -positions of the BINOL backbone.46 Chan and co-workers achieved high enantioselectivity in the same reaction (up to 94% ee) by using chiral copper diphosphite catalyst (R,R,R)-41 48,48a 48d Hoveyda and co-workers used ligand 46 to realize excellent enantiocontrol (97% ee) in the 1,4-additions of 2-cyclopentenones,52 which may be used in the practical asymmetric synthesis of some substituted cyclopentanes (including prostaglandins). 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

Although biaryl-based chiral molecules are the most extensively studied, also other molecular frameworks are associated to high twisting powers and thus suitable for stereochemical studies. One of the first chiral structures used in... 

This chapter describes atropisomeric biaryl bisphosphine ligands modified DIOP-type ligands P-chiral bisphosphane ligands other bisphosphane ligands and their applications in the enantioselective hydrogenation of olefins. 

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