Improved Catalysts for the Palladium-Catalyzed Synthesis of Oxindoles by Amide α-Arylation. Rate Acceleration, Use of Aryl Chloride Substrates, and a New Carbene Ligand for Asymmetric Transformations

Enantioselective Palladium-Catalyzed α-Arylation of Acyclic Esters

A Pd-catalyzed enantioselective α-arylation of α,α-disubstituted esters with aryl bromides is established for the first time by employing P-chiral monophosphorus ligand 3-Pent-BIDIME as a chiral ligand, leading to a series of enantioenriched α,α-diaryl esters possessing quaternary carbon stereocenters in moderate to good yields and high enantioselectivities. The method features a broad substrate scope, mild conditions, excellent functional group compatibility, and low Pd loadings (as low as 1 mol%). The synthetic power of this protocol is exemplified by the efficient preparation of a chiral α,α-diaryl substituted γ-lactone and asymmetric synthesis of (R)-amolanone. DFT calculation revealed an NaBr-bridged downstream transmetallation and the importance of noncovalent interaction in controlling the enantioselectivity.

Pd-catalyzed enantioselective access to hydrocarbazolones containing contiguous quaternary and tertiary stereocenters

The hydrocarbazole scaffold represents the core structure of numerous monoterpenoid indole alkaloids. The development of catalytic methods that provide efficient access to enantioenriched hydrocarbazole derivatives is central for the synthesis of these bioactive alkaloids. We report here a palladium-catalyzed enantioselective formal 5-endo arylative cyclization of enaminones, facilitating the construction of hexahydrocarbazol-4-ones containing contiguous C4a-quaternary and C9a-tertiary stereocenters with high enantioselectivities (86.5 : 13.5-99 : 1 er) and diastereoselectivities (>20 : 1 dr). Notably, enaminone substrates bearing an α-allyl group undertake an arylation/Cope rearrangement cascade, offering a unique route to C1-substituted tetrahydrocarbazol-4-ones. A stereodivergent approach to all four stereoisomers of the quaternary/tertiary chiral center set is achieved by combining the catalyst with Z/E allyl substituents, yielding excellent enantioselectivity. The N-methyl group of the hydrocarbazolone products is readily removed under oxidation conditions. The utility of the method is demonstrated by the access to a variety of hydrocarbazole derivatives and the efficient syntheses of four Aspidosperma alkaloids/analogs, (+)-N-methyl aspidospermidine, (+)-C20-epi-N-methyl aspidospermidine, (+)-N-methyl fendleridine, and (+)-N-methyl limaspermidine from a hexahydrocarbazol-4-one in 3-5 steps.

Illuminating Palladium Catalysis

ConspectusThe past decade has witnessed significant advancements of visible-light-induced photocatalysis, establishing it as a powerful and versatile tool in organic synthesis. The major focus of this field has centered on the development of methodologies that either rely solely on photocatalysts or combine photocatalysis with other catalytic methods, such as transition metal catalysis, to address a broader and more diverse array of transformations. Within this rapidly evolving area, a subfield that we refer to as transition metal photocatalysis has garnered significant attention due to its growing impact and mechanistic uniqueness. A distinguishing feature of this subfield is the dual functionality of a single transition metal complex, which not only acts as a photocatalyst to initiate photochemical processes but also functions as a traditional catalyst, facilitating key bond-breaking and bond-forming events. As such, an exogenous photocatalyst is not required in transition metal photocatalysis. However, the implications of harnessing both the excited- and ground-state reactivities of the transition metal complex can extend beyond this simplification. One of the most compelling aspects of this area is that photoexcited transition metal complexes can exhibit unique reactivities inaccessible through conventional thermal or dual photocatalytic approaches. These distinct reactivities can be leveraged to accomplish novel transformations either by engaging an entirely different substrate pool or by unlocking new reactivities of known substrates.In 2016, our group pioneered the use of phosphine-ligated palladium catalysts that can be photoexcited upon visible-light irradiation to engage diverse substrates in radical reactions. In our initial discovery, we showed that photoexcitation can redirect the well-established oxidative addition of a Pd(0) complex into aryl iodides toward an unprecedented radical process, generating hybrid aryl Pd(I) radical species. We subsequently extended this novel strategy to the formation of alkyl radicals from alkyl halides. These reactive radical intermediates have been harnessed in a wide variety of transformations, including desaturation, alkyl Heck reactions, and alkene difunctionalization cascades, among others.Seeking to further expand this new avenue, we achieved the first example of asymmetric palladium photocatalysis in the context of allylic C-H amination, where the palladium catalyst now plays triple duty by additionally controlling the stereochemical outcome of the reaction. In parallel to reaction discovery, we have also established that diazo compounds, strained molecules, and electron-deficient alkenes can serve as alkyl radical precursors beyond organic halides and redox-active esters. Notably, the engagement of electron-deficient alkenes has been made possible by the photoinduced hydricity enhancement of Pd-H species, representing a new mode of photoexcited reactivity.This Account presents our discovery and development of visible-light-induced palladium catalysis, organized by the type of transformations explored. Given the rapid progress in the field, we anticipate that this Account will provide readers with guiding principles and inspiration for designing and developing more efficient and novel transformations.

