Sulfenamide-enabled ortho thiolation of aryl iodides via palladium/norbornene cooperative catalysis

Lewis Acid-Catalyzed Formal [4 + 2] Reaction of Alkynyl Sulfides and 2-Pyrones To Access Polysubstituted Aryl Sulfides

A practical and efficient method to access polysubstituted aryl sulfides has been discovered via a Lewis acid-catalyzed reaction between alkynyl sulfide and 2-pyrone, involving a Diels-Alder/retro-Diels-Alder pathway. Alkynyl sulfide as an electron-rich dienophile and 2-pyrones as electron-poor dienes are conjunctively transformed into a series of polysubstituted aryl sulfides with broad functional group compatibility in good to excellent yields (40 examples, 43-88% yield). The robustness and practicality of the protocol has been demonstrated through gram-scale synthesis and the ease of transformation of the resulting products.

Template Synthesis to Solve the Unreachable Ortho C-H Functionalization Reaction of Aryl Iodide

This report describes the use of a simple Pd/NBE catalytic system to achieve ortho C-H oxylation and phosphonylation and other functionalizations of aryl iodide through templated conversion reactions. Dimethylamine is introduced in the ortho-site of aryl iodide through C-H amination, and aryl dimethylamine is quickly converted to methyl quaternary ammonium salt precipitation. Methyl quaternary ammonium salt avoids Hofmann elimination in subsequent functionalization. This method solves various ortho functionalization reactions of aryl iodide that have not been achieved for a long time in the field of Pd/NBE chemistry indirectly.

Ortho-C-H methoxylation of aryl halides enabled by a polarity-reversed N-O reagent

Oxygen-substituted arenes widely exist in biologically important molecules and can serve as versatile handles to install other functional groups. However, direct and site-selective installation of oxygen groups to common aromatic compounds remains challenging, especially when additional arene functionalization is simultaneously required. Current arene C-H oxidation strategies generally require directing groups or precisely prefunctionalized substrates to control site-selectivity. While palladium/norbornene cooperative catalysis is promising for site-specific arene vicinal difunctionalization through simultaneous reactions with an electrophile and a nucleophile, the electrophile scope has been limited to species based on relatively 'soft' elements, such as carbon, nitrogen and sulfur. Here we report the development of an ortho oxygenation reaction with common aryl halides to rapidly deliver diverse aryl ethers. The coupling of the 'hard' oxygen electrophile is enabled by a stable, polarity-reversed, conformationally predistorted N-O reagent and facilitated by a C7-bromo-substituted norbornene mediator. Mechanistic studies reveal a unique SN2-type pathway between the N-O reagent as the oxygen electrophile and an electron-rich Pd(II) nucleophile.

Crystal structure of S-(benzo[d]thiazol-2-yl)-N-(tert-butyl)thiohydroxylamine, C11H14N2S2

C11H14N2S2, monoclinic, I2/a (no. 15), a = 11.4278(8) Å, b = 8.5816(6) Å, c = 25.1115(18) Å, β = 95.173(6)°, V = 2452.6(3) Å3, Z = 8, Rgt (F) = 0.0416, wRref (F 2) = 0.1169, T = 293 K.

Access to Sulfur-Containing Bisheterocycles through Base-Promoted Consecutive Tandem Cyclization/Sulfenylation with Elemental Sulfur

A convenient and efficient tandem cyclization/sulfenylation of o-alkynyl-phenols/-anilines/enaminones for the synthesis of diverse sulfur-containing bisheterocycles has been developed using stable, odorless, and easy-to-handle elemental S₈ as a building block under green chemistry conditions. Notably, a one-step simple base-mediated organic transformation affords a benzofuran (indole or chromone) ring and two C-S bonds. Attractive features of this methodology include the absence of a metal catalyst, mild conditions, good functional group tolerance, and valuable product structures.

