Chiral NHC Research Articles

Aerobic Oxidation/Annulation Cascades through Synergistic Catalysis of RuCl3 and N-Heterocyclic Carbenes.

Cooperative catalysis combining a transition metal with an N-heterocyclic carbene is challenging due to strong binding of NHCs towards late transition metals. We report the first example of synergistic catalysis by a chiral NHC and a coordinatively unsaturated ruthenium compound. RuCl3 was found to mediate efficient aerobic oxidation of homoenolates generated from enals and the N-heterocyclic carbene. The resulting α,β-unsaturated acylazolium intermediate reacts selectively with 1,3-dicarbonyl compounds or ketones at either the β- or γ-carbon, yielding polysubstituted chiral lactones in high yield and with excellent enantioselectivity (up to 98 % yield, 94 % ee). This protocol can be applied to structurally sophisticated substrates.

Chiral NHC ligands bearing a pyridine moiety in copper-catalyzed addition of diethylzinc to nitroalkenes

Chiral N-heterocyclic carbenes, derived from amino acids containing a pyridine ring, are effective ligands in the enantioselective copper-catalyzed addition of diethylzinc to β-nitroalkenes, which provides access to chiral nitroalkanes. The advantages of this process include high yields, broad and complementary substrate scope, and good to high enantioselectivities.

Synthesis, Characterization, and Catalytic Studies of Unsymmetrical Chiral NCC Pincer Pd(II) and Ni(II) Complexes Bearing (Imidazolinyl)aryl NHC Ligands

A series of palladium(II) and nickel(II) complexes based on unsymmetrical chiral (imidazolinyl)aryl NHC ligands are reported. The new ligand presursors 3a–g were prepared from commercially available 3-bromobenzoic acid, in which the carboxyl and bromo functionalities were converted to chiral imidazoline and NHC moieties, respectively, in three steps. Treatment of 3a–g with Pd(OAc)2 in dimethylacetamide in the presence of NaOAc, followed by addition of LiCl, gave the seven palladium(II) pincer complexes 4a–g via direct aryl C–H activation. Likewise, the six nickel(II) pincer complexes 5a–f were obtained by reacting 3a–f with anhydrous NiCl2. All of the ligand precursors and pincer complexes are bench-stable and were fully characterized. The absolute configurations of palladium complexes 4a,c and 5e were further determined by X-ray diffraction. Finally, Pd pincer complex catalyzed asymmetric Friedel–Crafts alkylation and aza-Morita–Baylis–Hillman reactions were briefly assessed.

Well-Defined Chiral Copper NHC Complex in the Asymmetric Conjugated β-Borylation and One-Pot Metathesis-Asymmetric β-Borylation Reactions.

Highly stereoselective conjugate β-borylation, using a new chiral NHC-based copper catalyst, has been achieved. The chiral NHC copper complex was prepared in gram scale and showed high enantioselectivity and activity (up to 10 000 turnovers at 100 ppm of catalyst loading). This method was employed in the synthesis of a chiral β-borylated ester from simple unconjugated alkenes though an unprecedented one-pot cross metathesis-asymmetric borylation sequence.

Inversions in asymmetric conjugate addition reaction of cyclic enones catalyzed by the Cu/NHC-AgX system: Factors affecting the stereoselective formation of both enantiomers

A switchable enantioselectivity was achieved in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. The ethylene-bridged, hydroxyamide-functionalized NHC-AgI complex, readily accessible from a chiral β-amino alcohol, was found to be a versatile chiral ligand precursor for dual enantioselective control. The stereocontrol of the catalytic ACA reaction depended on the order of addition of the substrates. To a THF solution containing CuOTf (6 mol %), NHC-AgI complex (1b, R1 = Et, R2 = Bn, 4 mol %), and 2-cyclohexen-1-one (3) was added Et2Zn to yield (R)-3-ethylcyclohexanone ((R)-4) in 74% enantioselectivity (method A). In stark contrast, when 3 was added as a last component to a mixture of CuOTf (4 mol %), 1b (10 mol %), and Et2Zn in THF (method B), the ACA reaction afforded (S)-4 with 87% ee. In order to obtain an insight into various aspects of the present ACA reaction, the chiral NHC ligand precursors, substrates, and NHC skeletons were fully evaluated. A study of the relationship between the catalyst ee (eecat) and the product ee (eepro) revealed a chiral amplification phenomenon. Moreover, it was found that the ACA reaction using method A was not affected by the halide ion on the NHC-AgX complex, whereas the halide ion on the silver complex showed a significant influence on the enantiocontrol of the ACA reaction using method B. In particular, a Cu/NHC/I catalytic ratio of 1/1/2 was needed for a successful ACA reaction using method B. According to these factors affecting the reversal of enantioselectivity in the ACA reaction, a possible reaction pathway was proposed.

