Pd(II)/N,N'-Disulfonyl Bisimidazoline-Catalyzed Enantioselective Synthesis of Cyclic Quaternary Centers and Mechanistic Investigations

A Pd(II)/N,N'-disulfonyl bisimidazoline-catalyzed asymmetric 1,4-conjugate addition reaction of low-cost arylboronic acids with readily available β-substituted cyclic enones is described, providing a straightforward way of constructing cyclic all-carbon quaternary stereocenters with high enantioselectivity, in which ≥96% ee was obtained in most cases. The reaction proceeded without the protection of inert gas, making the operation process simple. Theoretical calculations have been applied to understand the origins of enantioselectivity.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Rhodium-Catalyzed Asymmetric Arylation of Cyclobutenone Ketals

Complex cyclobutanes are important motifs in both bioactive molecules and natural products, yet their enantioselective preparation has not been widely explored. In this work, we describe rhodium-catalyzed enantioselective additions of aryl and vinyl boronic acids to cyclobutenone ketals. This transformation involves enantioselective carbometalation to give cyclobutyl-rhodium intermediates, followed by β-oxygen elimination to afford enantioenriched enol ethers. Overall, this addition serves as a surrogate for Rh-catalyzed 1,4-additions to cyclobutenone.

Highly regioselective synthesis of lactams via cascade reaction of α,β-unsaturated ketones, ketoamides, and DBU as a catalyst

Herein, the aldol/Michael cascade reaction on the β,γ-positions of α,β-unsaturated ketones with ketoamides to construct bicyclic lactams via DBU catalysis has been developed. The substrates were well-tolerated with high regio- and diastereoselectivities in moderate to good yields (32 examples). The control experiments revealed that the hydrogen of the amide was the key factor.

Asymmetric Synthesis of Nortropanes via Rh-Catalyzed Allylic Arylation

Tropane derivatives are extensively used in medicine, but catalytic asymmetric methods for their synthesis are underexplored. Here, we report Rh-catalyzed asymmetric Suzuki–Miyaura-type cross-coupling reactions between a racemic N-Boc-nortropane-derived allylic chloride and (hetero)aryl boronic esters. The reaction proceeds via an unexpected kinetic resolution, and the resolved enantiopure allyl chloride can undergo highly enantiospecific reactions with N-, O-, and S-containing nucleophiles. The method was applied in a highly stereoselective formal synthesis of YZJ-1139(1), a potential insomnia treatment that recently completed Phase II clinical trials. Our report represents an asymmetric catalytic method for the synthesis of YZJ-1139(1) and related compounds.

Enantioselective construction of ortho-substituted benzylic quaternary centers using a phenanthroline-Pd catalyst

An efficient and enantioselective construction of ortho-substituted benzylic all-carbon quaternary centers via chiral phenanthroline-Pd complex catalyzed conjugate addition has been developed.

Synthesis of Cyclopentenones with C4-Quaternary Stereocenters via Stereospecific [3,3]-Sigmatropic Rearrangement and Applications in Total Synthesis of Sesquiterpenoids

A cationic gold(I)-catalyzed asymmetric [3,3]-sigmatropic rearrangement of sulfonium leads after cyclization to cyclopentenones with a C4-quaternary stereocenter. Starting with simple vinyl sulfoxides and propargyl silane, numerous compounds were isolated with moderate to good yields and excellent enantiomeric excesses (26 examples). The application of this simple methodology allowed the efficient total synthesis of five natural sesquiterpenoids, including enokipodin A and B, hitoyopodin A, lagopodin A, and isocuparene-3,4-diol.

A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Recent advances in palladium-catalysed asymmetric 1,4-additions of arylboronic acids to conjugated enones and chromones

The transition metal (palladium)-catalysed asymmetric 1,4-addition of arylboronic acids to conjugated enones belong to the most important and emerging strategies for the construction of C-C bonds in an asymmetric fashion. This review covers known catalytic systems used for this transformation. For clarity, we are using the type of ligand as a sorting criterion. Finally, we attempted to create a flowchart facilitating the selection of a suitable ligand for a given combination of enone and arylboronic acid.

