Palladium-Catalyzed Coupling of Arene C–H Bonds with Methyl- and Arylboron Reagents Assisted by the Removable 2-Pyridylsulfinyl Group

Aryl Sulfoxides: A Traceless Directing Group for Catalytic C-H Activation of Arenes

The transition metal-catalyzed C-H activation of arenes directed by sulfoxides represents a compelling strategy in organic synthesis, owing to its exceptional regioselectivity and high efficiency. This innovative approach stands out for its traceless character, enabling the direct functionalization of arenes, before the easy removal or conversion of the key sulfinyl moiety. Beyond their utility as a directing group, sulfoxides have proved particularly valuable to mediate as chiral auxiliaries, presenting exciting prospects for the synthesis of stereo-enriched compounds upon C-H functionalization. The versatility demonstrated by the method paves the way to different structures with potential applications ranging from medicinal chemistry to organic electronics.

Electro-oxidative Methylation of 2-Isocyanobiaryls Using N,N-dimethylformamide (DMF) as Carbon Source: Synthesis of 6-Methylphenanthridines

A benign electrochemical method to access 6-methylphenanthridines from 2-isocyanobiaryls using N,N-dimethylformamide (DMF) as a methyl source is reported. The protocol operates at ambient temperature without the need for harmful methylating reagents. Mechanistic studies suggested that DMF delivered a methylene synthon, followed by reduction at the cathode and tautomerization. The method offers environmental benefits by avoiding metal-based reagents and harsh conditions.

Palladium-Catalyzed Direct C-H Thiolation of 2-Pyridyl Sulfoxide with Disulfides

Palladium-catalyzed C-H thiolation of aryl 2-pyridyl sulfoxide with diaryl disulfides has been achieved. The reaction proceeds selectively at the ortho-position of masked thiophenol. The directing group 2-pyridyl sulfoxide can be...

Transition-Metal-Catalyzed Directing Group Assisted (Hetero)aryl C-H Functionalization: Construction of C-C/C-Heteroatom Bonds

Transition-metal-catalyzed C-H functionalization is one of the fascinating scientific fronts in organic synthesis for the formation of conjugated arenes and has emerged as a benchmark to revolutionize the synthetic enterprise since past decades. In this realm, chelation-guided functionalization of C-H bonds using an exogenous directing group has received considerable attention recently for the expedient regioselective construction of C-C and C-heteroatom bonds as an efficient and sustainable alternative. This article outlines our contribution towards a wide variety of transformations that have been achieved by the directed C-H functionalization through the fine tuning of catalytic systems.

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Pd-Catalyzed ipso,meta-Dimethylation of ortho-Substituted Iodoarenes via a Base-Controlled C-H Activation Cascade with Dimethyl Carbonate as the Methyl Source

A methyl group can have a profound impact on the pharmacological properties of organic molecules. Hence, developing methylation methods and methylating reagents is essential in medicinal chemistry. We report a palladium-catalyzed dimethylation reaction of ortho-substituted iodoarenes using dimethyl carbonate as a methyl source. In the presence of K₂CO₃ as a base, iodoarenes are dimethylated at the ipso- and meta-positions of the iodo group, which represents a novel strategy for meta-C-H methylation. With KOAc as the base, subsequent oxidative C(sp³)-H/C(sp³)-H coupling occurs; in this case, the overall transformation achieves triple C-H activation to form three new C-C bonds. These reactions allow expedient access to 2,6-dimethylated phenols, 2,3-dihydrobenzofurans, and indanes, which are ubiquitous structural motifs and essential synthetic intermediates of biologically and pharmacologically active compounds.

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

Palladium-Catalyzed Methylation of Aryl, Heteroaryl, and Vinyl Boronate Esters

A method for the direct methylation of aryl, heteroaryl, and vinyl boronate esters is reported, involving the reaction of iodomethane with aryl-, heteroaryl-, and vinylboronate esters catalyzed by palladium and PtBu₂Me. This transformation occurs with a remarkably broad scope and is suitable for late-stage derivatization of biologically active compounds via the boronate esters. The unique capabilities of this method are demonstrated by combining carbon-boron bond-forming reactions with palladium-catalyzed methylation in a tandem transformation.

Pd-Catalyzed, ortho C-H Methylation and Fluorination of Benzaldehydes Using Orthanilic Acids as Transient Directing Groups

The direct, Pd-catalyzed ortho C-H methylation and fluorination of benzaldehydes have been accomplished using commercially available orthanilic acids as transient directing groups. In these reactions, the 1-fluoro-2,4,6-trimethylpyridinium salts can be either a bystanding F⁺ oxidant or an electrophilic fluorinating reagent. An X-ray crystal structure of a benzaldehyde ortho C-H palladation intermediate was obtained using triphenylphosphine as the stabilizing ligand.

Cobalt-catalyzed directed ortho-methylation of arenes with methyl tosylate

A cobalt catalyst enables directed ortho-methylation of arenes using readily available methyl tosylate as well as its deuterated analogue.