Asymmetric [3+3] Cycloaddition for Heterocycle Synthesis

Asymmetric syntheses of six-membered ring heterocycles are important research targets not only in synthetic organic chemistry but also in pharmaceuticals. The [3+3]-cycloaddition methodology is a complementary strategy to [4+2] cycloaddition for the synthesis of heterocyclic compounds. Recent progress in [3+3]-cycloaddition processes provide powerful asymmetric methodologies for the construction of six-membered ring heterocycles with one to three heteroatoms in the ring. In this account, synthetic efforts during the past five years toward the synthesis of enantioenriched six-membered ring heterocycles through asymmetric [3+3] cycloaddition are reported. Asymmetric organocatalysis uses chiral amines, thioureas, phosphoric acids, or NHC catalysis to achieve high enantiocontrol. Transition-metal catalysts used as chiral Lewis acids to activate a dipolar species is an alternative approach. The most recent advance, chiral transition-metal-catalyzed reactions of enoldiazo compounds, has contributed toward the versatile and highly selective synthesis of six-membered heterocyclic compounds.1 Introduction2 Asymmetric Formal [3+3]-Cycloaddition Reactions by Organo­catalysis2.1 By Amino-Catalysis2.2 By N-Heterocyclic Carbenes2.3 By Bifunctional Tertiary Amine-thioureas2.4 By Chiral Phosphoric Acids3 Asymmetric Formal [3+3]-Cycloaddition Reactions by Transition-Metal Catalysis3.1 Copper Catalysis3.2 Other Transition-Metal Catalysis4 Asymmetric [3+3]-Cycloaddition Reactions of Enoldiazo Compounds4.1 Asymmetric [3+3]-Cycloaddition Reactions of Nitrones with Electrophilic Metallo-enolcarbene Intermediates4.2 Dearomatization in Asymmetric [3+3]-Cycloaddition Reactions of Enoldiazoacetates4.3 Asymmetric Stepwise [3+3]-Cycloaddition Reaction of Enoldiazoacetates with Hydrazones5 Summary and Outlook

Catalytic Generation of Donor‐Acceptor Cyclopropanes under N‐Heterocyclic Carbene Activation and their Stereoselective Reaction with Alkylideneoxindoles

AbstractFormylcyclopropanes undergo activation in the presence of an N‐heterocyclic carbene catalyst generating a donor‐acceptor cyclopropane intermediate with the ability to undergo ring‐opening followed by formal [4+2] cycloaddition with alkylideneoxindoles. This enables the direct enantio‐ and diastereoselective synthesis of tetrahydropyrano[2,3‐b]indoles through the use of a chiral NHC catalyst.magnified image

Ruthenium-NHC-Diamine Catalyzed Enantioselective Hydrogenation of Isocoumarins.

A novel and practical chiral ruthenium-NHC-diamine system is disclosed for the enantioselective hydrogenation of isocoumarins, which provides a new concept to apply (chiral) NHC ligands in asymmetric catalysis. A variety of optically active 3-substituted 3,4-dihydroisocoumarins were obtained in excellent enantioselectivities (up to 99% ee). Moreover, this methodology was utilized in the synthesis of O-methylmellein, mellein, and ochratoxin A.

Chiral Bidentate NHC Ligands Based on the 1,1'-Binaphthyl Scaffold: Synthesis and Application in Transition-Metal-Catalyzed Asymmetric Reactions.

