Chiral monodentate phosphorus ligands for rhodium-catalyzed asymmetric hydrogenation

Stereoisomeric Tris-BINOL-Menthol Bulky Monophosphites: Synthesis, Characterisation and Application in Rhodium-Catalysed Hydroformylation

Four stereoisomeric monoether derivatives, based on axially chiral (R)- or (S)-BINOL bearing a chiral (+)- or (−)-neomenthyloxy group were synthesised and fully characterised by NMR spectroscopy and X-ray crystallography. The respective tris-monophosphites were thereof prepared and fully characterised. The coordination ability of the new bulky phosphites with Rh(CO)₂(acac), was attested by ³¹P NMR, which presented a doublet in the range of δ = 120 ppm, with a ¹J(¹⁰³Rh-³¹P) coupling constant of 290 Hz. The new tris-binaphthyl phosphite ligands were further characterised by DFT computational methods, which allowed us to calculate an electronic (CEP) parameter of 2083.2 cm⁻¹ and an extremely large cone angle of 345°, decreasing to 265° upon coordination with a metal atom. Furthermore, the monophosphites were applied as ligands in rhodium-catalysed hydroformylation of styrene, leading to complete conversions in 4 h, 100% chemoselectivity for aldehydes and up to 98% iso-regioselectivity. The Rh(I)/phosphite catalytic system was also highly active and selective in the hydroformylation of disubstituted olefins, including (E)-prop-1-en-1-ylbenzene and prop-1-en-2-ylbenzene.

Leveraging Chiral Zr(IV)-Based Metal-Organic Frameworks To Elucidate Catalytically Active Rh Species in Asymmetric Hydrogenation Reactions

One of the most widely employed strategies to produce chiral molecules involves the asymmetric hydrogenation of functionalized olefins using rhodium catalysts. Despite their excellent performance, the exact identity of the active Rh species is still ambiguous as each site may plausibly feature one or two phosphorus ligands. In this work, we used a sequential postsynthetic modification approach to successfully incorporate single-site Rh species into a zirconium-based metal-organic framework comprised of chiral spinol-based ligands. These Rh species feature one phosphorus ligand per Rh, which contrasts with the molecular analogue that contains two phosphorus ligands per Rh site. Following extensive characterization of the Rh-monophosphorus material using techniques including solid-state NMR and extended X-ray absorption fine-structure (EXAFS) spectroscopy, we studied their catalytic performance in the asymmetric hydrogenations of enamides and α-dehydroamino acid esters and observed excellent yields and enantioselectivities (up to 99.9% ee). Notably, the Rh-monophosphorus catalyst is 5 times more active than the homogeneous Rh-biphosphorus control, which we attributed to the higher activity of the single-site Rh-monophosphorus species and the confined MOF cavities that can enrich reactants. In addition, we observed a unique topology-dependent behavior in which linker expansion leads to the formation of a novel Zr-MOF with a distinct 4,8-connected net that cannot be phosphorylated, presumably due to intense tensile strain and steric repulsion present within this framework. Finally, we demonstrate the utility of this single-site Rh-monophosphorus catalyst in the gram-scale synthesis of (R)-cinacalcet hydrochloride, a first-in-class drug in the therapy of secondary hyperparathyroidism and parathyroid carcinoma, with 99.1% ee.

Atypical and Asymmetric 1,3-P,N Ligands: Synthesis, Coordination and Catalytic Performance of Cycloiminophosphanes

Novel seven-membered cyclic imine-based 1,3-P,N ligands were obtained by capturing a Beckmann nitrilium ion intermediate generated in situ from cyclohexanone with benzotriazole, and then displacing it by a secondary phosphane under triflic acid promotion. These "cycloiminophosphanes" possess flexible non-isomerizable tetrahydroazepine rings with a high basicity; this sets them apart from previously reported iminophophanes. The donor strength of the ligands was investigated by using their P-κ1 - and P,N-κ2 -tungsten(0) carbonyl complexes, by determining the IR frequency of the trans-CO ligands. Complexes with [RhCp*Cl2 ]2 demonstrated the hemilability of the ligands, giving a dynamic equilibrium of κ1 and κ2 species; treatment with AgOTf gives full conversion to the κ2 complex. The potential for catalysis was shown in the RuII -catalyzed, solvent-free hydration of benzonitrile and the RuII - and IrI -catalyzed transfer hydrogenation of cyclohexanone in isopropanol. Finally, to enable access to asymmetric catalysts, chiral cycloiminophosphanes were prepared from l-menthone, as well as their P,N-κ2 -RhIII and a P-κ1 -RuII complexes.

Diamidophosphites from β-hydroxyamides: readily assembled ligands for Pd-catalyzed asymmetric allylic substitution

Novel diamidophosphites based on β-hydroxyamides were prepared, and their individual and in situ formed complexes were tested in Pd-mediated allylations.

Oxalamide-based bisdiamidophosphites: synthesis, coordination, and application in asymmetric metallocatalysis

A new group of P*-chiral bisdiamidophosphites has been developed for Pd- and Rh-catalyzed asymmetric reactions with good to excellent enantioselectivities.

