Highly Selective Hydrogenation of Aromatic Ketones and Phenols Enabled by Cyclic (Amino)(alkyl)carbene Rhodium Complexes

A Dual Cobalt-Photoredox Catalytic Approach for Asymmetric Dearomatization of Indoles with Aryl Amides via C-H Activation

In this study, we unveil a novel method for the asymmetric dearomatization of indoles under cobalt/photoredox catalysis. By strategically activating C-H bonds of amides and subsequent migratory insertion of π-bonds present in indole as reactive partner, we achieve syn-selective tetrahydro-5H-indolo[2,3-c]isoquinolin-5-one derivatives with excellent yields and enantiomeric excesses of up to >99 %. The developed method operates without a metal oxidant, relying solely on oxygen as the oxidant and employing an organic dye as a photocatalyst under irradiation. Control experiments and stoichiometric studies elucidate the reversible nature of the enantiodetermining C-H activation step, albeit not being rate-determining. This study not only expands the horizon of cobalt-catalyzed asymmetric C-H bond functionalization, but also showcases the potential synergy between cobalt and photoredox catalysis in enabling asymmetric synthesis of complex molecules.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

Pd/C-Catalyzed Stereoselective Arene Hydrogenation of Benzocyclobutenes Enabled by π-Bond Localization

We developed here a Pd/C-catalyzed diastereoselective cis-hydrogenation of benzocyclobutene derivatives under mild conditions to deliver an array of bicyclo[4.2.0]octane scaffolds with up to five stereocenters. The π-bond localization enabled hydrogenation of the arene moiety to occur even at room temperature under 1 atm of a H2 atmosphere.

Ring Contraction by Rearrangement of Sterically Congested Cyclic (Amino)(aryl)carbenes

The rearrangement of sterically congested cyclic (amino)(aryl)carbenes (CAArCs) by the reaction of related iminium salts with potassium bis(trimethylsilyl)amide is reported, allowing for forming benzocyclobutanimines via a ring contraction process. Mechanistic studies by theoretical calculations indicate that the formation of conjugated ketenimines as intermediates could be considered, in which steric hindrance caused by N-alkyl motifs of CAArCs plays an important role in promoting the ring-opening by the cleavage of C-N bond.

Arene Reduction by Rh/Pd or Rh/Pt under 1 atm Hydrogen Gas and Room Temperature

Current methods for arene hydrogenation generally need either harsh reaction conditions or complex catalyst preparation. Here we describe a mild and convenient protocol that only utilizes commercially available catalysts. Using [Rh(nbd)Cl]2 and Pd/C together as catalysts, arenes bearing various functional groups can be hydrogenated under 1 atm of H2 at room temperature. This arene hydrogenation can also be achieved using catalysts of [Rh(cod)Cl]2 and PtO2, thus avoiding glovebox manipulations and simplifying the reaction procedure.

Chemoselective Heterogeneous Hydrogenation of Sulfur Containing Quinolines under Mild Conditions

Sulfur, alongside oxygen and nitrogen, holds a prominent position as one of the key heteroatoms in nature and medicinal chemistry. Its significance stems from its ability to adopt different oxidation states, rendering it valuable as both a polarity handle and a hydrogen bond donor/acceptor. Nevertheless, the poisonous nature of its free electron pairs makes sulfur containing substrates inaccessible for many catalytic protocols. Strong and (at low temperatures) irreversible chemisorption to the catalyst's surface is in particular detrimental for heterogeneous catalysts, possessing only few catalytically active sites. Herein, we present a novel heterogeneous Ru-S catalyst that tolerates multiple sulfur functionalities, including thioethers, thiophenes, sulfoxides, sulfones, sulfonamides, and sulfoximines, in the hydrogenation of quinolines. The utility of the products was further demonstrated by subsequent diversifications of the sulfur functionalities.