Stereoselectivity Predictions for the Pd-Catalyzed 1,4-Conjugate Addition Using Quantum-Guided Molecular Mechanics

The conjugate addition of aryl boronic acids to enones is a powerful synthetic tool to introduce quaternary chiral centers, but the experimentally observed stereoselectivities vary widely, and the identification of suitable substrate-ligand combinations requires significant effort. We describe the development and application of a transition-state force field (TSFF) by the quantum-guided molecular mechanics (Q2MM) method that is validated using an automated screen of 9 ligands, 38 aryl boronic acids, and 22 enones, leading to a MUE of 1.8 kJ/mol and a R² value of 0.877 over 82 examples. A detailed error analysis identified the structural origin for the deviations in the small group of outliers. The TSFF was then used to predict the stereoselectivity for 27 ligands and 59 enones. The vast majority of the virtual screening results are in line with the expected results. Selected results for 6-substituted pyrox ligands, which were not part of the training set, were followed up by density functional theory and experimental studies.

Can Donor Ligands Make Pd(OAc)2 a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria

Palladium(II)-catalyzed oxidation reactions represent an important class of methods for selective modification and functionalization of organic molecules. This field has benefitted greatly from the discovery of ancillary ligands that expand the scope, reactivity, and selectivity in these reactions; however, ancillary ligands also commonly poison these reactions. The different influences of ligands in these reactions remain poorly understood. For example, over the 60-year history of this field, the Pd^II/0 redox potentials for catalytically relevant Pd complexes have never been determined. Here, we report the unexpected discovery of (L)Pd^II(OAc)₂-mediated oxidation of hydroquinones, the microscopic reverse of quinone-mediated oxidation of Pd⁰ commonly employed in Pd^II-catalyzed oxidation reactions. Analysis of redox equilibria arising from the reaction of (L)Pd(OAc)₂ and hydroquinones (L = bathocuproine, 4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones, provides the basis for determination of (L)Pd^II(OAc)₂ reduction potentials. Experimental results are complemented by density functional theory calculations to show how a series of nitrogen-based ligands modulate the (L)Pd^II(OAc)₂ reduction potential, thereby tuning the ability of Pd^II to serve as an effective oxidant of organic molecules in catalytic reactions.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Asymmetric Synthesis of 1-Tetralones Bearing A Remote Quaternary Stereocenter through Rh-Catalyzed C-C Activation of Cyclopentanones

Herein, we describe the preparation of 1-tetralones bearing a remote quaternary stereocenter in a highly enantioselective manner. A sequence of Pd-catalyzed asymmetric 1,4-addition and Rh-catalyzed enantiospecific C-C/C-H activation delivers diverse 1-tetralones with a C4 quaternary stereocenter, which are prepared in good overall yields and high enantioselectivity.

Stepwise approach for sterically hindered Csp3-Csp3 bond formation by dehydrogenative O-alkylation and Lewis acid-catalyzed [1,3]-rearrangement towards the arylalkylcyclopentane skeleton of sesquiterpenes

A stepwise dehydrogenative cross-coupling method was developed for the formation of sterically hindered Csp3-Csp3 bonds. Intramolecular dehydrogenative O-alkylation of a β-ketoester by 2,3-dichloro-5,6-dicyano-p-benzoquinone to form an oxolane followed by Lewis acid-catalyzed [1,3]-rearrangement furnished the sesquiterpene arylmethylcyclopentane skeleton. The formal syntheses of herbertane-type β-herbertenol, cuparane-type enokipodins A and B were also achieved.

