Palladium-catalyzed arylation and heteroarylation of indolizines

Copper-Catalyzed Nucleophilic Cycloisomerization Cascade Constructing Azepinoindolizine

Numerous effective bioisosteric replacements have been identified through substituting scaffolds and functional groups in lead molecules with alternative ones that preserve or enhance the desired biological activity of the original compound. Here, a copper-catalyzed nucleophilic cycloisomerization was developed to access potential bioisosteric replacements of azepinoindole. In this process, "tetra-alkene" characteristic of indolizine undergoes a 12π electrocyclization, offering a complementary method to obtain azepinoindolizine derivatives that are otherwise challenging to prepare through conventional means.

Palladium(II)-catalyzed annulation of N-methoxy amides and arynes: computational mechanistic insights and substituents effects

CONTEXT

The combined use of transition metal-catalyzed C-H activation with aryne annulation reactions has emerged as an important strategy in organic synthesis. In this study, the mechanisms of the palladium(II)-catalyzed annulation reaction of N-methoxy amides and arynes were computationally investigated by density functional theory. The role of methoxy amide as a directing group was elucidated through the calculation of three different pathways for the C-H activation step, showing that the pathway where amide nitrogen acts as a directing group is preferable. At the reductive elimination transition state, an unstable seven-membered ring is formed preventing the lactam formation. A substituent effect study based on an NBO analysis, Hammet, and using a More O'Ferall-Jenks plot indicates that the C-H activation step proceeds via an electrophilic concerted metalation-deprotonation (eCMD) mechanism. The results show that electron-withdrawing groups increase the activation barrier and contribute to an early Pd-C bond formation and a late C-H bond breaking when compared with electron-donating substituents. Our computational results are in agreement with the experimental data provided in the literature.

METHODS

All calculations were performed using Gaussian 16 software. Geometry optimizations, frequency analyses at 393.15 K, and IRC calculations were conducted at the M06L/Def2-SVP level of theory. Corrected electronic energies, NBO charges, and Wiberg bond indexes were computed at the M06L/Def2-TZVP//M06L/Def2-SVP level of theory. Implicit solvent effects were considered in all calculations using the SMD model, with acetonitrile employed as the solvent.

Thermodynamic Controlled Regioselective C1-Functionalization of Indolizines with 3-Hydroxyisoindolinones via Brønsted Acid Catalyzed aza-Friedel-Crafts Reaction

A Brønsted acid catalyzed aza-Friedel-Crafts reaction of indolizines with 3-hydroxyisoindolinones has been established, which constructs isoindolinone derivatives bearing a tetrasubstituted stereocenter in good to high yields and enantioselectivities. Notably, this strategy provides a new access to C1-functionalization of indolizines with excellent regioselectivities. Moreover, this intriguing C1-regioselective transformation was induced under thermodynamic control.

Opening a Pandora's Flask on a Prototype Catalytic Direct Arylation Reaction of Pentafluorobenzene: The Ag2CO3/Pd(OAc)2/PPh3 System

Direct C-H functionalization reactions have opened new avenues in catalysis, removing the need for prefunctionalization of at least one of the substrates. Although C-H functionalization catalyzed by palladium complexes in the presence of a base is generally considered to proceed by the CMD/AMLA-6 mechanism, recent research has shown that silver(I) salts, frequently used as bases, can function as C-H bond activators instead of (or in addition to) palladium(II). In this study, we examine the coupling of pentafluorobenzene 1 to 4-iodotoluene 2a (and its analogues) to form 4-(pentafluorophenyl)toluene 3a catalyzed by palladium(II) acetate with the commonplace PPh3 ligand, silver carbonate as base, and DMF as solvent. By studying the reaction of 1 with Ag2CO3/PPh3 and with isolated silver (triphenylphosphine) carbonate complexes, we show the formation of C-H activation products containing the Ag(C6F5)(PPh3)n unit. However, analysis is complicated by the lability of the Ag-PPh3 bond and the presence of multiple species in the solution. The speciation of palladium(II) is investigated by high-resolution-MAS NMR (chosen for its suitability for suspensions) with a substoichiometric catalyst, demonstrating the formation of an equilibrium mixture of Pd(Ar)(κ1-OAc)(PPh3)2 and [Pd(Ar)(μ-OAc)(PPh3)]2 as resting states (Ar = Ph, 4-tolyl). These two complexes react stoichiometrically with 1 to form coupling products. The catalytic reaction kinetics is investigated by in situ IR spectroscopy revealing a two-term rate law and dependence on [Pdtot/nPPh3]0.5 consistent with the dissociation of an off-cycle palladium dimer. The first term is independent of [1], whereas the second term is first order in [1]. The observed rates are very similar with Pd(PPh3)4, Pd(Ph)(κ1-OAc)(PPh3)2, and [Pd(Ph)(μ-OAc)(PPh3)]2 catalysts. The kinetic isotope effect varied significantly according to conditions. The multiple speciation of both AgI and PdII acts as a warning against specifying the catalytic cycles in detail. Moreover, the rapid dynamic interconversion of AgI species creates a level of complexity that has not been appreciated previously.