Hydrogenation of functionalised pyridines with a rhodium oxide catalyst under mild conditions

Piperidines are one of the most widely used building blocks in the synthesis of pharmaceutical and agrochemical compounds. The hydrogenation of pyridines is a convenient method to synthesise such compounds as it only requires reactant, catalyst, and a hydrogen source. However, this reaction still remains difficult for the reduction of functionalised and multi-substituted pyridines. Here we report the use of a stable, commercially available rhodium compound, Rh2O3, for the reduction of various unprotected pyridines. The reaction only requires mild conditions, and the substrate scope is broad, making it practically useful.

Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

(Hetero)arene reduction is one of the key avenues for synthesizing related cyclic alkenes and alkanes. While catalytic hydrogenation and Birch reduction are the two broadly utilized approaches for (hetero)arene reduction across academia and industry over the last century, both methods have encountered significant chemoselectivity challenges. We hereby introduce a highly chemoselective quinoline and isoquinoline reduction protocol operating through selective energy transfer (EnT) catalysis, which enables subsequent hydrogen atom transfer (HAT). The design of this protocol bypasses the conventional metric of reduction reaction, that is, the reductive potential, and instead relies on the triplet energies of the chemical moieties and the kinetic barriers of energy and hydrogen atom transfer events. Many reducing labile functional groups, which were incompatible with previous (hetero)arene reduction reactions, are retained in this reaction. We anticipate that this protocol will trigger the further advancement of chemoselective arene reduction and enable the current arene-rich drug space to escape from flatland.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

Arene Insertion with Pincer-Supported Molybdenum-Hydrides: Determination of Site Selectivity, Relative Rates, and Arene Complex Formation

The synthesis of phosphino(oxazoline)pyridine-supported molybdenum(0) cycloocta-1,5-diene complexes is described. Exposure of these complexes to dihydrogen in the presence of an arene resulted in insertion of the substrate into the molybdenum hydride bond and afforded the corresponding molybdenum cyclohexadienyl hydrides. For mono- and disubstituted arenes, the site selectivity for insertion of the most substituted bond increases with increasing size of the substituent from methyl to ethyl, iso-propyl, and tert-butyl. In contrast, 1,3,5-trisubstituted arenes underwent insertion with exclusive site selectivity. Relative rates of insertion were determined by competition experiments and established faster insertions for electron-rich arenes. Introduction of electron-withdrawing trifluoromethyl groups on the arene resulted in decreased relative rates of insertion and an increased rate for H2 reductive elimination, favoring formation of the corresponding molybdenum η6-arene complex. Studies on the reductive elimination of the cyclohexadienyl ligand with the hydride enabled the synthesis of an enantioenriched cyclohexa-1,3-diene. This study provides new insights into the ligand requirements for catalytic arene hydrogenation and a new strategy for selective arene reduction.

Ligand-Free Ultrasmall Recyclable Iridium(0) Nanoparticles for Regioselective Aromatic Hydrogenation of Phosphine Oxide Scaffolds: An Easy Access to New Phosphine Ligands

Herein, we developed the recyclable ligand-free iridium (Ir)-hydride based Ir0 nanoparticles (NPs) for the first regioselective partial hydrogenation of PV -substituted naphthalenes. Both the isolated and in situ generated NPs are catalytically active. A control nuclear magnetic resonance (NMR) study revealed the presence of metal-surface-bound hydrides, most likely formed from Ir0 species. A control NMR study confirmed that hexafluoroisopropanol as a solvent was accountable for substrate activation via hydrogen bonding. High-resolution transmission electron microscopy of the catalyst supports the formation of ultrasmall NPs, and X-ray photoelectron spectroscopy confirmed the dominance of Ir0 in the NPs. The catalytic activity of NPs is broad as showcased by highly regioselective aromatic ring reduction in various phosphine oxides or phosphonates. The study also showcased a novel pathway toward preparing bis(diphenylphosphino)-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl (H8 -BINAP) and its derivatives without losing enantioselectivity during catalytic events.