Synthesis of ɑ-Aryl-oxindoles via Grignard Reaction with Isatin in the Presence of Diphenyl Phosphite

A protocol has been developed for the synthesis of α-aryl-oxindoles from isatin and Grignard reagents in the presence of diphenyl phosphite for the first time. This reaction was conveniently carried out under mild conditions in a one-pot fashion with moderate to excellent yields.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Multicomponent Assembly of Complex Oxindoles by Enantioselective Cooperative Catalysis

Chiral oxindoles are important chemical scaffolds found in many natural products, and their enantioselective synthesis thus attracts considerable attention. Highly diastereo- and enantioselective synthetic methods for constructing C3 quaternary oxindoles have been well-developed. However, the efficient synthesis of chiral 3-substituted tertiary oxindoles has been rarely reported due to the ease of racemization of the tertiary stereocenter via enolization. Therefore, we herein report on the multicomponent assembly (from N-aryl diazoamides, aldehydes, and enamines/indoles) of complex oxindoles by enantioselective cooperative catalysis. These reactions proceed under mild conditions and show broad substrate scope, affording the desired coupling products (>90 examples) with good to excellent stereocontrol. Additionally, this research also demonstrates the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of Speradine A.

Nitrogen-Containing Heterocyclic Compounds Obtained from Monoterpenes or Their Derivatives: Synthesis and Properties

Directed transformation of available natural compounds with native biological activity is a promising area of research in organic and medicinal chemistry aimed at finding effective drug substances. The number of scientific publications devoted to the transformation of natural compounds and investigations of their pharmacological properties, in particular, monoterpenes and their nearest derivatives, increases every year. At the same time, the chemistry of nitrogen-containing heterocyclic compounds has been actively developed since the 1950s after the news that the benzimidazole core is an integral part of the structure of vitamin B₁₂. At the time of writing this review, the data on chemical modifications of monoterpenes and their nearest derivatives leading to formation of compounds with a nitrogen-containing heterocycle core have not been summarized and systematized in terms of chemical transformations. In this review, we tried to summarize the literature data on the preparation and properties of nitrogen-containing heterocyclic compounds synthesized from monoterpenes/monoterpenoids and their nearest derivatives for the period from 2000 to 2021.

Synthesis of 3-substituted 2-oxindoles from secondary α-bromo-propionanilides via palladium-catalyzed intramolecular cyclization

In contrast to aromatic halides, coupling reactions involving oxidative addition of alkyl halides, especially secondary or tertiary halides, to transition metals tend to be more challenging. Herein a palladium-catalyzed intramolecular cyclization of α-bromo-propionanilides has been developed, delivering a series of 3-substituted 2-oxindoles in high yields. The method features easy to prepare starting materials, broad substrate scope and excellent functional group tolerance. A detailed mechanistic investigation has been performed.