Productivity and bioactivity of enokipodins A-D of Flammulina rossica and Flammulina velutipes

Enokipodins are antimicrobial sesquiterpenes produced by Flammulina velutipes in a mycelial culture medium. To date, enokipodin production has not been reported in other members of the genus Flammulina. Hence, in this study, the production of enokipodins A, B, C, and D by F. velutipes and F. rossica was investigated. Some strains of F. rossica were confirmed to produce at least one of the four enokipodins in the culture medium. However, some strains of F. velutipes did not produce any of the enokipodins. In an antibacterial assay using liquid medium, enokipodin B showed the strongest growth inhibitory activity against Bacillus subtilis among the four types of enokipodins. Enokipodin B inhibited the spore germination of some plant pathogenic fungi. Enokipodins B and D exerted moderate anti-proliferative activity against some cancer cell lines, and enokipodins A and C inhibited the proliferation of the malarial parasite, Plasmodium falciparum.

Construction of all-carbon quaternary stereocenters by catalytic asymmetric conjugate addition to cyclic enones in natural product synthesis

This review discusses the construction of all-carbon quaternary stereocenters using catalytic asymmetric conjugate addition and its application in natural product synthesis.

Pd(II)-Catalyzed Enantioselective Desymmetrization of Polycyclic Cyclohexenediones: Conjugate Addition versus Oxidative Heck

Pd(II)-catalyzed desymmetrization of polycyclic cyclohexenediones has been achieved with high enantio- and diastereoselectivities. Up to five contiguous stereocenters are desymmetrized, while simultaneously, an additional stereocenter is created by the enantioselective conjugate addition. Surprisingly, the conjugate addition products dominate even under typical oxidative Heck conditions, and these observations may provide some insight into the competition between the two related reactions.

Operando Spectroscopic and Kinetic Characterization of Aerobic Allylic C-H Acetoxylation Catalyzed by Pd(OAc)2/4,5-Diazafluoren-9-one

Allylic C-H acetoxylations are among the most widely studied palladium(II)-catalyzed C-H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd^I complex [Pd^I(DAF)(OAc)]₂, followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd^I dimer into a π-allyl-Pd^II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O₂ as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.

Formation of quaternary centres by copper catalysed asymmetric conjugate addition to β-substituted cyclopentenones with the aid of a quantitative structure-selectivity relationship

A new asymmetric conjugate addition method was developed for β-substituted cyclopentenones to form quaternary centres using alkylzirconocene nucleophiles giving up to 97% yield and 92% ee.

A new asymmetric conjugate addition method was developed for β-substituted cyclopentenones to form quaternary centres using alkylzirconocene nucleophiles giving up to 97% yield and 92% ee. Key to the reaction's success was the design of suitable phosphoramidite ligands which was aided by a linear quantitative structure–selectivity relationship (QSSR). QSSR models were created from the ligand screening data (a total of 36 ligands) which revealed important electronic and steric requirements and led to the synthesis of more enantioselective ligands. DFT calculations of competing transition structures enable the interpretation of the electronic and steric terms present in the QSSR models.

Palladium-polymer nanoreactors for the aqueous asymmetric synthesis of therapeutic flavonoids

Polymeric core–shell micelles incorporating a chiral palladium pyridinooxazoline catalyst are presented as nanoreactors for the aqueous asymmetric synthesis of flavanones, a class of flavonoids, with therapeutic properties.

Synthesis and Structural Analysis of Palladium(II) Complexes Containing Neutral or Anionic C 2-Symmetric Bis(oxazoline) Ligands: Effects of Substituents in the 5-Position

