Rh(I)/NHC*-Catalyzed Site- and Enantioselective Functionalization of C(sp3)–H Bonds Toward Chiral Triarylmethanes

Chiral Primary Amine-Catalyzed Asymmetric Photochemical Reactions of Pyridotriazoles with Boronic Acids to Access Triarylmethanes

Imine-containing azaarene-based triarylmethanes are vital molecular motifs that are prevalent in a wide array of bioactive compounds. Recognizing the limitations of current synthetic methodologies─marked by a scarcity of examples and difficulties in flexible functional group modulation─we have developed an efficient and modular asymmetric photochemical strategy employing pyridotriazoles and boronic acids as substrates. Utilizing novel chiral diamine-derived pyrroles and primary amines as catalysts, we successfully synthesized a diverse range of triarylmethanes with high yields and excellent enantioselectivities. This method not only exhibits a broad substrate scope and outstanding functional group tolerance but also enables the precise synthesis of deuterated derivatives using inexpensive D2O as the deuterium source. Mechanistic studies reveal that an unusual 1,4-boron shift is a critical step in generating the boronated enamine intermediate, while also shedding light on the potential enantiocontrol mechanisms facilitated by the chiral catalyst.

Asymmetric Migratory Tsuji-Wacker Oxidation Enables the Enantioselective Synthesis of Hetero- and Isosteric Diarylmethanes

Diarylmethanes play, in part, a pivotal role in the design of highly potent, chiral, nonracemic drugs whose bioactivity is typically affected by the substitution pattern of their arene units. In this context, certain arenes such as para-substituted benzenes or unsubstituted heteroarenes cause particular synthetic challenges, since such isosteric residues at the central methane carbon atom are typically indistinguishable for a chiral catalyst. Hence, the stereoselective incorporation of isosteric (hetero)arenes into chiral methane scaffolds requires the use of stoichiometrically differentiated building blocks, which is typically realized through preceding redox-modifying operations such as metalation or halogenation and thus associated with disadvantageous step- and redox-economic traits. As a counter-design, we report herein a generalized enantioselective synthesis of chiral diarylmethanes by means of an asymmetric migratory Tsuji-Wacker oxidation of simple stilbenes. The title protocol relies on the well-adjusted interplay of aerobic photoredox and selenium-π-acid catalysis to allow for the installation of a broad variety of arenes, including isosteric ones, into the methane core. Facial differentiation and regioselectivity are solely controlled by the selenium catalyst, which (a) renders the E/Z-configuration of the stilbene substrates inconsequential and (b) permits the stereodivergent synthesis of both product enantiomers from a single catalyst enantiomer, simply by employing constitutionally isomeric starting materials. Altogether, this multicatalytic platform offers the target structures with high levels of enantioselectivity in up to 97% ee, which has also been successfully exploited in expedited syntheses of antihistaminic (R)- and (S)-neobenodine.

Chiral Bis(binaphthyl) Cyclopentadienyl Ligands for Rhodium-Catalyzed Desymmetrization of Diarylmethanes via Selective Arene Coordination

Owing to substantial advances in the past several decades, transition-metal-catalyzed asymmetric reactions have garnered considerable attention as pivotal methods for constructing chiral molecules from abundant, readily available achiral counterparts. These advances are largely attributed to the development of chiral ligands that control stereochemistry through steric repulsion and other noncovalent interactions between the ligands and functional groups or prochiral centers on the substrates. However, stereocontrol weakens dramatically with increasing distance between the reaction site and the functional group or prochiral center. Herein, we report a symphonic strategy for remote stereocontrol of Rh(III)-catalyzed asymmetric benzylic C-H bond addition reactions of diarylmethanes in which the two aryl motifs differ at the meta and/or para position. Specifically, catalysts bearing a new type of chiral cyclopentadienyl (Cp) ligand differentiate between the two aromatic rings of the diarylmethane by arene-selective η6 coordination, setting up an opportunity for ligand-controlled stereoselective benzylic deprotonation and subsequent stereoselective addition to the 1,1-bis(arylsulfonyl)ethylene.

Dual C(sp3)-H and C(sp2)-H Activation of 8-Methylquinoline N-Oxides: A Route to Access C7-H Bond

A Pd(II)-catalyzed regioselective dual C(sp3)-H/C7(sp2)-H activation and annulation of 8-methylquinoline N-oxides with maleimide has been accomplished. The use of N-oxide as a weak directing group under Pd(II)-complex catalysis activates the initial C(sp3)-H and triggers a relayed, second C7(sp2)-H activation. The dual C-H bond activation, [3 + 2]-annulation, facile introduction and removal of the directing group, substrate scope, and functional group diversity are the important practical features.