The use of the chiral 1,1'-binaphthyl scaffold to construct chiral ligands can be traced back for a long time. However, the development of bidentate NHC ligands based on the same backbone has only appeared recently. In this account, we describe the design and synthesis of a new family of chiral NHC ligands based on the 1,1'-binaphthyl scaffold and demonstrate the applications of these chiral NHC-metal complexes in the catalyzed oxidative kinetic resolution of secondary alcohols, asymmetric carbon-carbon bond formations, hydrosilylations, and cyclizations of 1,6-enynes. The chiral NHC ligands containing the 1,1'-binaphthyl backbone can be synthesized in good yields from enantiomerically pure 1,1'-binaphthyl-2,2'-diamine. These transition metals coordinated with chiral bidentate NHC ligands exhibit high catalytic activities and good enantioselectivities for a wide range of metal-catalyzed asymmetric reactions.

Structure–enantioselectivity correlation in NHC–Au(I) catalysis for 1,6-enynecyclizations

A direct correlation of the NHC ligand structure and the induced enantioselectivity has been uncovered for a series of new chiral gold(I) complexes. The Au(I)Cl complexes were identified by 1H, 13C NMR spectroscopy, and X-ray structure analysis. The corresponding activated Au(I) catalysts with a phenyl nitrile dynamic ligand are prepared and used in the catalysis of 1,6-enyne cyclization reactions. High chemical yields and good enantioselectivity have been obtained with 6%mol of the chiral NHC–Au(I) catalysts. The products in these reactions reach up to 75% ee. The structure of the chiral NHC ligand, where an ortho-biphenyl moiety is connected to the nitrogen atoms, the 3,5-dialkyl distal phenyl group plays a determining role in enantioselectivity.

ChemInform Abstract: Synthesis of a New Chiral C2‐Symmetric NHC‐AuCl Complex.

AbstractA new chiral C2 symmetric NHC ligand bearing two seven‐membered rings that connect the imidazolylidene with the binaphthyl units is synthesized and utilized in a AuCl‐complex in catalytic asymmetric ene‐yne cyclizations.

ChemInform Abstract: Copper‐Catalyzed Asymmetric Conjugate Addition of Dimethylzinc to Acyl‐N‐methylimidazole Michael Acceptors: A Powerful Synthetic Platform.

AbstractThe enantioselective addition of dimethylzinc to α,β‐unsaturated ketones of type (I) in the presence of Cu(OTf)2 and the chiral NHC (IMD) as catalysts affords the corresponding 1,4‐adducts (III) in high regio‐ and enantioselectivities.

Nickel(0)-Catalyzed Enantio- and Diastereoselective Synthesis of Benzoxasiloles: Ligand-Controlled Switching from Inter- to Intramolecular Aryl-Transfer Process.

A highly enantioselective synthesis of 3-aryl-, vinyl-, and alkynyl-2,1-benzoxasiloles (up to 99.9% ee and 99% yield) was achieved via the sequential activation of an aldehyde and a silane by nickel(0). This strategy was applied to a simultaneous generation of carbon- and silicon-stereogenic centers with excellent selectivity (dr = 99:1) via diastereotopic aryl transfer. Initial mechanistic studies revealed the complete switching of an aryl-transfer process from an intermolecular (racemic synthesis in the presence of IPr) to an intramolecular (enantioselective synthesis using chiral NHC, L5) fashion. A plausible rationale for the switching of the aryl-transfer process is given by a preliminary DFT calculation, which suggests that the coordination of 1 to the nickel(0)/L5 fragment in an η(2)-arene:η(2)-aldehyde fashion would be a key to the intramolecular process, while the formation of the corresponding intermediate is not possible in the presence of IPr. Owing to the chemically labile nature of its C-Si and O-Si bonds, enantioenriched benzoxasiloles are utilized for the synthesis of chiral building blocks and antihistaminic and anticholinergic drug molecules such as (R)-orphenadrine and (S)-neobenodine with no erosion of the enantiomeric excess.

Synthesis and characterization of novel chiral [(NHC)Au(I)Cl] complexes: Featuring ortho-biphenyl substituents

A series of new chiral [(NHC)Au(I)Cl] are synthesized. The chiral NHC ligands feature pairs of ortho-biphenyl groups. The distal phenyl groups of the biphenyl substituents are varied from simple phenyl, 3,5-dimethylphenyl, to 3,5-di-t-butylphenyl. X-Ray structure analysis were performed for two such [(NHC)Au(I)Cl] complexes. High percent buried volumes were calculated for two novel chiral [(NHC)Au(I)Cl]complexes whose structure were solved by an X-ray analysis. The rotational motion in the biphenyl group is slow on the NMR time scale. Rotational barriers were determined using WINDNMR for two of the chiral gold complexes.