Regioselective Direct C-H Bond Heteroarylation of Thiazoles Enabled by an Iminopyridine-Based α-Diimine Nickel(II) Complex Evaluated by DFT Studies

Direct C-H bond arylation is a highly effective method for synthesizing arylated heteroaromatics. This method reduces the number of synthetic steps and minimizes the formation of impurities. We report an air- and moisture-stable iminopyridine-based α-diimine nickel(II) complex for direct C5-H bond arylation of thiazole derivatives. Under a low catalyst loading and performing the reactions at lower temperatures (80 °C) under aerobic conditions, we produced mono- and diarylated thiazole units. Competition experiments and density functional theory calculations revealed that the mechanism of C-H activation in 4-methylthiazole involves an electrophilic aromatic substitution.

Metal-free visible-induced C(sp2)-C(sp2) coupling of quinoxalin-2(H)-ones via oxidative cleavage of the C-N bond

A C(sp²)-C(sp²) reaction between aromatic hydrazines and quinoxalines has been developed through a photocatalytic system. The protocol is established for C(sp²)-N bond cleavage and direct C(sp²)-H functionalization for the coupling of C(sp²)-C(sp²) via photocatalysis under mild and ideal air conditions without the presence of a strong base and metal. The mechanistic studies reveal that the generation of a benzene radical via the oxidative cleavage of aromatic hydrazines for the cross-coupling of C(sp²)-C(sp²) with the assistance of a photocatalyst is essential. The process exhibits excellent compatibility with functional groups and provides convenient access to various 3-arylquinoxalin-2(1H)-ones in good to excellent yields.

Mechanistic studies of the palladium-catalyzed S,O-ligand promoted C-H olefination of aromatic compounds

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has been shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions.

Palladium-Catalyzed Regioselective C3-Allylic Alkylation of 2-Aryl Imidazopyridines with MBH Carbonates

Imidazopyridine is an important framework that constitutes several pharmaceutical drugs and biologically active molecules. Herein, we present the palladium-catalyzed regioselective C3-allylic alkylation of 2-aryl imidazopyridines with MBH carbonates. This strategy furnishes a broad spectrum of C3-allylated imidazopyridines, and their structures have been unequivocally established using X-ray analysis. Besides, the reaction can be easily scaled up on a gram scale, and the ensuing product can be smoothly manipulated into synthetically useful entities.

Rational ligand modification maximizes turnover rate in a model Pd-catalyzed C-H arylation

The direct cross-coupling of (hetero)aromatics without prior functionalization is a promising reaction for the chemical and pharmaceutical industries, enabling the conversion of inexpensive feedstocks in a highly step-efficient manner. However, many C-H arylations rely on high loadings of a Pd catalyst that preclude their use in low-cost applications. In this work, we have maximized the turnover rate of a Pd-catalyzed C-H arylation reaction through rational tuning of the ligands. Starting from a computational study of the catalytic mechanism, a kinetic model was derived that accurately explains the experimental time profiles. Quantitative structure-activity relationships between the substituents on the ligands and the resulting catalytic activity for various C-H arylations were obtained. This study demonstrates that, depending on the coupling partner, the C-H activation is not the sole rate-limiting step, and that the ligands can be adapted accordingly to further accelerate catalytic turnover.

Heteroarylation of Congested α-Bromoamides with Imidazo-Heteroarenes and Indolizines via Aza-Oxyallyl Cations: Enroute to Dibenzoazepinone and Zolpidem Analogues

Herein, we report a highly efficient and unprecedented approach for heteroarylation of congested α-bromoamides via electrophilic aromatic substitution of imidazo-heteroarenes and indolizines under mild reaction conditions (room temperature, metal, and oxidant free). The participation of an in situ generated aza-oxyallyl cation as an alkylating agent is the hallmark of this transformation. The method was readily adapted to synthesize novel imidazo-heteroarene-fused dibenzoazepinone architectures of potential medicinal value.

B2pin2-Mediated Cascade Cyclization/Aromatization Reaction: Facial Access to Functionalized Indolizines

Herein, a B₂pin₂-mediated radical cascade cyclization/aromatization reaction of enaminone with pyridine is described. This strategy provides a practical way for the construction of valuable functionalized indolizines under metal-, external oxidant-, and base-free conditions, which could be compatible with various kinds of functional groups, such as halogen, π-system, heterocycle, ferrocenyl, etc. A preliminary mechanism investigation indicated that the pyridine-boryl radical formed in situ triggered the reaction to occur.