Directed, nickel-catalyzed 1,2-alkylsulfenylation of alkenyl carbonyl compounds

We report a regioselective, nickel-catalyzed syn-1,2-carbosulfenylation of non-conjugated alkenyl carbonyl compounds with alkyl/arylzinc nucleophiles and tailored N-S electrophiles. This method allows the simultaneous installation of a variety of C(sp³) and S(Ar) (or Se(Ar)) groups onto unactivated alkenes, which complements previously developed 1,2-carbosulfenylation methodology in which only C(sp²) nucleophiles are compatible. A bidentate directing auxiliary controls regioselectivity, promotes high syn-stereoselectivity with a variety of E- and Z-internal alkenes, and enables the use of an array of electrophilic sulfenyl (and seleno) electrophiles. Among compatible electrophiles, those with N-alkyl-benzamide leaving groups were found to be especially effective, as determined through comprehensive structure-reactivity mapping.

Electrophilic Sulfur Reagent Design Enables Directed syn-Carbosulfenylation of Unactivated Alkenes

A multi-component approach to structurally complex organosulfur products is described via the nickel-catalyzed 1,2-carbosulfenylation of unactivated alkenes with organoboron nucleophiles and tailored organosulfur electrophiles. The key to the development of this transformation is the identification of a modular N-alkyl-N-(arylsulfenyl)arenesulfonamide family of sulfur electrophiles. Tuning the electronic and steric properties of the leaving group in these reagents controls pathway selectivity, favoring three-component coupling and suppressing side reactions, as examined via computational studies. The unique syn-stereoselectivity differs from traditional electrophilic sulfenyl transfer processes involving a thiiranium ion intermediate and arises from the directed arylnickel(I) migratory insertion mechanism, as elucidated through reaction kinetics and control experiments. Reactivity and regioselectivity are facilitated by a collection of monodentate, weakly coordinating native directing groups, including sulfonamides, alcohols, amines, amides, and azaheterocycles.

Synthesis of C8-Aminated Pyrrolo-Phenanthridines or -Indoles via Series C(sp2 or sp3)-H Activation and Fluorescence Study

This report developed a method for the synthesis of C8-aminated pyrrolo-phenanthridines or -indoles by series ortho C(sp²)-H amination/ipso C(sp²)-H or C(sp³)-H arylation. N-benzoyloxyamines, as electrophilic amination reagents, did not undergo an electrophilic substitution reaction with the pyrrole side, but they did undergo a site-selective C-H amination reaction with the benzene side via Pd/NBE catalysis. The C8-aminated pyrrolo-phenanthridines have strong fluorescence in solution and solid state. X-ray single crystal diffraction shows that the steric hindrance of amino and ortho benzene ring may inhibit aggregation-caused quenching (ACQ).

Polysubstituted Indole Synthesis via Palladium/Norbornene Cooperative Catalysis of Oxime Esters

Polysubstituted indoles are prevalent in pharmaceuticals, agrochemicals, and organic materials. Presented herein is the fact that polyfunctionalized indoles can be efficiently constructed from easily accessible oxime esters and aryl iodides, involving a palladium/norbornene synergistic synthesis. The reaction is enabled by a unique class of electrophiles in palladium/norbornene cooperative catalysis, which are oxime esters derived from simple ketone. The broad substrate scope and high functional group tolerance could make this method attractive for the synthesis of polysubstituted indoles.

Trifunctionalization of aryl iodides via intermolecular C–H acylation/intramolecular C–H alkylation achieved using palladium/norbornene cooperative catalysis

Herein, we describe a palladium/norbornene-catalyzed trifunctionalization of ortho-unsubstituted aryl iodides via a highly chemoselective cascade process involving intermolecular ortho acylation/intramolecular ortho alkylation/ipso alkenylation.

Redox-Neutral Vicinal Difunctionalization of Five-Membered Heteroarenes with Dual Electrophiles

A new reaction mode of palladium/norbornene (Pd/NBE) cooperative catalysis is reported involving the selective coupling of two different carbon-based electrophiles for vicinal double C-H functionalization of five-membered heteroarenes in a site-selective and redox-neutral manner. The key is to use alkynyl bromides as the second electrophile, which allows vicinal difunctionalization of a wide range of heteroarenes including pyrroles, thiophenes and furans at their C4 and C5 positions. One- or two-step tetrafunctionalizations of simple pyrrole and thiophene have also been realized. The C2-substituted NBEs prove most effective in these reactions, and the mechanistic exploration discloses the origin of the high selectivity of this transformation. Synthetic utility of this method has been exemplified in the concise preparations of thiophene-containing organic materials and a protein kinase inhibitor analogue. Preliminary success has also been achieved in a direct annulation event, using a tethered ketone as the second electrophile.

