Redox-Neutral ortho Functionalization of Aryl Boroxines via Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene-catalyzed diversified trifunctionalization of aryl-thianthreniums

A novel Catellani-type conversion is reported using aryl-thianthreniums (aryl-TTs) instead of aryl halides. Three classes of ortho-dual C-H functionalization involving alkylation, amination, and deuterated methylation and five types of ipso-operation including alkenylation, cyanation, methylation, hydrogenation, and alkynylation all proceed well in this procedure. In this conversion, aryl-TTs exhibit satisfactory reactivity and feature the advantage that the leaving TT unit can be recovered. More strikingly, this finding represents a new chemistry conversion of aryl-TTs, wherein contiguous tri-functionalization in a single chemical manipulation is realized.

A Practical and Regioselective Strategy for Aromatic C-H Difunctionalization via Site-Selective C-H Thianthrenation

Herein, we present a novel Catellani-type reaction that employed aryl-thianthrenium salts as aryl substrates to trigger the subsequent palladium/norbornene cooperatively catalyzed progress. This strategy can achieve site-selective C-H difunctionalization of aryl compounds without directing groups or a known initiating reagent. A series of functionalized syntheses of bioactive molecules further demonstrated the potential of this strategy.

Palladium/Norbornene-Catalyzed Direct Vicinal Di-Carbo-Functionalization of Indoles: Reaction Development and Mechanistic Study

Methods that can simultaneously install multiple different functional groups to heteroarenes via C-H functionalizations are valuable for complex molecule synthesis, which, however, remain challenging to realize. Here we report the development of vicinal di-carbo-functionalization of indoles in a site- and regioselective manner, enabled by the palladium/norbornene (Pd/NBE) cooperative catalysis. The reaction is initiated by the Pd(II)-mediated C3-metalation and specifically promoted by the C1-substituted NBEs. The mild, scalable, and robust reaction conditions allow for a good substrate scope and excellent functional group tolerance. The resulting C2-arylated C3-alkenylated indoles can be converted to diverse synthetically useful scaffolds. The combined experimental and computational mechanistic study reveals the unique role of the C1-substituted NBE in accelerating the turnover-limiting oxidative addition step.

Functionalized Cycloolefin Ligand as a Solution to Ortho-Constraint in the Catellani-Type Reaction

The Catellani reaction, i.e., the Pd/norbornene (NBE) catalysis, has been evolved into a versatile approach to multisubstituted arenes via the ortho-functionalization/ipso-termination process of a haloarene. Despite significant advances over the past 25 years, this reaction still suffered from an intrinsic limitation in the substitution pattern of haloarene, referred to as "ortho-constraint". When an ortho substituent is absent, the substrate often fails to undergo an effective mono ortho-functionalization process, and either ortho-difunctionalization products or NBE-embedded byproducts predominate. To tackle this challenge, structurally modified NBEs (smNBEs) have been developed, which were proved effective for the mono ortho-aminative, -acylative, and -arylative Catellani reactions of ortho-unsubstituted haloarenes. However, this strategy is incompetent for solving the ortho-constraint in Catellani reactions with ortho-alkylation, and to date there lacks a general solution to this challenging but synthetically useful transformation. Recently, our group developed the Pd/olefin catalysis, in which an unstrained cycloolefin ligand served as a covalent catalytic module to enable the ortho-alkylative Catellani reaction without NBE. In this work, we show that this chemistry could afford a new solution to ortho-constraint in the Catellani reaction. A functionalized cycloolefin ligand bearing an amide group as the internal base was designed, which allowed for mono ortho-alkylative Catellani reaction of iodoarenes suffering from ortho-constraint before. Mechanistic study revealed that this ligand is capable of both accelerating the C-H activation and inhibiting side reactions, which accounts for its superior performance. The present work showcased the uniqueness of the Pd/olefin catalysis as well as the power of rational ligand design in metal catalysis.

Steric and electronic effects of arsa-Buchwald ligands on Suzuki-Miyaura coupling reaction

The Suzuki-Miyaura coupling (SMC) reaction is one of the most commonly used cross-coupling reactions. Bulky biaryldialkyl monophosphine ligands, i.e., Buchwald ligands, are beneficial for the SMC reaction. We recently developed a synthetic procedure for arsa-Buchwald ligands, arsenic analogs of Buchwald ligands, and found that these ligands are effective for sterically hindered substrates because of facilitating the transmetalation step owing to the longer arsenic-palladium bond. However, the relationship between the structure and steric/electronic properties of the arsa-Buchwald ligands has not yet been studied in detail. In this study, a series of arsa-Buchwald ligands with various alkyl substituents were synthesized. The cyclopentyl group afforded the highest catalytic activity for the SMC reaction, particularly with sterically hindered substrates. Furthermore, the steric/electronic properties of the arsa-Buchwald ligands were computationally analyzed.