Synthesis of novel macrocyclic planar chiral carbene–Ag complexes derived from [2.2]paracyclophane for Rh-catalyzed asymmetric 1,2-additions of arylboronic acids to aromatic aldehydes

A successful development had been achieved in the asymmetric 1,2-addition of arylboronic acids to aromatic aldehydes by the use of macrocyclic planar chiral NHC–Rh complexes derived from [2.2]paracyclophane. A variety of chiral diarylmethanols were obtained in excellent yields and with moderate enantioselectivities.

Temperature-Dependent Direct Enantioconvergent Silylation of a Racemic Cyclic Allylic Phosphate by Copper(I)-Catalyzed Allylic Substitution

The near-quantitative transformation of a racemic cyclic allylic phosphate to a highly enantiomerically enriched allylic silane by allylic substitution with a silicon nucleophile is reported. The reaction is catalyzed by a chiral NHC–copper(I) complex. Experimental analysis revealed a rare case of a direct enantioconvergent transformation where the enantiomeric allylic phosphates converge to the same allylic silane by two distinctive S_N2′ pathways with opposite diastereofacial selectivity.

Asymmetric hydrogenation of quinazolinium salts catalysed by halide-bridged dinuclear iridium complexes bearing chiral diphosphine ligands.

Asymmetric hydrogenation of quinazolinium salts was catalysed by halogen-bridged dinuclear iridium complexes bearing chiral diphosphine ligands, yielding tetrahydroquinazoline and 3,4-dihydroquinazoline with high enantioselectivity. A derivative of chiral dihydroquinazoline was used as a chiral NHC ligand.

Nickel(0)‐Catalyzed Enantioselective Annulations of Alkynes and Arylenoates Enabled by a Chiral NHC Ligand: Efficient Access to Cyclopentenones

AbstractCyclopentenones are versatile structural motifs of natural products as well as reactive synthetic intermediates. The nickel‐catalyzed reductive [3+2] cycloaddition of α,β‐unsaturated aromatic esters and alkynes constitutes an efficient method for their synthesis. Here, nickel(0) catalysts comprising a chiral bulky C1‐symmetric N‐heterocyclic carbene ligand were shown to enable an efficient asymmetric synthesis of cyclopentenones from mesityl enoates and internal alkynes under mild conditions. The bulky NHC ligand provided the cyclopentenone products in very high enantioselectivity and led to a regioselective incorporation of unsymmetrically substituted alkynes.

Nickel(0)-catalyzed enantioselective annulations of alkynes and arylenoates enabled by a chiral NHC ligand: efficient access to cyclopentenones.

Cyclopentenones are versatile structural motifs of natural products as well as reactive synthetic intermediates. The nickel-catalyzed reductive [3+2] cycloaddition of α,β-unsaturated aromatic esters and alkynes constitutes an efficient method for their synthesis. Here, nickel(0) catalysts comprising a chiral bulky C1-symmetric N-heterocyclic carbene ligand were shown to enable an efficient asymmetric synthesis of cyclopentenones from mesityl enoates and internal alkynes under mild conditions. The bulky NHC ligand provided the cyclopentenone products in very high enantioselectivity and led to a regioselective incorporation of unsymmetrically substituted alkynes.

Catalytic enantioselective protoboration of disubstituted allenes. Access to alkenylboron compounds in high enantiomeric purity.

Proto-boryl additions to 1,1-disubstituted allenes in the presence of 1.0–5.0 mol % of chiral NHC–Cu complexes, B2(pin)2, and t-BuOH proceed to afford alkenyl–B(pin) products in up to 98% yield, >98:2 site selectivity, and 98:2 er. The enantiomerically enriched alkenylboron products can be converted to otherwise difficult-to-access alkenyl bromides, methyl ketones or carboxylic acids. What’s more, the corresponding boronic acids may be used in highly stereoselective NHC−Cu-catalyzed allylic substitution reactions.