Organophotocatalysed synthesis of 2-piperidinones in one step via [1 + 2 + 3] strategy

Six-membered N-containing heterocycles, such as 2-piperidinone derivatives, with diverse substitution patterns are widespread in natural products, drug molecules and serve as key precursors for piperidines. Thus, the development of stereoselective synthesis of multi-substituted 2-piperidinones are attractive. However, existing methods heavily rely on modification of pre-synthesized backbones which require tedious multi-step procedure and suffer from limited substitution patterns. Herein, an organophotocatalysed [1 + 2 + 3] strategy was developed to enable the one-step access to diverse substituted 2-piperidinones from easily available inorganic ammonium salts, alkenes, and unsaturated carbonyl compounds. This mild protocol exhibits exclusive chemoselectivity over two alkenes, tolerating both terminal and internal alkenes with a wide range of functional groups.

The Emergence and Properties of Selectively Fluorinated 'Janus' Cyclohexanes

This account describes the evolution of a research programme that started by linking fluoromethylene (-CHF-) groups along aliphatic chains and then progressing to alicyclic rings with contiguous fluorine atoms. Different stereoisomers of aliphatic chains tend to adopt low polarity conformations. In order to force polar conformations, the programme began to address ring systems and in particular cyclohexanes, to restrain conformational freedom and co-aligned C-F bonds. The flagship molecule, all-cis-1,2,3,4,5,6-hexafluorocyclohexane 7, emerged to be the most polar aliphatic compound recorded. The polarity arises because there are three co-aligned triaxial C-F bonds and the six fluorines occupy one face of the ring. Conversely the electropositive hydrogens occupy the other face. These have been termed Janus face cyclohexanes after the Roman god with two faces. The review outlines progress by our group and others in preparing derivatives of the parent cyclohexane 7, in order to explore properties and potential applications of these Janus cyclohexanes.

ItOct (ItOctyl) - pushing the limits of ItBu: highly hindered electron-rich N-aliphatic N-heterocyclic carbenes

ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene) represents the most important and most versatile N-alkyl N-heterocyclic carbene available in organic synthesis and catalysis. Herein, we report the synthesis, structural characterization and catalytic activity of ItOct (ItOctyl), C₂-symmetric, higher homologues of ItBu. The new ligand class, including saturated imidazolin-2-ylidene analogues has been commercialized in collaboration with MilliporeSigma: ItOct, 929 298; SItOct, 929 492 to enable broad access of the academic and industrial researchers within the field of organic and inorganic synthesis. We demonstrate that replacement of the t-Bu side chain with t-Oct results in the highest steric volume of N-alkyl N-heterocyclic carbenes reported to date, while retaining the electronic properties inherent to N-aliphatic ligands, such as extremely strong σ-donation crucial to the reactivity of N-alkyl N-heterocyclic carbenes. An efficient large-scale synthesis of imidazolium ItOct and imidazolinium SItOct carbene precursors is presented. Coordination chemistry to Au(i), Cu(i), Ag(i) and Pd(ii) as well as beneficial effects on catalysis using Au(i), Cu(i), Ag(i) and Pd(ii) complexes are described. Considering the tremendous importance of ItBu in catalysis, synthesis and metal stabilization, we anticipate that the new class of ItOct ligands will find wide application in pushing the boundaries of new and existing approaches in organic and inorganic synthesis.

Reactivities of α-Oxo BMIDA Gold Carbenes Generated by Gold-Catalyzed Oxidation of BMIDA-Terminated Alkynes

An oxidative strategy is reported to access α-oxo BMIDA gold carbenes directly from BMIDA-terminated alkynes. Besides offering expedient access to seldom studied boryl metal carbenes, these BMIDA gold carbene species undergo facile insertions into methyl, methylene, methine, and benzylic C-H bonds in the absence of the Thorpe-Ingold effect. They also undergo efficient OH insertion, cyclopropanation, and F-C alkylations. This chemistry provides rapid access to structurally diverse α-BMIDA ketones, which are scarcely documented. In combination with DFT studies, the role of BMIDA is established to be an electron-donating group that attenuates the high electrophilicity of the gold carbene center.