Synthetic Strategy for AB2-Type Arsines via Bidentate Dithiolate Leaving Groups

Despite their potential for several transition-metal-catalyzed reactions, arsenic ligands are poorly diversified. In this work, we developed an efficient synthetic methodology for AB₂-type ligands, which is a typical motif in phosphorus systems, for example, in Buchwald ligands. The introduction of 1,2-benzenedithiol to tribromoarsine reduces the reactivity of two of the three reaction sites. After the substitution reaction with the first nucleophile involving the elimination of bromide, the substitution reaction with the second nucleophile produced AB₂-type arsines through the elimination of the dithiolate anion. Among the various types of obtained AB₂-type arsines, the arsa-Buchwald ligands, which are arsenic analogues of Buchwald ligands, were applied to the Suzuki-Miyaura cross-coupling reaction. Some of the arsa-Buchwald ligands showed activity comparable to that of the well-known Buchwald ligand, SPhos. Furthermore, the arsenic analogue of SPhos showed higher activity and stability than SPhos under open-air conditions.

Nickel-Catalyzed Arylative Substitution of Homoallylic Alcohols

Direct coupling of unactivated alcohols remains a challenge in synthetic chemistry. Current approaches to cross-coupling of alcohol-derived electrophiles often involve activated alcohols such as tosylates or carbonates. We report the direct arylative substitution of homoallylic alcohols catalyzed by a nickel-bisphosphine complex as a facile method to generate allylic arenes. These reactions proceed via formation of an allylic alcohol intermediate. Subsequent allylic substitution with arylboroxine nucleophiles enables the formation of a variety of allylic arenes. The presence of p-methoxyphenylboronic acid is crucial to activate the allylic alcohol to achieve high product yields.

Arylative substitutions of homoallylic alcohols with arylboron nucleophiles demonstrate the utility of unactivated alcohols as coupling partners in transition metal-catalyzed cross-coupling chemistry.

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/Norbornene-Catalyzed Decarbonylative Difunctionalization of Thioesters

The transition-metal-catalyzed decarboxylation of aryl carboxylic acids has drawn significant attention as an efficient and practical tool for the synthesis of substituted arenes. However, the decarboxylative construction of polysubstituted arenes with different contiguous substituents has not been widely reported. Herein, we describe a novel decarbonylative Catellani reaction via palladium-catalyzed, norbornene (NBE)-mediated polyfunctionalization of aromatic thioesters, which serve as readily available carboxylic acid derivatives. A variety of alkenyl, alkyl, aryl, and sulfur moieties could be conveniently introduced into the ipso-positions of the aromatic thioesters. By combining carboxyl-directed C-H functionalization and the classical Catellani reaction, our protocol allows for the construction of 1,2,3-trisubstituted and 1,2,3,4-tetrasubstituted arenes from simple aromatic acids. Furthermore, the late-stage functionalization of a series of drug molecules highlights the potential utility of the reaction.

Chiral Indoline-2-carboxylic Acid Enables Highly Enantioselective Catellani-type Annulation with 4-(Bromomethyl)cyclohexanone

Chiral indoline-2-carboxylic acid has been identified to enable a highly enantioselective Catellani-type annulation of (hetero)aryl, alkenyl triflate and conjugated vinyl iodides with 4-(bromomethyl)cyclohexanone, directly assembling a diverse range of chiral all-carbon bridged ring systems. Control experiments and DFT calculations suggest that the coordinating orientation of the chiral amino acid to the arylpalladium(II) center allows for high levels of stereochemical control.

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.

Modular Entry to Functionalized Tetrahydrobenzo[b]azepines via the Palladium/Norbornene Cooperative Catalysis Enabled by a C7-Modified Norbornene

Tetrahydrobenzo[b]azepines (THBAs) are commonly found in many bioactive compounds; however, the modular preparation of functionalized THBAs remains challenging to date. Here, we report a straightforward method to synthesize THBAs directly from simple aryl iodides via palladium/norbornene (Pd/NBE) cooperative catalysis. Capitalizing on an olefin-tethered electrophilic amine reagent, an ortho amination followed by 7-exo-trig Heck cyclization furnishes the seven-membered heterocycle. To overcome the difficulty with ortho-unsubstituted aryl iodide substrates, we discovered a unique C7-bromo-substituted NBE (N1) to offer the desired reactivity and selectivity. In addition to THBAs, synthesis of other benzo-seven-membered ring compounds can also be promoted by N1. Combined experimental and computational studies show that the C7-bromo group in N1 plays an important and versatile role in this catalysis, including promoting β-carbon elimination, suppressing benzocyclobutene formation, and stabilizing reaction intermediates. The mechanistic insights gained could guide future catalyst design. The synthetic utility has been demonstrated in a streamlined synthesis of tolvaptan and forming diverse pharmaceutically relevant THBA derivatives. Finally, a complementary and general catalytic condition to access C6-substituted THBAs from ortho-substituted aryl iodides has also been developed.