Synthesis and application of a new chiral monodentate spiro phosphoramidite ligand based on hexamethyl-1,1'-spirobiindane backbone in asymmetric hydroamination/arylation of alkenes

The design and synthesis of a new chiral monodentate spiro phosphoramidite ligand based on a hexamethyl-1,1'-spirobiindane scaffold has been accomplished. The ligand could serve as an elegant chiral monodentate ligand in the Pd-catalyzed asymmetric hydroamination/arylation of alkenes leading to chiral imidazolidin-2-ones with good enantioselectivities.

Tailored trisubstituted chiral Cp x RhIII catalysts for kinetic resolutions of phosphinic amides

A trisubstituted chiral Cp^x ligand family is introduced.

A trisubstituted chiral Cp^x ligand family is introduced. Based on the disubstituted atropchiral Cp^x ligand scaffold, the introduction of a bulky third substituent at the central position of the Cp ring leads to substantially increased selectivities for rhodium(iii)-catalyzed kinetic resolutions and allowed for s-factors of up to 50. Their superiority is demonstrated by kinetic resolutions of phosphinic amides providing access to compounds with stereogenic phosphorus(v) atoms. The unreacted acyclic phosphinic amide and the cyclized product are both obtained in good yields and enantioselectivities. The ligand synthesis capitalizes on a late stage modification and expands the accessible ligand Cp^x ligand portfolio.

Ligand libraries for high throughput screening of homogeneous catalysts

This review describes different approaches to construct ligand libraries towards high throughput screening of homogeneous metal catalysts.

By-design enantioselective self-amplification based on non-covalent product-catalyst interactions

The synthesis of enantiomerically pure compounds is of great importance in pharmaceuticals, fragrances and biological applications, and functions as a key to many processes in nature. Asymmetric catalysis using enantiomerically pure catalysts represents an efficient synthetic method to achieve this goal. The enantiomeric excess of the reaction product correlates with the enantiomeric purity of the catalysts, except for nonlinear behaviour, therefore the use of stereochemically flexible catalysts seems to complicate the control of stereoselectivity. Self-amplifying catalytic reactions are attractive, but a general rational design is highly challenging. Here we show that product interaction with chiral recognition sites attached to structurally flexible phoshoramidite-type catalysts can sense the chirality and induce enantioselectivity in the catalyst. Structural flexibility along with sensing of the chirality of the product molecules results in a rapid increase of enantioselectivity of the dynamic catalysts (Δe.e. of up to 76%) and a shift out of equilibrium. In contrast to stereodynamic catalysts controlled with cleavable chiral auxiliaries, the enantioselectivity does not decrease.

Ligand scaffold optimization of a supramolecular hydrogenation catalyst: Analyzing the influence of key structural subunits on reactivity and selectivity

Results are reported for the catalytic asymmetric hydrogenation of two prototypical substrates with a series of more than 150 closely related supramolecular catalysts differing in only their ligand/catalyst scaffold. These modular catalysts are constructed from four subunits and vary widely in their reactivity (no reaction to quantitative yield) and enantioselectivity (racemic to 96% ee). Analysis of the ligand/catalyst scaffold optimization data reveals how each subunit contributes to the effectiveness of the modular supramolecular catalyst. The results suggest that a balance between key elements of rigidity and flexibility is required for the successful catalysts and, moreover, that this balance is required to enable effective fine-tuning via catalyst scaffold optimization.

Coordination studies on supramolecular chiral ligands and application in asymmetric hydroformylation

In this study we introduce a series of monodentate pyridine-based ligands for which the phosphorus coordination mode to rhodium can be controlled by the binding of Zn(II)-templates to the pyridyl group. A series of monodentate phosphoroamidite and phosphite ligands have been prepared and studied under hydroformylation conditions by in situ high-pressure NMR and IR techniques. These studies reveal the exclusive formation of rhodium hydride complexes in which the phosphorus atom of the ligand resides in an axial position, trans to the hydride, but only after addition of Zn(II)-template. In the absence of these templates the usual mono-coordinated rhodium hydrido complexes are formed, with the phosphorus ligated in the equatorial plane, cis to the hydride. The catalytic performance of these complexes is evaluated in asymmetric hydroformylation of unfunctionalised internal alkenes in which the supramolecular change is reflected in higher activity and selectivity.

FeCl3-PTSA Co-Catalysed Highly Regio- and Stereo-Selective Synthesis of β-Functionalised Enamine Derivatives

β-Enaminone derivatives are useful synthetic precursors. β- N-Substituted ( E)-enaminones and β-N-substituted ( E)-aminoacrylates were synthesised with high regio- and stereo-selectivity via using the co-catalytic system of FeCl3/PTSA, which also provides a new way to the formation of C–O bond.

Rhodium-catalyzed asymmetric hydrogenation of olefins with PhthalaPhos, a new class of chiral supramolecular ligands

A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (PhthalaPhos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2-acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1-yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3-phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect.

Asymmetric hydroamination catalyzed by in situ generated chiral amidate and ureate complexes of zirconium — Probing the role of the tether in ligand design

Simple chiral proligands have been synthesized from inexpensive chiral starting materials. These amidate and ureate ligands support zirconium complexes that successfully catalyze intramolecular hydroamination with up to 26% ee. Several elements necessary for successful ligand design are highlighted and discussed. In particular, the strict control of metal geometry through multidentate tethered ligands is determined to be an essential aspect of future ligand development.