Rh(iii)-catalyzed sp3/sp2-C-H heteroarylations via cascade C-H activation and cyclization

The development of an efficient strategy for facile access to quinoline-based bis-heterocycles holds paramount importance in medicinal chemistry. Herein, we describe a unified approach for accessing 8-(indol-3-yl)methyl-quinolines by integrating Cp*Rh(iii)-catalyzed C(sp3)-H bond activation of 8-methylquinolines followed by nucleophilic cyclization with o-ethynylaniline derivatives. Remarkably, methoxybiaryl ynones under similar catalytic conditions delivered quinoline tethered spiro[5.5]enone scaffolds via a dearomative 6-endo-dig C-cyclization. Moreover, leveraging this method for C8(sp2)-H bond activation of quinoline-N-oxide furnished biologically relevant oxindolyl-quinolines. This reaction proceeds via C(sp2)-H bond activation, regioselective alkyne insertion, oxygen-atom-transfer (OAT) and intramolecular nucleophilic cyclization in a cascade manner. One C-C, one C-N and one C[double bond, length as m-dash]O bond were created with concomitant formation of a quaternary center.

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Enantioselective Rh(I)-Catalyzed C-H Arylation of Ferroceneformaldehydes

As an important class of platform molecules, planar chiral ferrocene carbonyl compounds could be transformed into various functional groups offering facile synthesis of chiral ligands and catalysts. However, developing efficient and straightforward methods for accessing enantiopure planar chiral ferrocene carbonyl compounds, especially ferroceneformaldehydes, remains highly challenging. Herein, we report a rhodium(I)/phosphoramidite-catalyzed enantioselective C-H bond arylation of ferroceneformaldehydes. Readily available aryl halides such as aryl iodides, aryl bromides, and even aryl chlorides are suitable coupling partners in this transformation, leading to a series of planar chiral ferroceneformaldehydes in good yields and excellent enantioselectivity (up to 83% yield and >99% ee). The aldehyde group could be transformed into diverse functional groups smoothly, and enantiopure Ugi's amine and PPFA analogues could be synthesized efficiently. The latter was found to be a highly efficient ligand in Pd-catalyzed asymmetric allylic alkylation reactions. Mechanistic experiments supported the formation of imine intermediates as the key step during the reaction.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Highly efficient α-arylation of aryl ketones with aryl chlorides by using bulky imidazolylidene-ligated oxazoline palladacycles

α-Aryl derivatives of carbonyl compounds are important building blocks. Herein, we presented an efficient catalytic system for the α-arylation of aryl ketones with inactive aryl chlorides by firstly using N,N'-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr)-ligated chiral oxazoline palladacycles, and tolerated a wide range of substrates at low catalyst loadings, leading to the desired products in good to excellent yields.

Transition-Metal-Free Synthesis of Polyfluoro-Polyarylmethanes via Direct Cross-Coupling of Polyfluoroarenes and Benzyl Chlorides

The transition-metal-free direct cross-coupling between polyfluoroarenes and benzyl chlorides is reported. In this strategy, a variety of polyfluoro di-, tri- and tetra-arylmethanes was efficiently prepared with good to excellent yields in the presence of Mg turnings via a one-pot procedure. Significantly, this method provides a general approach for the synthesis of polyfluorinated polyarylmethanes.

Enantioselective synthesis of triarylmethanes via organocatalytic transfer hydrogenation of para-quinone methides

A new organocatalytic asymmetric method for the synthesis of enantioenriched triarylmethanes is developed. Different from the conventional approaches featuring asymmetric arylation, the present study employs asymmetric reduction via C-H bond formation as the key step. This approach does not require the presence of a heteroaryl ring or the presynthesis of unstable para-quinone methides. Instead, the stable racemic triarylmethanols were used as substrates for the in situ generation of the intermediates with a suitable chiral phosphoric acid catalyst.

Different Chiral Ligands Assisted Enantioselective C-H Functionalization with Transition-Metal Catalysts

C–H bonds are common in organic molecules, and the functionalization of these inactive C–H bonds has become one of the most powerful methods used to assemble complicated bioactive molecules from readily available starting materials. However, a central challenge in these reactions is controlling their stereoselectivity. Recently, significant progress has been made in the development of enantioselective C–H activation enabled by different chiral ligands for the formation of C–C and C–X bonds bearing a chiral center. In this paper, we focus on some archetypal chiral ligands for enantioselective C–H functionalization developed in recent years and analyze the mechanism of these methods, aiming to accelerate related research and to search for more efficient strategies.

Switchable, Reagent-Controlled C(sp3)-H Selective Iodination and Acetoxylation of 8-Methylquinolines

An efficient Pd-catalyzed C(sp³)-H selective iodination of 8-methylquinolines is reported herein for the first time. Because of the versatility of organic iodides, the method offers a facile access to various C8-substituted quinolines. By slightly switching the reaction conditions, an efficient C(sp³)-H acetoxylation of 8-methylquinolines has also been enabled. Both approaches feature mild reaction conditions, good tolerance of functional groups, and a broad substrate scope.

