Axially chiral bidentate ligands in asymmetric catalysis

Organocatalysts for enantioselective synthesis of fine chemicals: definitions, trends and developments

<p>Organocatalysis, that is the use of small organic molecules to catalyse organic transformations, has been included among the most successful concepts in asymmetric catalysis and it has been used for the enantioselective construction of C-C, C-N, C-O, C-S, C-P, and C-halide bonds. Since the seminal works in early 2000, the scientific community has been paying an ever-growing attention to the use of organocatalysts for the synthesis, with high yields and remarkable stereoselectivities, of optically active fine chemicals of interest for the pharmaceutical industry. A brief overview is here presented about the two main classes of substrate activation by the catalyst: covalent organocatalysis and non-covalent organocatalysis, with a more stringent focus on some recent outcomes in the field of the latter and of hydrogen-bond-based catalysis. Finally, some successful examples of heterogenisation of organocatalysts are also discussed, in the view of a potential industrial exploitation.</p>

Regioselective derivatizations of a tribrominated atropisomeric benzamide scaffold

The enantioselective synthesis of atropisomeric, tribrominated benzamides and subsequent regioselective transformations to afford derivatized, axially chiral molecules is reported. The enantioenriched tribromides were carried through sequential Pd-catalyzed cross-coupling and lithium-halogen exchange with high regioselectivity and enantioretention. A variety of complexity-generation functional group installations were performed to create a library of homochiral benzamides. The potential utility of these molecules is demonstrated by using a phosphino benzamide derivative as an asymmetric ligand in a Pd-catalyzed allylic alkylation.

Multi-nuclear NMR of axially chiral biaryls in polypeptide orienting solvents: spectral discriminations and enantiorecognition mechanisms

The analytical potential of multinuclear NMR using chiral liquid crystals to discriminate between enantiomers of axially chiral biaryls is explored.

Enantioselective transformation of Na2SO3into allylic sulfonic acids under Pd catalysis

Pd-catalyzed asymmetric allylic sulfonation of di-aryl-substituted allylic acetates with sodium sulfite (Na₂SO₃) was accomplished under mild conditions. This method directly provided allylic sulfonic acids in up to excellent yield and enantioselectivity.

Construction of axial chirality by rhodium-catalyzed asymmetric dehydrogenative Heck coupling of biaryl compounds with alkenes

Enantioselective construction of axially chiral biaryls by direct C-H bond functionalization reactions has been realized. Novel axially chiral biaryls were synthesized by the direct C-H bond olefination of biaryl compounds, using a chiral [Cp*Rh(III)] catalyst, in good to excellent yields and enantioselectivities. The obtained axially chiral biaryls were found as suitable ligands for rhodium-catalyzed asymmetric conjugate additions.

Enzymatic desymmetrising redox reactions for the asymmetric synthesis of biaryl atropisomers

Atropisomeric biaryls carrying ortho-hydroxymethyl and formyl groups were made enantioselectively by desymmetrisation of dialdehyde or diol substrates. The oxidation of the symmetrical diol substrates was achieved using a variant of galactose oxidase (GOase), and the reduction of the dialdehydes using a panel of ketoreductases. Either M or P enantiomers of the products could be formed, with absolute configurations assigned by time-dependent DFT calculations of circular dichroism spectra. The differing selectivities observed with different biaryl structures offer an insight into the detailed structure of the active site of the GOase enzyme.

Rhodium-catalyzed direct coupling of biaryl pyridine derivatives with internal alkynes

Axially chiral biaryls were synthesized by an isoquinoline or 2-pyridine-directed Rh(iii)-catalyzed dual C–H cleavage and coupling with internal alkynes.

An Enantiomerically Pure Pyridine NC-Palladacycle Derived from [2.2]Paracyclophane

An enantiomerically pure planar chiral pyridine-based palladacycle was prepared from [2.2]paracyclophane in just four steps. The palladacycle shows potential in catalysis, mediating the Suzuki coupling of an aryl chloride. It also permits the ortho bromination of [2.2]paracyclophane, a reaction that can be hard to achieve selectively.

Design, preparation, and implementation of an imidazole-based chiral biaryl P,N-ligand for asymmetric catalysis

A new strategy for increasing the barrier to rotation in biaryls has been developed that allows for the incorporation of 5-membered aromatic heterocycles into chiral atropisomers. Using this concept, an imidazole-based biaryl P,N-ligand has been designed and prepared as a single enantiomer. This ligand performs exceptionally well in the enantioselective A(3)-coupling, demonstrating the potential of this new design element.

Rhodium-catalyzed oxidative cyclization of arylphosphonic acid monoethyl esters with alkenes: efficient synthesis of benzoxaphosphole 1-oxides

Rhodium-catalyzed tandem oxidative alkenylation and an intramolecular oxy-Michael reaction were developed using arylphosphonic acid monoethyl esters and alkenes under aerobic conditions, which produced benzoxaphosphole 1-oxides in good to excellent yields.

Synthesis of new BINAP-based aminophosphines and their ³¹P-NMR spectroscopy

BINAP aminophosphines are prevalent N,P-bidentate, chiral ligands for asymmetric catalysis. While modification via the BINAP-nitrogen linkage is well explored and has provided a diverse body of derivatives, modification of the other substituents of the phosphorous center is another avenue in generating new congeners of this important class of chiral ligands. Herein reported are new BINAP aryl aminophosphines with electron rich or deficient substituents on the aryl rings. This scalable synthesis converted readily available starting material, (S)-BINOL, to a key intermediate (S)-NOBIN, from which the final chiral aminophosphines were prepared via a palladium-catalyzed, phosphonylation reaction. The aryl substituents are able to modify the electronic properties of the phosphorous center as indicated by the range of ³¹P-NMR shifts of these new ligands. A computational analysis was performed to linearly quantitate contributions to the ³¹P-NMR shifts from both resonance and field effects of the substituents. This correlation may be useful for designing and preparing other related aminophosphines with varying ligand properties.