Palladium-Catalyzed C-H Amination/[2 + 3] or [2 + 4] Cyclization via C(sp3 or sp2)-H Activation

This report describes a palladium-catalyzed Catellani reaction consisting of amination/[2 + 3] or [2 + 4] cyclization via a carboxylate ligand-exchange strategy. This method effectively activates ortho-substituents that avoid a second C-H palladation. The scope of substrates was broad, o-methyl-substituted iodoarenes were applied to the reaction smoothly, and o-phenyl-substituted iodoarenes can also be obtained by this method. In terms of mechanism, density functional theory calculations proved the sequence of the key five-membered aryl-norbornene-palladacycle intermediate formation and C(sp³ or sp²)-H activation.

Transition Metal-Free C-H Thiolation via Sulfonium Salts Using β-Sulfinylesters as the Sulfur Source

We disclose a direct C(sp)-, C(sp²)-, and C(sp³)-H thiolation reaction using β-sulfinylesters as the versatile sulfur source. The key step of this protocol is chemoselective C-S bond cleavage of the sulfonium salts that are formed in situ from the corresponding alkenes, alkynes, and 1,3-dicarboxyl compounds with β-sulfinylesters. The successful capture of the acrylate byproduct supports a retro-Michael reaction mechanism.

Sequential C-H activation enabled expedient delivery of polyfunctional arenes

Modular construction of polyfunctional arenes from abundant feedstocks stands as an unremitting pursue in synthetic chemistry, accelerating the discovery of drugs and materials. Herein, using the multiple C-H activation strategy with versatile imidate esters, the expedient delivery of molecular libraries of densely functionalized sulfur-containing arenes was achieved, which enabled the concise construction of biologically active molecules, such as Bipenamol.

Modular Synthesis of Aryl Thio/Selenoglycosides via the Catellani Strategy

We described a novel palladium-catalyzed domino procedure for the preparation of (hetero)aryl thio/selenoglycosides. Readily available (hetero)aryl iodides and easily accessible 1-thiosugars/1-selenosugars are utilized as the substrates. Meanwhile, 10 types of sugars are quite compatible with this reaction with good regio- and stereoselectivity, high efficiency, and broad applicability (up to 89%, 53 examples). This method enables the straightforward formation of the C(sp²)-S/Se bond of (hetero)aryl thio/selenoglycosides.

A Catellani and retro-Diels–Alder strategy to access 1-amino phenanthrenes via ortho- and interannular C–H activation of 2-iodobiphenyls

A palladium-catalyzed three-component domino reaction for the construction of 1-amino phenanthrene derivatives by ortho- and interannular C–H activation of 2-iodobiphenyls has been developed.

C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates

In this review article, we summarized recent advances in C–H hetero-functionalization of arenes through palladacyclopentane-type intermediates.

Synthesis of Chiral-Substituted 2-Aryl-ferrocenes by the Catellani Reaction

A palladium-catalyzed and norbornene-mediated methodology has been developed for the synthesis of chiral 2-aryl-ferroceneamides from chiral 2-iodo-N,N-diisopropylferrocencarboxamide, iodoarenes, and alkenes using a JohnPhos ligand and potassium carbonate as a base in dimethylformamide at 105 °C. The developed three-component coupling protocol allows the compatibility of electron-withdrawing fluoro, chloro, ester, and nitro and electron-donating methyl, methoxy, dimethoxy, benzyl ether-substituted iodo-benzenes, other iodoarenes, such as iodo-naphthalene, heteroarenes, such as iodothiophene, and terminating substrates, such as methyl, ethyl, tert-butyl acrylates, and substituted styrenes with 2-iodo-N,N-diisopropylferrocencarboxamide. Furthermore, the developed three-component Catellani method proceeded with the retention of the configuration of the planar chiral ferrocene, which depends on the role of the participating carbon-iodine bond in ferrocene. Consequently, the developed protocol enabled the formation of densely substituted chiral 2-aryl ferroceneamides, exhibiting good to excellent enantioselectivity. The conversion of an ester of the synthesized chiral 2-aryl ferroceneamides has also been carried out to further accommodate the easily expendable acid and alcohol functionalities.