Chromium-catalyzed stereodivergent E- and Z-selective alkyne hydrogenation controlled by cyclic (alkyl)(amino)carbene ligands

The hydrogenation of alkynes allows the synthesis of olefins, which are important feedstock for the materials, pharmaceutical, and petrochemical industry. Thus, methods that enable this transformation via low-cost metal catalysis are desirable. However, achieving stereochemical control in this reaction is a long-standing challenge. Here, we report on the chromium-catalyzed E- and Z-selective olefin synthesis via hydrogenation of alkynes, controlled by two carbene ligands. A cyclic (alkyl)(amino)carbene ligand that contains a phosphino anchor enables the hydrogenation of alkynes in a trans-addition manner, selectively forming E-olefins. With an imino anchor-incorporated carbene ligand, the stereoselectivity can be switched, giving mainly Z-isomers. This ligand-enabled geometrical stereoinversion strategy by one metal catalysis overrides common methods in control of the E- and Z-selectivity with two different metal catalysis, allowing for highly efficient and on-demand access to both E- and Z-olefins in a stereo-complementary fashion. Mechanistic studies indicate that the different steric effect between these two carbene ligands may mainly dominate the selective forming E- or Z-olefins in control of the stereochemistry.

cis-Selective Hydrogenation of Aryl Germanes: A Direct Approach to Access Saturated Carbo- and Heterocyclic Germanes

A catalytic approach of synthesizing the cis-selective saturated carbo- and heterocyclic germanium compounds (3D framework) is reported via the hydrogenation of readily accessible aromatic germanes (2D framework). Among the numerous catalysts tested, Nishimura's catalyst (Rh2O3/PtO2·H2O) exhibited the best hydrogenation reactivity with an isolated yield of up to 96%. A broad range of substrates including the synthesis of unprecedented saturated heterocyclic germanes was explored. This selective hydrogenation strategy could tolerate several functional groups such as -CF3, -OR, -F, -Bpin, and -SiR3 groups. The synthesized products demonstrated the applications in coupling reactions including the newly developed strategy of aza-Giese-type addition reaction (C-N bond formation) from the saturated cyclic germane product. These versatile motifs can have a substantial value in organic synthesis and medicinal chemistry as they show orthogonal reactivity in coupling reactions while competing with other coupling partners such as boranes or silanes, acquiring a three-dimensional structure with high stability and robustness.

Homogeneous Dearomative Hydrogenation with a Co/P4 N2 Catalyst: A Nucleophilic Approach

Arene hydrogenation is the most straightforward method to prepare carbo- and heterocycles. However, the high resonance energy prevents aromatic substrates from hydrogenation. Herein the homogeneous, nucleophilic hydrogenation of less electron-rich arenes and heteroarenes is reported. The Co(P₄ N₂ )H species that has been demonstrated to be a strong hydride donor could deliver a hydride ion to the cyano (hetero)arene substrates. Deuterium labeling experiments supported a Michael-type reaction pathway. Theoretical analyses have been conducted to investigate the hydricity of the catalytically active CoH species and the electrophilicity of the arene substrates. An outlook for the synthesis of more challenging substituted benzenes was proposed based on the in silico modification of the CoH species.