Palladium-Catalyzed Enantioselective C(sp3)-H Arylation of 2-Propyl Azaaryls Enabled by an Amino Acid Ligand

A palladium(II)-catalyzed enantioselective arylation of unbiased secondary C(sp³)-H bonds was developed. The enantioselectivity was controlled by the combination of a pyridyl or isoquinolinyl directing group and an amino acid, N-Boc-2-pentyl proline. A variety of 2-propyl azaaryls and biaryl iodides were employed to provide arylated products in moderate to good yields (up to 82%) with high enantioselectivities (up to 93:7 er). This reaction is a rare example of an amino-acid-enabled enantioselective acyclic methylene C(sp³)-H arylation. Furthermore, the reaction proceeded with high enantioselectivity even on a gram scale, and the product was transformed to a 5,6,7,8-tetrahydroisoquinoline bioactive molecule. Kinetic isotope effect (KIE) experiments indicated that C-H activation is the rate-determining step for the enantioselective C(sp³)-H arylation.

Chiral polycyclic benzosultams from photocatalytic diastereo- and enantioselective benzylic C–H functionalization

Photocatalytic diastereo- and enantioselective C(sp3)–H functionalization/intramolecular cyclization reactions have been achieved, delivering optically active polycyclic benzosultams and fused tetrahydroisoquinolines.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Pd(II)-Catalyzed Arylation/Oxidation of Benzylic C-H of 8-Methylquinolines: Access to 8-Benzoylquinolines

An efficient access to 8-benzoylquinoline was developed by a sequential arylation/oxidation of 8-methylquinolines with aryl iodides in the presence of Pd(OAc)₂. This transformation demonstrates good tolerance of a wide range of functional groups on aryl iodides, providing good to excellent yields of 8-benzoylquinolines.

Organocatalytic asymmetric formal oxidative coupling for the construction of all-aryl quaternary stereocenters

A new catalytic asymmetric formal cross dehydrogenative coupling process for the construction of all-aryl quaternary stereocenters is disclosed, which provides access to rarely explored chiral tetraarylmethanes with excellent enantioselectivity. The suitable oxidation conditions and the hydrogen-bond-based organocatalysis have enabled efficient intermolecular C–C bond formation in an overwhelmingly crowded environment under mild conditions. para-Quinone methides bearing an ortho-directing group serve as the key intermediate. The precise loading of DDQ is critical to the high enantioselectivity. The chiral products have also been demonstrated as promising antiviral agents.

A one-pot oxidation of racemic triarylmethanes to form para-quinone methides followed by enantioselective construction of all-aryl quaternary stereocenters has been developed.

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC ^C Cyclometalated Ligands

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp² -H or Csp³ -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20^th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based C_NHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

Triarylmethanes and their Medium-Ring Analogues by Unactivated Truce-Smiles Rearrangement of Benzanilides

Intramolecular nucleophilic aromatic substitution (Truce–Smiles rearrangement) of the anions of 2‐benzyl benzanilides leads to triarylmethanes in an operationally simple manner. The reaction succeeds even without electronic activation of the ring that plays the role of electrophile in the S_NAr reaction, being accelerated instead by the preferred conformation imposed by the tertiary amide tether. The amide substituent of the product may be removed or transformed into alternative functional groups. A ring‐expanding variant (n to n+4) of the reaction provided a route to doubly benzo‐fused medium ring lactams of 10 or 11 members. Hammett analysis returned a ρ value consistent with the operation of a partially concerted reaction mechanism.

Multicomponent Synthesis of Unsymmetrical 4,5-Disubstituted Imidazolium Salts as N-Heterocyclic Carbene Precursors: Applications in Palladium-Catalyzed Cross-Coupling Reactions

Various novel (a)chiral 4,5-disubstituted 1-aryl-3-alkyl-imidazolium salts were synthesized via the multicomponent reaction of diketone derivatives, sterically congested arylamines, and alkylamines. Moreover, two novel unsymmetrical bulky cycloalkyl-based NHC-Pd complexes proved highly active as catalysts for Suzuki-Miyaura and Negishi cross-coupling reactions.

Straightforward Construction and Functionalizations of Nitrogen-Containing Heterocycles Through Migratory Insertion of Metal-Carbenes/Nitrenes

Nitrogen-containing heterocycles are widely found in various biologically active substrates, pharmaceuticals, natural products and organic materials. Consequently, the continuous effort has been devoted towards the development of straightforward, economical, environmentally acceptable, efficient and ingenious methods for the synthesis of various N-containing heterocycles and their functionalizations. Arguably, one of the most prominent direct strategy is regioselective C-H bond functionalizations which provide the step and atom economical approaches in the presence of suitable coupling partners. In this context, site-selective migratory insertion of metal carbenes/nitrenes to the desired C-H bonds has proven as a useful tool to access various functionalized nitrogen heterocycles. In this personal account, we highlight some of our contemporary development toward constructing N-containing heterocycles and their direct functionalizations via transition metal catalysed C-H bond functionalizations based on migratory insertion of metal-carbenes and nitrenes.