Synthesis of α-Aryl Secondary Amides via Nickel-Catalyzed Reductive Coupling of Redox-Active Esters

The transition-metal-catalyzed α-arylation of secondary amides remains a synthetic challenge due to the presence of a free N-H bond. We report a strategy to synthesize secondary α-aryl amides via a Ni-catalyzed reductive arylation of redox-active N-hydroxyphthalimide (NHP) esters of malonic acid half amides. This transformation proceeds under mild conditions and displays excellent chemoselectivity for amide α-arylation in the presence of other enolizable carbonyls. The NHP ester substrates are readily prepared from Meldrum's acid.

Remote Construction of N-Heterocycles via 1,4-Palladium Shift-Mediated Double C-H Activation

In the past years, Pd0 -catalyzed C(sp3 )-H activation provided efficient and step-economical methods to synthesize carbo- and heterocycles via direct C(sp2 )-C(sp3 ) bond formation. We report herein that a 1,4-Pd shift allows access to N-heterocycles which are difficult to build via a direct reaction. It is shown that o-bromo-N-methylanilines undergo a 1,4-Pd shift at the N-methyl group, followed by intramolecular trapping by C(sp2 )-H or C(sp3 )-H activation at another nitrogen substituent and remote C-C bond formation to generate biologically relevant isoindolines and β-lactams. The product selectivity is influenced by the employed ligand, with NHCs favoring the product of remote C-C coupling against products arising from direct C-C coupling and N-demethylation.

BF3·OEt2 catalyzed decarbonylative arylation/C-H functionalization of diazoamides with arylaldehydes: synthesis of substituted 3-aryloxindoles

A metal-free BF₃·OEt₂ catalyzed direct decarbonylative arylation of diazoamides with readily accessible aryl aldehydes under an open-air atmosphere was developed to afford 3-aryloxindoles via 1,2-aryl migration with high selectivity. The reaction offers an efficient pathway for 3-arylation of diazoamides under relatively mild conditions, which shows a high level of functional group tolerance of both electron-donating and electron-withdrawing groups with a broad substrate scope. 3-Aryloxindoles were also obtained by a substituent-controlled chemo- and site-selective C-H bond functionalization of unprotected salicylaldehyde derivatives.

Cu(ii)-thiophene-2,5-bis(amino-alcohol) mediated asymmetric Aldol reaction and Domino Knoevenagel Michael cyclization: a new highly efficient Lewis acid catalyst

The highly efficient Lewis acid-catalytic system Cu(ii)-thiophene-2,5-bis(amino-alcohol) has been developed for enantioselective Aldol reaction of isatin derivatives with ketones. The new catalytic system also proved to be highly enantioselective for the one pot three-component Domino Knoevenagel Michael cyclization reaction of substituted isatin with malononitrile and ethylacetoacetate. The chiral ligand (2S,2'S)-2,2'-((thiophene-2,5-diylbis(methylene))bis(azanediyl))bis(3-phenylpropan-1-ol) (L1) in combination with Cu(OAc)2·H2O employed as a new Lewis acid catalyst, furnished 3-substituted-3-hydroxyindolin-2-ones derivatives (3a-s) in good to excellent yields (81-99%) with high enantioselectivities (up to 96% ee) and spiro[4H-pyran-3,3-oxindole] derivatives (6a-l) in excellent yields (89-99%) with high ee (up to 95%). These aldol products and spiro-oxindoles constitute a core structural motif in a large number of pharmaceutically active molecules and natural products.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Visible light promoted polyhalomethylation of alkenes: alkylation and cyclization

The report describes a visible light promoted carbochloromethylation of 2-bromomethyl acrylate and N-arylmethacrylamide, and a series of trihalomethyl substituted allylic acid esters and indolone derivatives were synthesized.

The catalytic decarboxylative allylation of enol carbonates: the synthesis of enantioenriched 3-allyl-3'-aryl 2-oxindoles and the core structure of azonazine

The catalytic asymmetric synthesis of 3-allyl-3'-aryl 2-oxindoles has been shown via the Pd(0)-catalyzed decarboxylative allylation of allylenol carbonates. This methodology provides access to a variety of 2-oxindole substrates (5a-v) with all-carbon quaternary stereocenters (up to 94% ee) at the pseudobenzylic position under additive-free and mild conditions. The synthetic potential of this method was shown by the asymmetric synthesis of the tetracyclic core of the diketopiparazine-based alkaloid azonazine (11).