Identification of Cyclohexadienyl Hydrides as Intermediates in Molybdenum-Catalyzed Arene Hydrogenation

Treatment of phosphino(imino)pyridine (PIP) molybdenum cyclooctadiene (COD) complexes [(PIP)Mo(COD)] with dihydrogen in the presence of benzene selectively furnished the molybdenum cyclohexadienyl hydrides [(PIP)MoH(η⁵ -C₆ H₇ )], which are precatalysts for the hydrogenation of benzene to cyclohexane. [(PIP)MoH(η⁵ -C₆ H₇ )] arises from a rarely observed insertion of benzene into a molybdenum-hydride bond, a key step in the molybdenum-catalyzed homogeneous hydrogenation of arenes. The reaction with toluene afforded a single isomer of the corresponding molybdenum cyclohexadienyl hydride while para-xylene predominantly formed the molybdenum η⁶ -arene complex with the insertion product being a minor component. Addition of carbon monoxide to a cyclohexane-d₁₂ solution of [(PIP)MoH(η⁵ -C₆ H₇ )] liberated cyclohexadiene, providing experimental support for a higher kinetic barrier for the subsequent steps en route to cycloalkanes.

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Access to Unexplored 3D Chemical Space: cis ‐Selective Arene Hydrogenation for the Synthesis of Saturated Cyclic Boronic Acids

A new class of saturated boron‐incorporated cyclic molecules has been synthesized employing an arene‐hydrogenation methodology. cis‐Selective hydrogenation of easily accessible, and biologically important molecules comprising benzoxaborole, benzoxaborinin, and benzoxaboripin derivatives is reported. Among the various catalysts tested, rhodium cyclic(alkyl)(amino)carbene [Rh‐CAAC] (1) pre‐catalyst revealed the best hydrogenation activity confirming turnover number up to 1400 with good to high diastereoselectivity. A broad range of functional groups was tolerated including sensitive substituents such as −F, −CF₃, and −silyl groups. The utility of the synthesized products was demonstrated by the recognition of diols and sugars under physiological conditions. These motifs can have a substantial importance in medicinal chemistry as they possess a three‐dimensional structure, are highly stable, soluble in water, form hydrogen bonds, and interact with diols and sugars.

Molybdenum-Catalyzed Asymmetric Hydrogenation of Fused Arenes and Heteroarenes

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenum-catalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo- and enantioselectivities. For quinoline derivatives, selective hydrogenation of both the carbocycle and heterocycle was observed depending on the ring substitution. Spectroscopic and mechanistic studies established molybdenum η⁶-arene complexes as the catalyst resting state and that partial hydrogenation arises from dissociation of the substrate from the coordination sphere of molybdenum prior to complete reduction. A stereochemical model is proposed based on the relative energies of the respective coordination of the prochiral faces of the arene determined by steric interactions between the substrate and the chiral ligand, rather than through precoordination by a heteroatom.

Janus faced fluorocyclohexanes for supramolecular assembly: synthesis and solid state structures of equatorial mono-, di- and tri alkylated cyclohexanes and with tri-axial C-F bonds to impart polarity

Concise and general synthesis protocols are reported to generate all-syn mono-, di- and tri-alkylated cyclohexanes where a single fluorine is located on the remaining carbons of the ring. The alkyl groups are positioned to lie equatorially and to have triaxial C-F bonds imparting polarity to these ring systems. Intermolecular electrostatic interactions in the solid-state structure of the trialkylated systems are explored and the resultant supramolecular order opens up prospects for design in soft materials.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

Asymmetric Full Saturation of Vinylarenes with Cooperative Homogeneous and Heterogeneous Rhodium Catalysis

Homogeneous and heterogeneous catalyzed reactions can seldom operate synergistically under the same conditions. Here we communicate the use of a single rhodium precursor that acts in both the homogeneous and heterogeneous phases for the asymmetric full saturation of vinylarenes that, to date, constitute an unmet bottleneck in the field. A simple asymmetric hydrogenation of a styrenic olefin, enabled by a ligand accelerated effect, accounted for the facial selectivity in the consecutive arene hydrogenation. Tuning the ratio between the phosphine ligand and the rhodium precursor controlled the formation of homogeneous and heterogeneous catalytic species that operate without interference from each other. The system is flexible in terms of both the chiral ligand and the nature of the external olefin. We anticipate that our findings will promote the development of asymmetric arene hydrogenations.

