Axial chiral compounds ligands

Recently, two new axially chiral compounds 44 and 45 have been prepared [44, A-acryl-A-allyl-o-t-butylanilide 45, A-(o-t-butylphenyl)-2-methyl-maleimide]. This represents the first instance of using nonbiaryl axially chiral ligands in asymmetric Dield-Alder reactions. In the presence of iodine, high endo-facial and diastereofacial selectivities have been obtained in 44/45-medi-ated reactions.10... 

Reactions like these, in which stereoselectivity is the consequence of steric hindrance to bond rotation, are most well known among the biaryls, and derivatives of binaphthyl have provided chemists with a valuable range of chiral ligands [4-6]. But the biaryls are only a small subset of axially chiral compounds containing two trigonal centres linked by a rotationally restricted single bond. Many others are known, some with much greater barriers to rotation than Fuji s enol ether [7]. Yet until quite recently there were no reports of reactions in which nonbiaryl atropisomers were the source, conveyor, or product of asymmetric induction. 

Many axially chiral compounds are Privileged Chiral Ligands . It means that it is highly probable that their application will lead to high enantioselectivity and/or activity of the resulting chiral catalysts. It is no wonder that many efforts have been made in order to attach axially chiral substiments, for example easily available... 

Various acetylenes having functional groups such as halide, alcohol, ether, amine, alkene and nitrile, are tolerated in the reaction. An asymmetric (2+2+2) cydoaddition of a,03-diynes with alkyne was achieved by a [IrCl(cod)]2 catalyst combined with a chiral phosphine ligand such as MeDUPHOS and EtDUPHOS, and gave axially chiral aromatic compounds [20]. 

Axially chiral diaryl compounds have become increasingly important as ligands for a variety of effective chiral catalysts <1992CR1007>. For this reason, much attention has been focused on their enantioselective synthesis. The 1,4-dioxocine motif, as a chiral 1,4-dioxy moiety, is frequently used in the construction of chiral 2,2, 6,6 -tetrahydroxybiphenyls. 

A column loaded with the amide 58 that is held in place merely by hydrogen bonds and electrostatic forces is used preparatively to resolve the important axially chiral binaphthol13 59. You will meet compounds of this type as reagents and ligands in chiral catalysts in chapters 24-26. 

The first place in catalytic hydrogenation nowadays is taken by Rh or Ru complexes of BINAP. This ligand has axial chirality as the naphthalene rings cannot rotate past each other. These compounds were developed by Noyori, who with Knowles and Sharpless received the 2001 Nobel prize for their contributions to asymmetric synthesis. BINAP 20 is usually made from BINOL 19 and either 19 or 20 can be resolved. Rhodium complexes similar to those we have met include a molecule of cyclooctadiene and, as these are Rh(I) compounds, a counterion, often triflate 21. Both enantiomers of BINAP are available commercially.8... 

Kinetic resolutions. Baylis-Hillman adducts are deracemized by exploiting their reactivity toward Pd(0)-catalyzed substitution, using chiral ligand 2. Both the planar chiral DMAP derivative 3 and the axially chiral analogue (4) ° and 5" have been developed as catalysts for enantioselective acylation. Benzylic alcohols undergo enantioselective acylation with the aid of 6. Methanolysis of meio-anhydrides in the presence of a cinchona alkaloids is a good way to desymmetrize such compounds. ... 

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]. 

Arylboranes with aryl bromides and iodides Coupling of arylboranes with aryl bromides and iodides offers an extremely useful synthetic method for biaryl compounds, and numerous examples are known. Asymmetric S-M aryl-aryl coupling of 33 with 34 in the presence of chiral 2-aminobinaphthyl-based phosphine (S)-VI-8 afforded the highly enantiomerically enriched biaryl 35 (92 % ee), which was converted to the axially chiral l-aryl-2-naphthylphosphine 36. BINAP is not an effective ligand for this asymmetric reaction [51]. 

Chiral ligand 651 is obtained from the appropriate natural amino-acid phenylalanine, whereas the corresponding derivatives of valine or leucine proved to be slightly less effective [46], Axially prochiral, enantiotopic, biaryl-2,6-diols have been converted to the respective chiral compounds via enzymatic desymmetrization. Thus Pseudomonas cepacia lipase (PCL) catalysed the atropisomerically-selective hydrolysis of diacetate 654 to give monoacetate 655 in 67% yield and 96% e. e. [47], Scheme 24. 

Biaryls with axial chirality are potentially important as chiral ligands for asymmetrical reactions and also as intermediates for synthesizing biologically active natural biaryl compounds, for example, michellamine in Eq. 66. (ii -Disubstituted arene)chromium tricarbonyl complexes exist in two enantiomeric forms based on planar chirality. Biaryl coupling provides a new approach to synthesizing both optically pure atropisom-ers starting from a single chiral arene chromium complex (Eq. 75). ... 

Axially chiral biaryls are an important class of molecules for both biologically active compounds and chiral ligands (78-80). The most common approach to obtain biaryls is by aryl coupling followed by resolution of the racemic product to afford enantiopure biaryls. Even though enantioselective partial intramolecular cyclotrimerization of diyne with alkynes (81,82) or nitriles (83) were developed with various transitional metals, it was difficult to carry out complete intermolecular reaction. Using a cationic chiral rhodium complex as catalyst, a regioselective intermolecular cross-cyclotrimerization of alkynes 72 and 73 for... 

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