Tunable C–H arylation and acylation of azoles with carboxylic acids by Pd/Cu cooperative catalysis

Direct C–H arylation and acylation of azoles with carboxylic acids are achieved selectively through Pd/Cu cooperative catalysis: biaryls are generated selectively with dppp as ligand, while biaryl ketones are obtained with high selectivity using dpph or Ph₂PCy as ligand.

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.

Modular and Stereoselective Synthesis of C-Aryl Glycosides via Catellani Reaction

In this work, we describe a Catellani-type C-H glycosylation to provide rapid access to various highly decorated α-C-(hetero)aryl glycosides in a modular and stereoselective manner (>90 examples). The termination step is flexible, which is demonstrated by ipso-Heck reaction, hydrogenation, Suzuki coupling, and Sonogashira coupling. Application of this methodology has been showcased by preparing glycoside-pharmacophore conjugates and a dapagliflozin analogue. Notably, the technology developed herein represents an unprecedented example of Catellani-type alkylation involving an S_N1 pathway.

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.

Direct synthesis of annulated indoles through palladium-catalyzed double alkylations

A facile, one-step synthesis of annulated indoles from (N–H) indoles and dibromoalkanes was developed through a palladium-catalyzed double alkylation process.

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.

Direct Vicinal Difunctionalization of Thiophenes Enabled by the Palladium/Norbornene Cooperative Catalysis

Herein we report a direct vicinal difunctionalization of thiophenes via the palladium/norbornene (Pd/NBE) cooperative catalysis. A series of mono- and disubstituted thiophenes can be difunctionalized site-selectively and regioselectively at the C4 and C5 positions in good yields, enabled by an arsine ligand and a unique amide-based NBE. The synthetic utility has been shown in derivatizations of complex bioactive compounds and an open-flask gram-scale preparation. Preliminary results have been obtained in the difunctionalization of furans and a direct C4-selective arylation of 2-substituted thiophenes.

Redox-neutral ortho-C-H amination of pinacol arylborates via palladium(ii)/norbornene catalysis for aniline synthesis

A palladium(ii)/norbornene cooperative catalysis enabled redox-neutral ortho-C-H amination of pinacol aryl- or heteroarylborates for the synthesis of structurally diverse anilines is reported. This method is scalable, robust (tolerance of air and moisture), phosphine ligand-free, and compatible with a wide range of functionalities. These practical features make this reaction amenable for industry. A plethora of synthetically very useful halogenated anilines, which often cannot be prepared via other transition-metal-catalyzed aminations, are readily produced using this method. Particularly, the orthogonal reactivity between pinacol arylborates and aryl iodides is demonstrated. Preliminary deuterium-labeling studies reveal a redox-neutral ipso-protonation mechanism of this process, which will surely inspire the future development of this field. Overall, the exceptionally broad scope (47 examples) and reliability of this procedure, together with the wide availability of pinacol arylborates, make this chemistry a valuable addition to the existing methods for aniline synthesis.

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

Poly-substituted aromatic sulfur compounds are widely found in pharmaceuticals, agrochemicals and organic materials. However, the position that a sulfur moiety can be introduced to is largely restricted to a pre-functionalized site; otherwise, use of electronically biased substrates or auxiliary groups that direct catalysis is required. Here we report a general ortho thiolation of common aryl and heteroaryl iodides via palladium-norbornene cooperative catalysis. Using this approach, an aryl or alky sulfur moiety can be site-selectively introduced at the arene ortho position without using sterically or electronically biased substrates. The arene ipso functionalization is simultaneously achieved through Heck, Suzuki or Sonogashira termination. The reaction is enabled by a unique class of electrophiles in palladium-norbornene cooperative catalysis, which are sulfenamides derived from seven-membered lactams. The broad substrates scope and high chemoselectivity could make this method attractive for synthesis of complex sulfur-containing aromatic compounds.

Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene cooperative catalysis has emerged as a distinct approach to construct polyfunctionalized arenes from readily available starting materials. This Review provides a comprehensive overview of this field, including the early stoichiometric investigations, catalytic reaction developments, as well as the applications in the syntheses of bioactive compounds and polymers. The section of catalytic reactions is divided into two parts according to the reaction initiation mode: Pd(0)-initiated reactions and Pd(II)-initiated reactions.

Chemoselective Borono-Catellani Arylation for Unsymmetrical Biaryls Synthesis

Reported is the borono-Catellani arylation process for unsymmetrical biaryls synthesis, utilizing the readily available pinacol ester of arylboronic acids, aryl bromides, and olefins as the reactants. The distinct reactivity of arylboronic ester and aryl bromides secures the excellent chemoselectivity in the pivotal arylation step. The reaction is enabled by the cooperative catalysis of Pd(OAc)₂ and the NBE derivative N⁷, with molecular oxygen as the terminal oxidant.