Sulfonamido−Phosphoramidite Ligands in Cooperative Dinuclear Hydrogenation Catalysis

Cooperative approaches in catalytic hydrogenation and dehydrogenation

Metal-ligand cooperativity (MLC) is an established strategy for developing effective hydrogenation and dehydrogenation catalysts. Metal-metal cooperativity (MMC) in bimetallic complexes is not as well understood, and to date has had limited implementation in (de)hydrogenation. Herein we use (de)hydrogenation processes as a platform to examine modes of cooperativity, with a particular focus on catalytic mechanisms. We investigate how lessons learnt from the extensive development of metal-ligand cooperative catalysts can aid the ongoing development of metal-metal cooperative catalysts.

Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh-H intermediates

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. These were studied as models for Rh/C hydrogenation catalysts.

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds of the trop ligand. This reaction is chemoselective and converts C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H₂ activation by interfacial sites of heterogeneous Rh/C catalysts.

Stereoelectronic Effects in Ligand Design: Enantioselective Rhodium-Catalyzed Hydrogenation of Aliphatic Cyclic Tetrasubstituted Enamides and Concise Synthesis of (R)-Tofacitinib

We herein report the development of a conformationally defined, electron-rich, C₂ -symmetric, P-chiral bisphosphorus ligand, ArcPhos, by taking advantage of stereoelectronic effects in ligand design. With the Rh-ArcPhos catalyst, excellent enantioselectivities and unprecedentedly high turnovers (TON up to 10 000) were achieved in the asymmetric hydrogenation of aliphatic carbocyclic and heterocyclic tetrasubstituted enamides, to generate a series of chiral cis-2-alkyl-substituted carbocyclic and heterocyclic amine derivatives in excellent enantiomeric ratios. This method also enabled an efficient and practical synthesis of the Janus kinase inhibitor (R)-tofacitinib.

Selective Carbanion-Pyridine Coordination of a Reactive P,N Ligand to RhI

Ligands with reactive carbon sites in the periphery of a metal center have emerged as a powerful approach for metal-ligand bond activation. These reactive carbon sites are commonly generated by deprotonation strategies. Carbon-silicon bond cleavage is a potential alternative to access such constructs. Herein, the monodesilylation of bis-silyl-substituted P,N scaffold PNSi2 in the coordination sphere of [RhI (Cl)(CO)(PNSi2 )] (1) with sodium azide is disclosed. This affords a unique dinucleating anionic κ2 -C,N-κ1 -P ligand with a carbanionic methine carbon atom directly bound to rhodium as part of a four-membered Rh-N-C-C rhodacycle. This dimer undergoes meta-pyridine C-H activation facilitated by weak bases, which leads to a desymmetrization of the system and provides a σ,π-bridging 3-pyridyl fragment bound to RhI . The facile Si-C cleavage strategy may pave the way to studying the reactivity and functionalization of a variety of κ2 -C,N-coordinated pyridine scaffolds for selective transformations.

Rhodium-catalyzed asymmetric hydrogenation of tetrasubstituted β-acetoxy-α-enamido esters and efficient synthesis of droxidopa

Rhodium-catalyzed asymmetric hydrogenation of challenging tetrasubstituted β-acetoxy-α-enamido esters was developed, giving chiral β-acetoxy-α-amido esters in high yields with excellent enantioselectivities. This protocol provides an efficient route for the synthesis of droxidopa.

Features and Application in Asymmetric Catalysis of Chiral Phosphine-Phosphite Ligands

Chiral phosphine-phosphites are a class of ligands for asymmetric catalysis characterized by two coordinating functionalities with different electronic properties. These ligands also possess a highly modular structure and, due to versatile synthetic processes, they can be tuned precisely in the catalyst optimization process. Research regarding the application of these ligands in several enantioselective catalytic processes has provided outstanding results in a good number of them. These processes include not only Rh catalyzed reactions, such as olefin hydrogenation and hydroformylation, but also other reactions, such as hydrogenation of olefins and imines by Ru complexes, of imines and N-heterocycles by Ir derivatives, allyl alkylation or conjugate addition by Cu catalysts, or hydrocyanation of olefins by Ni ones. Overall, the use of these ligands has led to the preparation of a wide variety of chiral building blocks with high enantiomeric excess. Therefore, phosphine-phosphites have become in an important class of ligands in asymmetric catalysis.

Hydrogenation of CO2 to formic acid with iridiumIII(bisMETAMORPhos)(hydride): the role of a dormant fac-IrIII(trihydride) and an active trans-IrIII(dihydride) species

Catalytic hydrogenation of CO₂ to formate with an Ir^III(METAMORPhos) complex in the presence of DBU requires a trans-dihydride for catalytic turnover, with an off-cycle trihydride as the dormant species.

Intermolecular C–H activation with an Ir-METAMORPhos piano-stool complex – multiple reaction steps at a reactive ligand

Reaction of phenylacetylene with an Ir^III(METAMORPhos) complex generates a unique four-membered IrPNC metallacyclic species with an exocyclic CC fragment.

Dehydrogenation of formic acid by Ir-bisMETAMORPhos complexes: experimental and computational insight into the role of a cooperative ligand

The synthesis of Ir-complexes with three bisMETAMORPhos ligands is reported. The activity of these systems towards HCOOH dehydrogenation and the dual role of the ligand during catalysis is discussed, using spectroscopic and computational methods.

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands (La–Lc) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir^I(L^H) species (1a), which convert via formal oxidative addition of the ligand to Ir^III(L) monohydride complexes 2a–c. The rate for this step strongly depends on the ligand employed. Ir^III complexes 2a–c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h^–1, respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by ¹H and ³¹P NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogen-bonding.

Transition metal-catalyzed enantioselective hydrogenation of enamides and enamines

Transition metal-catalyzed enantioselective hydrogenation of enamides and enamines is one of the most important methods for the preparation of optically active amines. This review describes the recent developments of highly efficient catalytic asymmetric hydrogenation of enamides, and enamines. It specifically focuses on the substrates because hydrogenation of enamides and enamines is highly dependent on the substrates although the chiral metal catalysts play a significant role.

Exchanging conformations of a hydroformylation catalyst structurally characterized using two-dimensional vibrational spectroscopy

Catalytic transition-metal complexes often occur in several conformations that exchange rapidly (<ms) in solution so that their spatial structures are difficult to characterize with conventional methods. Here, we determine specific bond angles in the two rapidly exchanging solution conformations of the hydroformylation catalyst (xantphos)Rh(CO)2H using two-dimensional vibrational spectroscopy, a method that can be applied to any catalyst provided that the exchange between its conformers occurs on a time scale of a few picoseconds or slower. We find that, in one of the conformations, the OC-Rh-CO angle deviates significantly from the canonical value in a trigonal-bipyramidal structure. On the basis of complementary density functional calculations, we ascribe this effect to attractive van der Waals interaction between the CO and the xantphos ligand.

Polymetallic complexes linked to a single-frame ligand: cooperative effects in catalysis

In an eco-friendly development, catalysis represents the key approach for converting raw materials into valuable products, particularly convenient for multi-step transformations. Mimicking Nature seems the smartest way to design poly-functional assemblies leading to the design of "synergic" catalysts. This perspective focuses on the advances in the conception of polymetallic catalysts bonded to a single ligand frame.