Synthesis of Azepinoindoles via Pd-Catalyzed C(sp2)-H Imidoylative Cyclization Reactions

An efficient and convenient method for the construction of diverse free (N-H)-benzazepinoindoles by Pd-catalyzed C(sp²)-H imidoylative cyclization of 3-(2-isocyanobenzyl)-1H-indoles was developed. The reaction shows a wide substrate scope and can be scaled up, providing a practical route to valuable bioactive azepinoindoles.

Acetic Acid-Catalyzed Regioselective C(sp2)-H Bond Functionalization of Indolizines: Concomitant Involvement of Synthetic and Theoretical Studies

An atom economical and environmentally benign protocol has been developed for the regioselective C(sp²)-H bond functionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing a wide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broad substrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played a crucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafast nature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated. Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzed pathway. Furthermore, regioselectivity has also been explained theoretically.

Rhodium-Catalyzed Atroposelective C-H/C-H Cross-Coupling Reaction between 1-Aryl Isoquinoline Derivatives and Indolizines

A rhodium-catalyzed asymmetric oxidative C-H/C-H cross-coupling reaction between 1-aryl isoquinolines and indolizines is disclosed. With a matched pair of SCpRh complex and chiral carboxylic acid, enantioselective two-fold C-H/C-H cross-coupling reactions between 1-aryl isoquinolines and indolizines provide a variety of axially chiral bi(hetero)aryls in excellent yields and enantioselectivity (up to 96% yield and 98% ee). Mechanistic studies suggest that both C-H cleavages are likely reversible.

Chiral Phosphoric Acid Catalyzed Desymmetrization of Cyclopentendiones via Friedel-Crafts Conjugate Addition of Indolizines

An organocatalytic highly diastero- and enantioselective Friedel-Crafts conjugate addition of indolizines to prochiral cyclopentenediones has been successfully developed. This desymmetric transformation provides a direct access to the desired indolizine-substituted cyclopentanediones in yields of 62-91% and excellent stereoselectivities. The utility of the approach was demonstrated by diverse late-stage functionalizations through reduction or oxidation. Importantly, the direct sp² C-H functionalization with nitromethane in one-pot process resulted in the indolizine-linked axially chiral styrene bearing a remote chiral center.

Direct Arylation in the Presence of Palladium Pincer Complexes

Direct arylation is an atom-economical alternative to more established procedures such as Stille, Suzuki or Negishi arylation reactions. In comparison with other palladium sources and ligands, the use of palladium pincer complexes as catalysts or pre-catalysts for direct arylation has resulted in improved efficiency, higher reaction yields, and advantageous reaction conditions. In addition to a revision of the literature concerning intra- and intermolecular direct arylation reactions performed in the presence of palladium pincer complexes, the role of these remarkably active catalysts will also be discussed.

Ligand-free Pd/Ag-mediated dehydrogenative alkynylation of imidazole derivatives

A variety of 2-alkynyl(benzo)imidazoles have been synthesized by dehydrogenative alkynylation of (benzo)imidazoles with terminal alkyne in NMP under air in the presence of Ag₂CO₃ as the oxidant and Pd(OAc)₂ as the catalyst precursor. The data obtained in this study support a reaction mechanism involving a non-concerted metalation deprotonation (n-CMD) pathway.

The regioselective synthesis of 2-alkynyl(benz)imidazoles was successfully achieved by Pd(ii)/Ag(i)-mediated dehydrogenative alkynylation of the corresponding (benz)imidazoles with terminal alkynes in an open vessel.

Ni/NHC catalysis in C–H functionalization using air-tolerant nickelocene and sodium formate for in situ catalyst generation

A facile method for Ni/NHC catalyzed C–H alkylation and alkenylation of heteroarenes with alkenes and internal alkynes using air-tolerant nickelocene, sodium formate and NHC·HCl salts for in situ catalyst generation has been developed.

Brønsted-acid-catalyzed selective Friedel-Crafts monoalkylation of isatins with indolizines in water

The controlled mono-addition of indolizines to isatins under very mild conditions is described. The reaction occurs in water using diphenylphosphate as the catalyst, which is dramatically accelerated by surfactant addition. 3-Hydroxy-3-indolizinyl-2-oxindole scaffolds were synthesized in up to >99% yield. Notably, in organic solvents only bis-addition products were observed.