A series of neutral and cationic palladium(II) complexes containing C ₂-symmetric bis(oxazoline) (BOX) ligands, (BOX)PdCl₂ (2a-d), (BOX)Pd(Me)Cl (3a-d), and [(BOX)PdMe(2,6-Me₂C₅H₃N)]⁺PF₆ ^- (4a-d) [BOX: 2,2'-(2-propylidene)bis{(4R)-4-phenyl-5,5-dimethyl-2-oxazoline}, 2,2'-methylenebis{(4R)-4-phenyl-5,5-dimethyl-2-oxazoline}, 2,2'-methylenebis{(4R)-4,5,5-triphenyl-2-oxazoline}, and 2,2'-methylenebis{(4R,5S)-4,5-diphenyl-2-oxazoline}], were prepared, and their structures were determined by X-ray crystallography. It was found that substituents at the 5-position (Ph, Me) in addition to substituents on the bridgehead carbon directly affect the structure around palladium, especially the BOX bite angle and the dihedral angles between the phenyl rings at the 4-position and the N₂Pd plane. Treatment of the bridged methylene proton in the BOX ligand (1b-d) with KH afforded the anionic BOX ligand; also, the neutral Pd complexes, (BOX)PdMe(2,6-Me₂C₅H₃N) (5b-d), could thus be prepared by reaction with Pd(Me)Cl(cod) (cod = 1,5-cyclooctadiene); 5b-d showed strong coordination to Pd, as demonstrated by X-ray crystallographic analysis.

Synthesis of functionalized alkyl substituted benzoquinones by Rh-catalyzed additions of boronic acids

An efficient, group tolerant one-pot process has been developed for the synthesis of functionalized-quinones, using a Rh-catalyzed addition as a key step.

Palladium-Catalyzed Aerobic Dehydrogenation of Cyclic Hydrocarbons for the Synthesis of Substituted Aromatics and Other Unsaturated Products

Catalytic dehydrogenation of saturated or partially saturated six-membered carbocycles into aromatic rings represents an appealing strategy for the synthesis of substituted arenes. Particularly effective methods have been developed for the dehydrogenation of cyclohexanones and cyclohexenes into substituted phenol, aniline, and benzene derivatives, respectively. In this Perspective, we present the contributions of our research group to the discovery and development of palladium-based catalysts for aerobic oxidative dehydrogenation methods, including general methods for conversion of cyclohexanones and cyclohexenones into substituted phenols and a complementary method for partial dehydrogenation cyclohexanones to cyclohexenones. The mechanistic basis for chemoselective conversion of cyclohexanones to phenols or enones is presented. These results are presented within the context of recent methods developed by others for the synthesis of aryl ethers, anilines and other substituted arenes. Overall, Pd-catalyzed dehydrogenation methods provide a compelling strategy for selective synthesis of aromatic and related unsaturated molecules.

Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

4,5-Diazafluoren-9-one (DAF) has been identified as a highly effective ligand in a number of Pd-catalyzed oxidation reactions, but the mechanistic basis for its utility has not been elucidated. Here, we present the complex coordination chemistry of DAF and palladium(II) carboxylate salts. Multiple complexes among an equilibrating mixture of species have been characterized by (1)H and (15)N NMR spectroscopy and X-ray crystallography. These complexes include monomeric and dimeric Pd(II) species, with monodentate (κ(1)), bidentate (κ(2)), and bridging (μ:κ(1):κ(1)) DAF coordination modes. Titration studies of DAF and Pd(OAc)2 reveal the formation of two dimeric DAF/Pd(OAc)2 complexes at low [DAF] and four monomeric species at higher [DAF]. The dimeric complexes feature two bridging acetate ligands together with either a bridging or nonbridging (κ(1)) DAF ligand coordinated to each Pd(II) center. The monomeric structures consist of three isomeric Pd(κ(1)-DAF)2(OAc)2 complexes, together with Pd(κ(2)-DAF)(OAc)2 in which the DAF exhibits a traditional bidentate coordination mode. Replacing DAF with the structurally related, but more-electron-rich derivative 9,9-dimethyl-4,5-diazafluorene (Me2DAF) simplifies the equilibrium mixture to two complexes: a dimeric species in which the Me2DAF bridges the two Pd centers and a monomeric species with a traditional κ(2)-Me2DAF coordination mode. The use of DAF in combination with other carboxylate ligands (CF3CO2(-) or tBuCO2(-)) also results in a simplified collection of equilibrating Pd(II)-DAF complexes. Collectively, the results highlight the ability of DAF to equilibrate rapidly among multiple coordination modes, and provide valuable insights into the utility of DAF as a ligand in Pd-catalyzed oxidation reactions.