Iridium-catalysed arylation of C-H bonds enabled by oxidatively induced reductive elimination

Direct arylation of C-H bonds is in principle a powerful way of preparing value-added molecules that contain carbon-aryl fragments. Unfortunately, currently available synthetic methods are not sufficiently effective to be practical alternatives to conventional cross-coupling reactions. We propose that the main problem lies in the late portion of the catalytic cycle where reductive elimination gives the desired carbon-aryl bond. Accordingly, we have developed a strategy where the Ir(III) centre of the key intermediate is first oxidized to Ir(IV). Density functional theory calculations indicate that the barrier to reductive elimination is reduced by nearly 19 kcal mol^-1 for this oxidized complex compared with that of its Ir(III) counterpart. Various experiments confirm this prediction, affording a new methodology capable of directly arylating C-H bonds at room temperature with a broad substrate scope and in good yields. This work highlights how the oxidation states of intermediates can be targeted deliberately to catalyse an otherwise impossible reaction.

DDQ-promoted direct C5-alkylation of oxazoles with alkylboronic acids via palladium-catalysed C–H bond activation

The first protocol for the direct C5-alkylation of oxazoles through transition-metal-catalysed C(5)–H bond activation.

Free-radical initiated cascade methylation or trideuteromethylation of isocyanides with dimethyl sulfoxides

The methylation or trideuteromethylation reaction of isocyanides with dimethyl sulfoxides in a radical way is developed, which offers a low-cost, easy-operation cascade methylation strategy for the synthesis of phenanthridines or isoquinolines.

Rhodium(III)-Catalyzed Site-Selective C-H Alkylation and Arylation of Pyridones Using Organoboron Reagents

In this study we developed a method for the pyridine-directed, rhodium-catalyzed, site-selective C-H alkylation and arylation of pyridones using commercially available trifluoroborate reagents. This simple and versatile transformation proceeded smoothly under relatively mild conditions with perfect site selectivity. The coupling groups in the boron reagents can be extended to primary alkyl, benzyl, and cycloalkyl. Moreover, direct C-H arylation products could also be obtained under similar conditions.

C-H Coupling Reactions Directed by Sulfoxides: Teaching an Old Functional Group New Tricks

Sulfoxides are classical functional groups for directing the stoichiometric metalation and functionalization of C-H bonds. In recent times, sulfoxides have been given a new lease on life owing to the development of modern synthetic methods that have arisen because of their unique reactivity. They have recently been used in catalytic C-H activation proceeding via coordination of an internal sulfoxide to a metal or through the action of an external sulfoxide ligand. Furthermore, sulfoxides are able to capture nucleophiles and electrophiles to give sulfonium salts, which subsequently enable the formation of C-C bonds at the expense of C-H bonds. This Review summarizes a renaissance period in the application of sulfoxides arising from their versatility in directing C-H functionalization.

Nickel-Catalyzed Methylation of Aryl Halides with Deuterated Methyl Iodide

A nickel-catalyzed methylation of aryl halides with cheap and readily available CH3 I or CD3 I is described. The reaction is applicable to a wide range of substrates and allows installation of a CD3 group under mild reaction conditions without deuterium scrambling to other carbon atoms. Initial mechanistic studies on the stoichiometric and catalytic reactions of the isolated [(dppp)Ni(C6 H4 -4-CO2 Et)Br] [dppp=1,3-bis(diphenylphosphanyl)propane] suggest that a Ni(0) /Ni(II) catalytic cycle is favored.

Amino acid-promoted C–H alkylation with alkylboronic acids using a removable directing group

Palladium-catalyzed C–H alkylation reaction with alkylboronic acids has been developed using a removable pyridyldiisopropylsilyl directing group.

Palladium-catalyzed decarboxylative ortho-arylation of 2-pyridyl sulfoxides with benzoyl peroxides

A palladium catalyzed efficient strategy for regio-selective ortho-arylation of sulfoxides with aryl acyl peroxides via decarboxylation has been developed.

Palladium-catalysed direct C-2 methylation of indoles

A direct C-2 methylation reaction of indoles bearing an N-2-pyrimidyl directing group has been developed with a palladium catalyst.

Iron-Catalyzed Directed C(sp(2))-H and C(sp(3))-H Functionalization with Trimethylaluminum

Conversion of a C(sp(2))-H or C(sp(3))-H bond to the corresponding C-Me bond can be achieved by using AlMe3 or its air-stable diamine complex in the presence of catalytic amounts of an inorganic iron(III) salt and a diphosphine along with 2,3-dichlorobutane as a stoichiometric oxidant. The reaction is applicable to a variety of amide substrates bearing a picolinoyl or 8-aminoquinolyl directing group, enabling methylation of a variety of (hetero)aryl, alkenyl, and alkyl amides. The use of the mild aluminum reagent prevents undesired reduction of iron and allows the reaction to proceed with catalyst turnover numbers as high as 6500.