Lambert Salt-Initiated Development of Friedel-Crafts Reaction on Isatin to Access Distinct Derivatives of Oxindoles

Herein, a mild metal-free and efficacious route for the synthesis of biologically important 3-aryl oxindole derivatives is described. Using Lambert salt-initiated hydroarylation of isatin, a diverse array of monoarylated products, symmetrical/unsymmetrical double-arylated products, and deoxygenated hydroarylated products could be synthesized from the single starting substrate in good to excellent yields. A preliminary mechanistic study revealed that the reaction proceeds via a monoarylated product followed by a nucleophilic attack by another electron-rich arene nucleophile under mild conditions. The potential of newly synthesized symmetric/unsymmetric 3,3-disubstituted oxindole, 3-substituted 3-hydroxy oxindoles, 3,3-di(indolyl)indolin-2-ones, and α-aryl oxindoles as valuable building blocks is further illustrated.

Designing successful monodentate N-heterocyclic carbene ligands for asymmetric metal catalysis

Chiral ligands are of particular importance in asymmetric transition-metal catalysis. Although the development of effective chiral monodentate N-heterocyclic carbenes (NHCs) has been slow, a growing amount of papers have been published in recent years showing their unique efficiency as chiral ancillary ligands. Herein we provide an overview of NHC structures that accomplish high levels of enantioselectivity (≥90% ee) and give guidelines to their use and thoughts on the future of this field.

Transition metal complexes bearing atropisomeric saturated NHC ligands

From achiral imidazolinium salts, chiral transition metal complexes containing an N-heterocyclic carbene (NHC) ligand were prepared (metal = palladium, copper, silver, gold, rhodium). Axial chirality in these complexes results from the formation of the metal-carbene bond leading to the restriction of rotation of dissymmetric N-aryl substituents about the C-N bond. When these complexes exhibited a sufficient configurational stability, a resolution by chiral high-performance liquid chromatography (HPLC) on preparative scale enabled isolation of enantiomers with excellent enantiopurities (>99% ee) and good yields. A study of the enantiomerization barriers revealed the effect of the backbone nature as well as the type of transition metal on its values. Nevertheless, the evaluation of palladium-based complexes in asymmetric intramolecular α-arylation of amides demonstrated that the ability to induce an enantioselectivity cannot be correlated to the configurational stability of the precatalysts.

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes

The Pd-catalyzed asymmetric α-arylation of carbonyl compounds is a valuable strategy to form benzylic stereocenters. However, the origin of the stereoselectivity of these reactions is poorly understood, and little is known about the reactivity of the putative diastereomeric arylpalladium enolate intermediates. To this end, we report the synthesis and characterization of a series of diphosphine-ligated arylpalladium fluoroenolate complexes, including complexes bearing a metal-bound, stereogenic carbon and an enantioenriched chiral diphosphine ligand. These complexes reductively eliminate to form chiral α-aryl-α-fluorooxindoles with enantioselectivities and rates that are relevant to those of the catalytic process with SEGPHOS as the ancillary ligand. Kinetic studies showed that the rate of reductive elimination is slightly slower than the rate of epimerization of the intermediate, causing the reductive elimination step to impart the greatest influence on the enantioselectivity. DFT calculations of these processes are consistent with these experimental rates and suggest that the minor diastereomer forms the major enantiomer of the product. The rates of reductive elimination from complexes containing a variety of electronically varied aryl ligands revealed the unusual trend that complexes bearing more electron-rich aryl ligands react faster than those bearing more electron-poor aryl ligands. Noncovalent Interaction (NCI) and Natural Bond Orbital (NBO) analyses of the transition-state structures for reductive elimination from the SEGPHOS-ligated complexes revealed key donor-acceptor interactions between the Pd center and the fluoroenolate fragment. These interactions stabilize the pathway to the major product enantiomer more strongly than they stabilize that to the minor enantiomer.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

I2-Mediated Cross-Dehydrogenative Coupling and Amidation of 3-Aryl Benzofuranones with Aryl Amines for the Synthesis of 3,3-Diaryl Indolin-2-ones

We have developed a protocol for efficient synthesis of indolin-2-ones from benzofuranones and aryl amines using iodine as a mediator. A diverse range of benzofuranones and aryl amines undergo cross-dehydrogenative coupling and amidation of 3-aryl benzofuranones for the cascade reaction to generate products in 24-93% yields. This reaction can be easily scaled-up to give an indolin-2-one in a gram scale. Further chemical manipulation of the products enabled useful transformations of the phenol ring including alkylation, arylation, etc.

