Asymmetric Hydrogenation of Olefins Using Chiral Crabtree-type Catalysts: Scope and Limitations

Adaptable P-X Biaryl Phosphite/Phosphoroamidite-Containing Ligands for Asymmetric Hydrogenation and C-X Bond-Forming Reactions: Ligand Libraries with Exceptionally Wide Substrate Scope

In this personal review, we present our efforts in the design of ligand libraries for the discovery of suitable metal catalysts for asymmetric hydrogenation and C-X bond-forming reactions.

Iridium-Catalyzed Selective Isomerization of Primary Allylic Alcohols

This Account presents the development of the iridium-catalyzed isomerization of primary allylic alcohols in our laboratory over the past 8 years. Our initial interest was driven by the long-standing challenge associated with the development of a general catalyst even for the nonasymmetric version of this seemingly simple chemical transformation. The added value of the aldehyde products and the possibility to rapidly generate molecular complexity from readily accessible allylic alcohols upon a redox-economical isomerization reaction were additional sources of motivation. Certainly influenced by the success story of the related isomerization of allylic amines, most catalysts developed for the selective isomerization of allylic alcohols were focused on rhodium as a transition metal of choice. Our approach has been based on the commonly accepted precept that hydrogenation and isomerization are often competing processes, with the latter being usually suppressed in favor of the former. The cationic iridium complexes [(Cy3P)(pyridine)Ir(cod)]X developed by Crabtree (X = PF6) and Pfaltz (X = BArF) are usually considered as the most versatile catalysts for the hydrogenation of allylic alcohols. Using molecular hydrogen to generate controlled amounts of the active form of these complexes but performing the reaction in the absence of molecular hydrogen enabled deviation from the typical hydrogenation manifold and favored exclusively the isomerization of allylic alcohols into aldehydes. Isotopic labeling and crossover experiments revealed the intermolecular nature of the process. Systematic variation of the ligand on the iridium center allowed us to identify the structural features beneficial for catalytic activity. Subsequently, three generations of chiral catalysts have been investigated and enabled us to reach excellent levels of enantioselectivity for a wide range of 3,3-disubstituted aryl/alkyl and alkyl/alkyl primary allylic alcohols leading to β-chiral aldehydes. The combination of the isomerization reaction with enamine catalysis in a sequential process gave access to α,β-chiral aldehydes in high diastereomeric ratio and excellent enantioselectivity. Catalyst-controlled diastereoselective isomerization of stereochemically complex steroid scaffolds has been achieved, giving access indifferently to derivatives with the natural and unnatural C20 configuration, a long-standing challenge in the field. Structural diversification at close proximity of the reactive site and within the polycyclic domain served to further demonstrate the generality and the potential of the method. Models based on quadrant diagrams enabled rationalization of the high levels of enantio- and diastereocontrol obtained in the isomerization of allylic alcohols.

Conjugate addition-enantioselective protonation reactions

The addition of nucleophiles to electron-deficient alkenes represents one of the more general and commonly used strategies for the convergent assembly of more complex structures from simple precursors. In this review the addition of diverse protic and organometallic nucleophiles to electron-deficient alkenes followed by enantioselective protonation is summarized. Reactions are first categorized by the type of electron-deficient alkene and then are further classified according to whether catalysis is achieved with chiral Lewis acids, organocatalysts, or transition metals.

NeoPHOX - a structurally tunable ligand system for asymmetric catalysis

A synthesis of new NeoPHOX ligands derived from serine or threonine has been developed. The central intermediate is a NeoPHOX derivative bearing a methoxycarbonyl group at the stereogenic center next to the oxazoline N atom. The addition of methylmagnesium chloride leads to a tertiary alcohol, which can be acylated or silylated to produce NeoPHOX ligands with different sterical demand. The new NeoPHOX ligands were tested in the iridium-catalyzed asymmetric hydrogenation and palladium-catalyzed allylic substitution. In both reactions high enantioselectivities were achieved, that were comparable to the enantioselectivities obtained with the up to now best NeoPHOX ligand derived from expensive tert-leucine.

Carbohydrates as enantioinduction components in stereoselective catalysis

Carbohydrate derivatives are readily available chiral molecules, yet they are infrequently employed as enantioinduction components in stereoselective catalysis. In this review, synthetic approaches to carbohydrate-based ligands and catalysts are outlined, along with example applications in enantioselective catalysis. A wide range of carbohydrate-based functionality is covered, and key trends and future opportunities are identified.