Diazafluorenone-Promoted Oxidation Catalysis: Insights into the Role of Bidentate Ligands in Pd-Catalyzed Aerobic Aza-Wacker Reactions

2,2'-Bipyridine (bpy), 1,10-phenanthroline (phen) and related bidentate ligands often inhibit homogeneous Pd-catalyzed aerobic oxidation reactions; however, certain derivatives, such as 4,5-diazafluoren-9-one (DAF), can promote catalysis. In order to gain insight into this divergent ligand behavior, eight different bpy- and phen-derived chelating ligands have been evaluated in Pd(OAc)₂-catalyzed oxidative cyclization of (E)-4-hexenyltosylamide. Two of the ligands, DAF and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me₂bpy), support efficient catalytic turnover, while the others strongly inhibit the reaction. DAF is especially effective and is the only ligand that exhibits "ligand-accelerated catalysis". Evidence suggests that the utility of DAF and 6,6'-Me₂bpy originates from the ability of these ligands to access κ¹-coordination modes via dissociation of one of the pyridyl rings. This hemilabile character is directly observed by NMR spectroscopy upon adding one equivalent of pyridine to solutions of 1:1 L/Pd(OAc)₂ (L = DAF and 6,6'-Me₂bpy), and is further supported by an X-ray crystal structure of Pd(py)(κ¹-DAF)OAc₂. DFT computational studies illuminate the influence of three different chelating ligands [DAF, 6,6'-Me₂bpy, and 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂phen)] on the energetics of the aza-Wacker reaction pathway. The results show that DAF and 6,6'-Me₂bpy destabilize the corresponding ground-state Pd(N~N)(OAc)₂ complexes, while stabilizing the rate-limiting transition state for alkene insertion into a Pd-N bond. Interconversion between κ²- and κ¹-coordination modes facilitate access to open coordination sites at the Pd^II center. The insights from these studies introduce new ligand concepts that could promote numerous other classes of Pd-catalyzed aerobic oxidation reaction.

Enantioselective oxidative boron Heck reactions

This review highlights the use of the oxidative boron Heck reaction in enantioselective Heck-type couplings.

MOF-253-Pd(OAc)2: a recyclable MOF for transition-metal catalysis in water

We report palladium(ii)-functionalized MOF-253 (MOF-253-Pd(OAc)₂) as a recyclable catalyst to form all-carbon quaternary centers via conjugate additions of arylboronic acids to β,β-disubstituted enones in aqueous media.

Palladium-Catalyzed Asymmetric Conjugate Addition of Arylboronic Acids to α,β-Unsaturated Cyclic Electrophiles

This account describes our laboratory's efforts in the development of a palladium-catalyzed asymmetric conjugate addition of arylboronic acids to cyclic conjugate acceptors. Specifically, we highlight the study of this transformation in the following areas: (a) construction of all-carbon quaternary stereocenters, (b) elucidation of the reaction mechanism,

Mechanistic analysis of an asymmetric palladium-catalyzed conjugate addition of arylboronic acids to β-substituted cyclic enones

An asymmetric palladium-catalyzed conjugate addition reaction of arylboronic acids to enone substrates was investigated mechanistically. Desorption electrospray ionization coupled to mass spectrometry was used to identify intermediates of the catalytic cycle and delineate differences in substrate reactivity. Our findings provide evidence for the catalytic cycle proceeding through formation of an arylpalladium(II) cation, subsequent formation of an arylpalladium-enone complex, and, ultimately, formation of the new C-C bond. Reaction monitoring in both positive and negative ion modes revealed that 4-iodophenylboronic acid formed a relatively stable trimeric species under the reaction conditions.