Structurally Modified Norbornenes: A Key Factor to Modulate Reaction Selectivity in the Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene (Pd/NBE) cooperative catalysis has received enormous attention and found numerous synthetic applications in the past two decades. Considering the critical roles that NBE plays in the catalytic cycle, the use of structurally modified NBEs (smNBEs), starting from 2015, has become an important approach to address limitations and modulate reaction selectivity in Pd/NBE catalysis. This Perspective highlights the development of three types of smNBEs: C1-substituted, C2-substituted, and C5-substituted or C5,C6-disubstituted NBEs, as well as their synthetic applications toward site-selective C-H functionalization. A focus is on the structure-activity relationship of smNBEs in these reactions, and rationales for using smNBEs in many cases have also been provided.

Compatibility Score for Rational Electrophile Selection in Pd/NBE Cooperative Catalysis

A mechanistically guided approach is developed to predict electrophile compatibility in the palladium/norbornene (Pd/NBE) cooperative catalysis for the ipso/ortho difunctionalization of aryl halides. A key challenge in these reactions is to identify orthogonal electrophile and aryl hali de starting materials that react selectively with different transition metal complexes in separate oxidative addition events in the catalytic cycle. We performed detailed experimental and computational mechanistic studies to identify the catalytically active Pd(II) intermediate and the substrate-dependent mechanisms in reactions with various types of carbon and nitrogen electrophiles. We introduced the concept of electrophile compatibility score (ECS) to rationally select electrophiles based on the orthogonal reactivity of electrophile and aryl halide towards the Pd(0) and Pd(II) complexes. This approach was applied to predict electrophile compatibility in the Pd/NBE cooperative catalysis with a variety of electrophilic coupling partners used in alkylation, arylation, amination, and acylation reactions.

Palladium-Catalyzed ortho-C-H Glycosylation/ipso-Alkenylation of Aryl Iodides

This report describes the first example of palladium-catalyzed ortho-C-H glycosylation/ipso-alkenylation of aryl iodides, and the easily accessible glycosyl chlorides are used as a glycosylation reagent. The reaction is compatible with the functional groups of the substrates, and a series of C-aryl glycosides have been synthesized in good to excellent yield and with excellent diastereoselectivity. It is found that a cheap 5-norbornene-2-carbonitrile as a transient mediator can effectively promote this reaction. In addition, ipso-arylation and cyanation were also realized by the strategy.

Catalyst and additive-free oxidative dual C-H sulfenylation of imidazoheterocycles with elemental sulfur using DMSO as a solvent and an oxidant

Dual C-H sulfenylation was used to obtain 3-vulcanized imidazoheterocycles using odorless elemental sulfur under catalyst- and additive-free conditions. C-H activation of both imidazoheterocycles and arylamines/arenols/indoles was realized by a practical protocol in which DMSO served as both a solvent and an internal oxidant.

Unexpected ortho-Heck Reaction under the Catellani Conditions

An unexpected ortho-Heck reaction has been discovered during the study of palladium/norbornene (Pd/NBE) catalysis. Under the Catellani reaction conditions in the presence of lithium salts and olefins, Heck coupling takes place at the ortho position instead of the commonly observed ipso position; meanwhile, a norbornyl group is introduced at the arene ipso position. Systematic deuterium labeling and crossover experiments suggest an unusual 1,4-palladium migration/intramolecular hydrogen transfer pathway. The knowledge gained in this study could provide insights for the future development of the Pd/NBE catalysis.

A practical ortho-acylation of aryl iodides enabled by moisture-insensitive activated esters via palladium/norbornene catalysis

Herein reported is a practical Catellani-type ortho-acylation of aryl iodides enabled by employing moisture-insensitive esters as the electrophile via C(O)–O bond cleavage.

Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides

We present a Pd/NBE-catalyzed sequential carbamoylation/olefination of aryl iodides under mild reaction conditions, which provide diverse 4-methylene-3,4-dihydro-1(2H)-isoquinolin-1-one analogues.

A carboxylate-assisted amination/unactivated C(sp2)–H arylation reaction via a palladium/norbornene cooperative catalysis

A carboxylate-assisted palladium-catalysed Catellani reaction, which is compatible with ortho-amination and unactivated C(sp²)–H arylation, synthesized a series of 1-amino substituted dihydrophenanthridines, phenanthridines and 6H-benzo[c]chromenes.