Chloride-Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral Diphosphine Ligands: Catalyst Precursors for Asymmetric Hydrogenation of Simple Olefins

Efficient rhodium(III) catalysts were developed for asymmetric hydrogenation of simple olefins. A new series of chloride-bridged dinuclear rhodium(III) complexes 1 were synthesized from the rhodium(I) precursor [RhCl(cod)]2 , chiral diphosphine ligands, and hydrochloric acid. Complexes from the series acted as efficient catalysts for asymmetric hydrogenation of (E)-prop-1-ene-1,2-diyldibenzene and its derivatives without any directing groups, in sharp contrast to widely used rhodium(I) catalytic systems that require a directing group for high enantioselectivity. The catalytic system was applied to asymmetric hydrogenation of allylic alcohols, alkenylboranes, and unsaturated cyclic sulfones. Control experiments support the superiority of dinuclear rhodium(III) complexes 1 over typical rhodium(I) catalytic systems.

Iron-Catalyzed Regioselective Transfer Hydrogenative Couplings of Unactivated Aldehydes with Simple Alkenes

An FeBr3 -catalyzed reductive coupling of various aldehydes with alkenes that proceeds through a direct hydride transfer pathway has been developed. With (i) PrOH as the hydrogen donor under mild conditions, previously challenging coupling reactions of unactivated alkyl and aryl aldehydes with simple alkenes, such as styrene derivatives and α-olefins, proceeded smoothly to furnish a diverse range of functionalized alcohols with complete linear regioselectivity.

A General Route to β-Substituted Pyrroles by Transition-Metal Catalysis

An atom-efficient route to pyrroles substituted in the β-position has been achieved in four high yielding steps by a combination of Pd, Ru, and Fe catalysis with only water and ethene as side-products. The reaction is general and gives pyrroles substituted in the β-position with linear and branched alkyl, benzyl, or aryl groups in overall good yields. The synthetic route includes a Pd-catalyzed monoallylation step of amines with substituted allylic alcohols that proceeds to yield the monoallylated products in moderate to excellent yields. In a second step, unsymmetrical diallylated aromatic amines are generated from the reaction of a second allylic alcohol with high selectivity in moderate to good yields by control of the reaction temperature. Ru-catalyzed ring-closing metathesis performed on the diallylated aromatic amines yields the pyrrolines substituted in the β-position in excellent yields. By addition of ferric chloride to the reaction mixture, a selective aromatization to yield the corresponding pyrroles substituted in the β-position was achieved. A reaction mechanism involving a palladium hydride, generated from insertion of palladium to O-H of an allyl alcohol, that is responsible for the C-O bond cleavage to generate the π-allyl intermediate is proposed.

Synthesis of chiral lactams via asymmetric hydrogenation of α,β-unsaturated nitriles

A highly efficient Rh-catalyzed asymmetric hydrogenation of α,β-unsaturated nitriles containing ester/amide groups is reported affording chiral nitriles with excellent enantioselectivities (up to 99.7% ee) and high activity (TON up to 10 000).

Highly efficient asymmetric hydrogenation of cyano-substituted acrylate esters for synthesis of chiral γ-lactams and amino acids

A highly efficient Rh-catalyzed asymmetric hydrogenation of 3-cyano acrylate esters is reported for the first time, which provides straightforward access to chiral cyano compounds with excellent enantioselectivities (up to 98% ee) and high turnover numbers (TON up to 10 000).

Photoorganocatalysed and visible light photoredox catalysed trifluoromethylation of olefins and (hetero)aromatics in batch and continuous flow

Trifluoromethylation of olefins and (hetero)aromatics with sodium triflinate as CF₃source and readily accessible benzophenone derivatives as photosensitisers has been developed in batch and flow.

Highly enantioselective hydrogenation of α-oxy functionalized α,β-unsaturated acids catalyzed by a ChenPhos–Rh complex in CF3CH2OH

A Chenphos–Rh complex is demonstrated to be a highly efficient catalyst for asymmetric hydrogenation of α-oxy functionalized α,β-unsaturated carboxylic acidsviaan ionic interaction between the ligand and the substrate.