Janus All-Cis 2,3,4,5,6-Pentafluorocyclohexyl Building Blocks Applied to Medicinal Chemistry and Bioactives Discovery Chemistry

Monoalkylated derivatives of the unusually polar all-cis 2,3,4,5,6- pentafluorocyclohexyl (Janus face) motif are prepared starting from an aryl hydrogenation of 2,3,4,5,6- pentafluorophenylacetate methyl ester 15. The method used Zeng's Rh(CAAC) carbene catalyst 4 in the hydrogenation following the protocol developed by Glorius. The resultant Janus pentafluorocyclohexylacetate methyl ester 16 was converted to the corresponding alcohol 18, aldehyde 13, bromide 29 and azide 14 through functional group manipulations, and some of these building blocks were used in Ugi-multicomponent and Cu-catalysed click reactions. NBoc protected pentafluoroarylphenylalanine methyl ester 35 was also subject to an aryl hydrogenation, and then deprotection to generate the Janus face β-pentafluorocyclohexyl-alanine amino acid 15, which was incorporated into representative members of an emerging class of candidate antiviral compounds. Log P measurements demonstrate that the all-cis 2,3,4,5,6-pentafluorocyclohexyl ring system is more polar than a phenyl ring. In overview the paper introduces new building blocks containing this Janus ring and demonstrates their progression to molecules typically used in bioactives discovery programmes.

Ru-NHC-Catalyzed Asymmetric Hydrogenation of 2-Quinolones to Chiral 3,4-Dihydro-2-Quinolones

Direct enantioselective hydrogenation of unsaturated compounds to generate chiral three-dimensional motifs is one of the most straightforward and important approaches in synthetic chemistry. We realized the Ru(II)-NHC-catalyzed asymmetric hydrogenation of 2-quinolones under mild reaction conditions. Alkyl-, aryl- and halogen-substituted optically active dihydro-2-quinolones were obtained in high yields with moderate to excellent enantioselectivities. The reaction provides an efficient and atom-economic pathway to construct simple chiral 3,4-dihydro-2-quinolones. The desired products could be further reduced to tetrahydroquinolines and octahydroquinolones.

IPr# - highly hindered, broadly applicable N-heterocyclic carbenes

IPr (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represents the most important NHC (NHC = N-heterocyclic carbene) ligand throughout the field of homogeneous catalysis. Herein, we report the synthesis, catalytic activity, and full structural and electronic characterization of novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept, including IPr#, Np# and BIAN-IPr#. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420, enabling broad access of the academic and industrial researchers to new ligands for reaction optimization and screening. In particular, the synthesis of IPr# hinges upon cost-effective, modular alkylation of aniline, an industrial chemical that is available in bulk. The generality of this approach in ligand design is demonstrated through facile synthesis of BIAN-IPr# and Np#, two ligands that differ in steric properties and N-wingtip arrangement. The broad activity in various cross-coupling reactions in an array of N–C, O–C, C–Cl, C–Br, C–S and C–H bond cross-couplings is demonstrated. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(i), Rh(i) and Pd(ii) is presented. Given the tremendous importance of NHC ligands in homogenous catalysis, we expect that this new class of NHCs will find rapid and widespread application.

We report novel, sterically-bulky, easily-accessible NHC ligands based on the hash peralkylation concept. The new ligands have been commercialized in collaboration with Millipore Sigma: IPr#HCl, 915653; Np#HCl; 915912; BIAN-IPr#HCl, 916420.