Design, scope and mechanism of highly active and selective chiral NHC-iridium catalysts for the intramolecular hydroamination of a variety of unactivated aminoalkenes

Chiral, cationic NHC-iridium complexes are introduced as catalysts for the intramolecular hydroamination reaction of unactivated aminoalkenes. The catalysts show high activity in the construction of a range of 5- and 6-membered N-heterocycles, which are accessed in excellent optical purity, with various functional groups being tolerated with this system. A major deactivation pathway is presented and eliminated by using alternative reaction conditions. A detailed experimental and computational study on the reaction mechanism is performed providing valuable insights into the mode of action of the catalytic system and pointing to future modifications to be made for this catalytic platform.

Evidence for “cocktail”-type catalysis in Buchwald–Hartwig reaction. A mechanistic study

The mechanism of the C–N cross-coupling reaction, catalyzed by Pd/NHC, was evaluated at the molecular and nanoscale levels. The first evidence for the involvement of a “cocktail”-type system in the Buchwald–Hartwig reaction is provided.

Crystal structures of [μ2-(Ra ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium di-chloro-methane monosolvate and [μ2-(Sa ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium

The title compounds, [Ir(C51H64N2)Cl(C8H12)]·CH2Cl2, (I), and [Ir(C51H64N2)Cl(C8H12)], (II), represent the first two examples of hexa-hydro-benzo-imidazole-based N-heterocyclic carbene (NHC) iridium complexes. The diastereomeric complexes differing only in their axial chirality, which could be separated via column chromatography, show noticeable differences in their 1H NMR spectra. Compound (I) crystallizes in the monoclinic system (P21) with two independent complexes and two half-occupied di-chloro-methane mol-ecules in the asymmetric unit, while compound (II) crystallizes in the ortho-rhom-bic system (P212121) with one complex in the asymmetric unit. The fused five-membered N-heterocycles of NHCs show unusually high backbone torsion angles of -34.1 (5) and -30.9 (5)° for (I) and -31.5 (7)° for (II), but the Ir-Ccarbene bond lengths of 2.046 (6) and 2.021 (6) Å for (I) and 2.045 (8) Å for (II) present typical NHC-Ir bond lengths. The solvent mol-ecule in the crystal of (I) was found to be highly disordered and its contribution to the scattering was masked using the solvent-masking routine smtbx.mask in OLEX2 [Dolomanov et al. (2009 ▸). J. Appl. Cryst. 42, 339-341]. The solvent contribution is not included in the reported chemical formula and other crystal data.

Visible-Light-Induced Palladium-Catalyzed Generation of Aryl Radicals from Aryl Triflates

A mild visible-light-induced Pd-catalyzed intramolecular C-H arylation of amides is reported. The method operates by cleavage of a C(sp2 )-O bond, leading to hybrid aryl Pd-radical intermediates. The following 1,5-hydrogen atom translocation, intramolecular cyclization, and rearomatization steps lead to valuable oxindole and isoindoline-1-one motifs. Notably, this method provides access to products with readily enolizable functional groups that are incompatible with traditional Pd-catalyzed conditions.

Synthesis of α-Aryl Oxindoles by Friedel-Crafts Alkylation of Arenes

α-Aryl oxindoles are accessed from isatin via a two-step procedure involving a phospha-Brook rearrangement and a Friedel-Crafts alkylation in a one-pot procedure. The use of 1,1,1,3,3,3-hexafluoro-2-propanol as solvent significantly extended the reaction substrate scope to include relatively less electron-rich arenes including benzene. This new alkylation method is fast and straightforward and allows for the direct introduction of the oxindole moiety onto a range of aromatic compounds including phenols. Additionally, the application of arylated products was shown in decarboxylative asymmetric allylation and protonation.