Organocatalytic C3-functionalization of indolizines: synthesis of biologically important indolizine derivatives

A Brønsted acid-catalyzed C3-alkylation of indolizines has been established with different electrophiles such as ortho-hydroxybenzyl alcohols, other precursors of para-quinone methides and ortho-quinone methides, and 2-indolylmethanol as well. By using this approach, a series of C3-functionalized indolizines were synthesized in overall good yields (up to 89%). These examples demonstrate that the Brønsted acid-catalyzed C3-alkylation of indolizines has a wide applicability, which can serve as a useful method for synthesizing C3-functionalized indolizine derivatives with structural diversity. This reaction has fulfilled the task of developing organocatalytic C3-functionalization of indolizines for the synthesis of biologically important indolizine derivatives.

Palladium(II)/Lewis Acid-Catalyzed Oxidative Olefination/Annulation of N-Methoxybenzamides: Identifying the Active Intermediates through NMR Characterizations

Although Pd(II)-catalyzed C-H activation in arenes has been widely successful in organic synthesis with many palladacycle compounds isolated as the intermediates in ligand-directed C-H activation, direct identification of the reaction intermediates such as the π-complex prior to the C-H activation is still not successful because of their instability. In the present study, we introduce a Pd(II)/LA (LA: Lewis acid)-catalyzed oxidative olefination/annulation reaction between N-methoxybenzamides and acrylates with oxygen as the oxidant source, in which two intermediates, including an unsymmetrical η⁶-complex and a palladacycle species without the proton releasing to the environment, were identified through NMR characterizations. The in situ formation of the heterobimetallic Pd(II)/LA species such as Pd(II)/Sc(III) may have enhanced the electrophilic properties of the Pd²⁺ cation, thus improving the stability of the π-complex, herein, an unsymmetrical η⁶-complex, and improving its catalytic efficiency. The observed insensitive electronic effect preferred the concerted metalation-deprotonation (CMD) mechanism for this C-H activation, and the detected palladacycle intermediate without the proton releasing to the environment offered an experimental clue to support the proposed CMD mechanism.

Catalyst-Free Conjugate Addition of Indolizines to In Situ-Generated Oxidized Morita-Baylis-Hillman Adducts

Sequential one-pot 2-iodoxybenzoic acid (IBX) oxidation of Morita-Baylis-Hillman (MBH) adducts followed by catalyst-free indolizine conjugate addition was developed. The wide scopes of MBH adducts and indolizines were investigated, and densely functionalized adducts were obtained in yields of up to 94%. The conjugate addition step occurred in less than a minute at room temperature with total regioselectivity toward indolizine C3 carbon. Less nucleophilic C1 carbon was also alkylated when C3-substituted indolizines were employed as the substrate.

Catalyst Control in Switching the Site Selectivity of C-H Olefinations of 1,2-Dihydroquinolines: An Approach to Positional-Selective Functionalization of Quinolines

A unique approach to achieve site-selective C-H olefinations exclusively at the C-3- or C-8-positions in the quinoline framework has been developed by catalyst control. Distal C(3)-H functionalization is achieved by using palladium catalysis, whereas proximal C(8)-H functionalization is obtained by employing ruthenium catalysis. Switching the site selectivity within a single substrate directly indicates two diverse pathways, which are operating under the palladium- and ruthenium-catalyzed reaction conditions.

Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (eCMD)

Oxidative C-H/C-H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is nevertheless hindered by the need for catalysts that excel in forging core semiconductor motifs, such as ubiquitous oligothiophenes, with high efficiency in the absence of metal reagents. We report a (thioether)Pd-catalyzed oxidative coupling method for the rapid assembly of both privileged oligothiophenes and challenging hindered cases, even at low catalyst loading under Ag- and Cu-free conditions. A combined experimental and computational mechanistic study was undertaken to understand how a simple thioether ligand, MeS(CH₂)₃SO₃Na, leads to such potent reactivity toward electron-rich substrates. The consensus from these data is that a concerted, base-assisted C-H cleavage transition state is operative, but thioether coordination to Pd is associated with decreased synchronicity (bond formation exceeding bond breaking) versus the "standard" concerted metalation-deprotonation (CMD) model that was formalized by Fagnou in direct arylation reaction. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term electrophilic CMD (eCMD) is introduced to distinguish this mechanism from the standard model, even though both mechanisms locate in a broad concerted continuum. More O'Ferrall-Jencks analysis further suggests eCMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring eCMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.

Pd-catalyzed regioselective intramolecular direct arylation of 3-indolecarboxamides: access to spiro-indoline-3,3′-oxindoles and 5,11-dihydro-6H-indolo[3,2-c]quinolin-6-ones

We report regioselective intramolecular direct C-3 and C-2 arylations of the indole rings of 3-indolecarboxamides for diastereospecific production of spiro-indoline-3,3′-oxindoles and 5,11-dihydro-6H-indolo[3,2-c]quinolin-6-ones, respectively, under different reaction conditions.