Mechanistic interpretation of selective catalytic hydrogenation and isomerization of alkenes and dienes by ligand deactivated Pd nanoparticles

Unsupported thiolate-capped palladium nanoparticle catalysts are found to be highly substrate-selective for alkene hydrogenation and isomerization. Steric and poisoning effects from thiolate ligands on the nanoparticle surface control reactivity and selectivity by influencing alkene adsorption and directing either di-σ or mono-σ bond formation. The presence of overlapping p orbitals and α protons in alkenes greatly influences the catalytic properties of deactivated palladium nanoparticles leading to easily predictable hydrogenation or isomerization products.

A Fluorescent Molecular Probe for the Detection of Hydrogen Based on Oxidative Addition Reactions with Crabtree-Type Hydrogenation Catalysts

A Crabtree-type Ir(I) complex tagged with a fluorescent dye (bodipy) was synthesized. The oxidative addition of H2 converts the weakly fluorescent Ir(I) complex (Φ=0.038) into a highly fluorescent Ir(III) species (Φ=0.51). This fluorogenic reaction can be utilized for the detection of H2 and to probe the oxidative addition step in the catalytic hydrogenation of olefins.

Synthesis and Exploration of Electronically Modified (R)-5,5-Dimethyl-(p-CF3)3-i-PrPHOX in Palladium-Catalyzed Enantio- and Diastereoselective Allylic Alkylation: A Practical Alternative to (R)-(p-CF3)3-t-BuPHOX

The synthesis of the novel electronically modified phosphinooxazoline (PHOX) ligand, (R)-5,5-dimethyl-(p-CF₃)₃-i-PrPHOX, is described. The utility of this PHOX ligand is explored in both enantio- and diastereoselective palladium-catalyzed allylic alkylations. These investigations prove (R)-5,5-dimethyl-(p-CF₃)₃-i-PrPHOX to be an effective and cost-efficient alternative to electronically modified PHOX ligands derived from the prohibitively expensive (R)-t-leucine.

Asymmetric hydrogenation of aromatic ketones by new recyclable ionic tagged ferrocene-ruthenium catalyst system

Newly developed ferrocene-oxazoline-phosphine ligands containing quaternary ammonium ionic groups exhibited excellent catalytic performance for the ruthenium-catalyzed hydrogenation of aromatic ketonic substrates to give chiral secondary alcohols with high levels of conversions and enantioselectivities. Simple manipulation process, water tolerance, high activity and good recyclable property make this catalysis practical and appealing.

Comparative Assessment of DFT Performances in Ru- and Rh-Promoted σ-Bond Activations

In this work, the performances of 19 density functional theory (DFT) methods are calibrated comparatively on Ru- and Rh-promoted σ-bond (C-H, O-H, and H-H) activations. DFT calibration reference is generated from explicitly correlated coupled cluster CCSD(T)-F12 calculations, and the 4s4p core-valence correlation effect of the two 4d platinum group transition metals is also included. Generally, the errors of DFT methods for calculating energetics of Ru-/Rh-mediated reactions appear to correlate more with the magnitude of energetics itself than other factors such as metal identity. For activation energy calculations, the best performing functionals for both Ru and Rh systems are MN12SX < CAM-B3LYP < M06-L < MN12L < M06 < ωB97X < B3LYP < LC-ωPBE (in the order of increasing mean unsigned deviations, MUDs, of less than 2 kcal/mol). For reaction energy calculations, best functionals with MUDs less than 2 kcal/mol are PBE0 < CAM-B3LYP ≈ N12SX. The effect of the DFT empirical dispersion correction on the performance of the DFT methods is beneficial for most density functionals tested in this work, reducing their MUDs to different extents. After including empirical dispersion correction, ωB97XD, B3LYP-D3, and CAM-B3LYP-D3 (PBE0-D3, B3LYP-D3, and ωB97XD) are the three best performing DFs for activation energy (reaction energy) calculations, from which B3LYP-D3 and ωB97XD can notably be recommended uniformly for both the reaction energy and reaction barrier calculations. The good performance of B3LYP-D3 in quantitative description of the energetic trends further adds value to B3LYP-D3 and singles this functional out as a reasonable choice in the Ru/Rh-promoted σ-bond activation processes.

Enantioselective desymmetrization via carbonyl-directed catalytic asymmetric hydroboration and Suzuki-Miyaura cross-coupling

The rhodium-catalyzed enantioselective desymmetrization of symmetric γ,δ-unsaturated amides via carbonyl-directed catalytic asymmetric hydroboration (directed CAHB) affords chiral secondary organoboronates with up to 98% ee. The chiral γ-borylated products undergo palladium-catalyzed Suzuki-Miyaura cross-coupling via the trifluoroborate salt with stereoretention.