Deciphering Reactivity and Selectivity Patterns in Aliphatic C-H Bond Oxygenation of Cyclopentane and Cyclohexane Derivatives

A kinetic, product, and computational study on the reactions of the cumyloxyl radical with monosubstituted cyclopentanes and cyclohexanes has been carried out. HAT rates, site-selectivities for C-H bond oxidation, and DFT computations provide quantitative information and theoretical models to explain the observed patterns. Cyclopentanes functionalize predominantly at C-1, and tertiary C-H bond activation barriers decrease on going from methyl- and tert-butylcyclopentane to phenylcyclopentane, in line with the computed C-H BDEs. With cyclohexanes, the relative importance of HAT from C-1 decreases on going from methyl- and phenylcyclohexane to ethyl-, isopropyl-, and tert-butylcyclohexane. Deactivation is also observed at C-2 with site-selectivity that progressively shifts to C-3 and C-4 with increasing substituent steric bulk. The site-selectivities observed in the corresponding oxidations promoted by ethyl(trifluoromethyl)dioxirane support this mechanistic picture. Comparison of these results with those obtained previously for C-H bond azidation and functionalizations promoted by the PINO radical of phenyl and tert-butylcyclohexane, together with new calculations, provides a mechanistic framework for understanding C-H bond functionalization of cycloalkanes. The nature of the HAT reagent, C-H bond strengths, and torsional effects are important determinants of site-selectivity, with the latter effects that play a major role in the reactions of oxygen-centered HAT reagents with monosubstituted cyclohexanes.

Supramolecular packing of alkyl substituted Janus face all-cis 2,3,4,5,6-pentafluorocyclohexyl motifs

This study uses X-ray crystallography, theory and Langmuir isotherm analysis to explore the conformations and molecular packing of alkyl all-cis 2,3,4,5,6-pentafluorocyclohexyl motifs, which are prepared by direct aryl hydrogenations from alkyl- or vinyl-pentafluoroaryl benzenes. Favoured conformations retain the more polar triaxial C-F bond arrangement of the all-cis 2,3,4,5,6-pentafluorocyclohexyl ring systems with the alkyl substituent adopting an equatorial orientation, and accommodating strong supramolecular interactions between rings. Langmuir isotherm analysis on a water subphase of a long chain fatty acid and alcohol carrying terminal all-cis 2,3,4,5,6-pentafluorocyclohexyl rings do not show any indication of monolayer assembly relative to their cyclohexane analogues, instead the molecules appear to aggregate and form higher molecular assemblies prior to compression. The study indicates the power and potential of this ring system as a motif for ordering supramolecular assembly.

Hydrogenation of fluorinated molecules: an overview

The review aims at providing an overview on the developments made in hydrogenation reactions of molecules having various fluorinated groups (F, CF3, CF2H, CF2Rf). Indeed, the hydrogenation of fluorine-containing molecules is a straightforward and atom-economical way to access challenging (chiral) fluorinated scaffolds. This promising field is still in its infancy and milestones are expected in the coming years. To illustrate that, the review will highlight the major contributions made in that field and will be organized by fluorinated groups.

Enantio- and Diastereoselective, Complete Hydrogenation of Benzofurans by Cascade Catalysis

We report an enantio- and diastereoselective, complete hydrogenation of multiply substituted benzofurans in a one-pot cascade catalysis. The developed protocol facilitates the controlled installation of up to six new defined stereocenters and produces architecturally complex octahydrobenzofurans, prevalent in many bioactive molecules. A unique match of a chiral homogeneous ruthenium-N-heterocyclic carbene complex and an in situ activated rhodium catalyst from a complex precursor act in sequence to enable the presented process.

Reducing Challenges in Organic Synthesis with Stereoselective Hydrogenation and Tandem Catalysis

Tandem catalysis enables the rapid construction of complex architectures from simple building blocks. This Perspective shares our interest in combining stereoselective hydrogenation with transformations such as isomerization, oxidation, and epimerization to solve diverse challenges. We highlight the use of tandem hydrogenation for preparing complex natural products from simple prochiral building blocks and present tandem catalysis involving transfer hydrogenation and dynamic kinetic resolution. Finally, we underline recent breakthroughs and opportunities for asymmetric hydrogenation.