Transition-Metal-Free Synthesis of Polyfunctional Triarylmethanes and 1,1-Diarylalkanes by Sequential Cross-Coupling of Benzal Diacetates with Organozinc Reagents

A variety of functionalized triarylmethane and 1,1-diarylalkane derivatives were prepared via a transition-metal-free, one-pot and two-step procedure, involving the reaction of various benzal diacetates with organozinc reagents. A sequential cross-coupling is enabled by changing the solvent from THF to toluene, and a two-step SN 1-type mechanism was proposed and evidenced by experimental studies. The synthetic utility of the method is further demonstrated by the synthesis of several biologically relevant molecules, such as an anti-tuberculosis agent, an anti-breast cancer agent, a precursor of a sphingosine-1-phosphate (S1P) receptor modulator, and a FLAP inhibitor.

Pd-Catalyzed sp3 C-H alkoxycarbonylation of 8-methylquinolines using Mo(CO)6 as a CO surrogate

A Pd(ii)-catalyzed three-component sp³ C-H alkoxycarbonylation of 8-methylquinonlines (8-MQs) with alcohols is accomplished using the colorless crystalline Mo(CO)₆ as a CO source. The protocol is compatible with a wide range of 8-MQs and alcohols, furnishing the carbonylated adducts in moderate to good yields. The substrate scope, functional group tolerance and natural product mutation are the important practical features.

Pyridylic anions are soft nucleophiles in the palladium-catalyzed C(sp3)-H allylation of 4-alkylpyridines

We report a mild palladium-catalyzed method for the selective allylation of 4-alkylpyridines in which highly basic pyridylic anions behave as soft nucleophiles. This method exploits alkylidene dihydropyridines, which are semi-stable intermediates readily formed using a ‘soft-enolization’ approach, in a new mechanistic manifold for decarboxylative allylation. Notably, the catalytic generation of pyridylic anions results in a substantially broader functional group tolerance compared to other pyridine allylation methods. Experimental and theoretical mechanistic studies strongly suggest that pyridylic anions are indeed the active nucleophiles in these reactions, and that they participate in an outer-sphere reductive elimination step. This finding establishes a new pK_a boundary of 35 for soft nucleophiles in transition metal-catalyzed allylations.

We report a mild palladium-catalyzed method for the selective allylation of 4-alkylpyridines in which highly basic pyridylic anions behave as soft nucleophiles.

Benzylic Methylene Functionalizations of Diarylmethanes

Diarylmethanes are cardinal scaffolds by virtue of their unique structural feature including the presence of a benzylic CH2 group that can be easily functionalized to generate a variety of fascinating molecules holding immense importance in pharmaceutical, agrochemical, and material sciences. While the originally developed protocols for benzylic C-H functionalization in diarylmethanes employing base-mediated and metal-catalyzed strategies are still actively used, they are joined by a new array of metal-free conditions, offering milder and benign conditions. With the recent surge of interest towards the synthesis of functionalized diarylmethanes, numerous choices are now available for a synthetic organic chemist to transform the benzylic C-H bond to C-C or C-X bond offering the synthesis of any molecule of choice. This review highlights benzylic methylene (CH2 ) functionalizations of diaryl/heteroarylmethanes utilizing various base-mediated, transition-metal-catalyzed, and transition-metal free approaches for the synthesis of structurally diverse important organic molecules, often with a high chemo-, regio- and enantio-selectivity. This review also attempts to provide analysis of the scope and limitations, mechanistic understanding, and sustainability of the transformations.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Catalytic asymmetric 1,4-type Friedel–Crafts (hetero)arylations of 1-azadienes: the highly enantioselective syntheses of chiral hetero-triarylmethanes

Strategies for achieving the direct catalytic asymmetric syntheses of benzofuran-containing hetero-triarylmethanes using a 1,4-type Friedel–Crafts (hetero)arylation reaction were developed.

Tritylium assisted iodine catalysis for the synthesis of unsymmetrical triarylmethanes

The combined Lewis acid catalytic system, generated from molecular iodine and tritylium tetrafluoroborate effectively catalyzed the Friedel-Crafts (FC) arylation of diarylmethyl sulfides providing an efficient access to various unsymmetrical triarylmethanes. The addition of tritylium and iodine created a more active catalytic system to promote the cleavage of sulfidic C-S bonds.