Palladium‐Catalyzed Direct Arylations of 1,2,3‐Triazoles with Aryl Chlorides using Conventional Heating

Generally applicable, palladium‐catalyzed direct arylations of 1,2,3‐triazoles with aryl chlorides were accomplished through conventional heating at reaction temperatures of 105–120 °C. Thereby, intra‐ and intermolecular C H bond functionalizations were achieved with a variety of differently substituted chlorides as electrophiles, bearing numerous valuable functional groups.     

Palladium‐Catalyzed and Hybrid Acids‐Assisted Synthesis of [60]Fulleroazepines in One Pot under Mild Conditions: Annulation of N‐Sulfonyl‐2‐aminobiaryls with [60]Fullerene through Sequential C‐H Bond Activation,C‐C and C‐N Bond Formation

An extraordinarily efficient hybrid acids‐assisted, palladium‐catalyzed and chelating‐group‐assisted C H bond activation of N‐sulfonyl‐2‐aminobiaryls and their annulations with [60]fullerene via sequential C C and C N bond formation at room temperature to afford [60]fulleroazepines is demonstrated. The formation of [60]fulleroazepines is highly regioselective and tolerant to both electron‐withdrawing and electron‐donating groups on the aryl moiety and the reaction gives monofunctionalized fullerenes in good yields (up to 54% isolated yield and 92% based on converted C₆₀).     

Mechanistic Studies on the Palladium‐Catalyzed Direct C‐5 Arylation of Imidazoles: The Fundamental Role of the Azole as a Ligand for Palladium

An in‐depth mechanistic study on the palladium‐catalyzed direct arylation of imidazoles at the C‐5 position is presented. The interactions of triphenylphosphine (PPh₃)‐ligated aryl‐Pd species with 1,2‐dimethyl‐1H‐imidazole (dmim) have been studied in detail. In contrast with previous suggestions, phosphine‐ligated organo‐Pd species are not active and the reaction proceeds through imidazole‐ligated organo‐Pd intermediates. The kinetics of the oxidative addition of aryl halides with dmim‐ligated Pd(0) species have been characterized in a Pd(dba)₂/dmim model system. A thorough study of the equilibria involving novel [ArPd(dmim)₂X] complexes (X=I, OAc) and the unexpected cationic [ArPd(dmim)₃]⁺ is also reported. The ability of these species to effect the C H arylation of dmim at room temperature in the presence of acetate is also demonstrated.

     

Copper(I)‐Catalyzed Dehydrogenative Amidation of Arenes Using Air as the Oxidant

The copper(I) bromide‐catalyzed intermolecular dehydrogenative amidation of arenes via C H bond activation assisted by a 2‐pyridyl or 1‐pyrazolyl chelating group using air as the terminal oxidant has been achieved at 140 °C. N‐Aryl amides, N‐alkyl amides, benzamide derivatives, imides, and lactams all are good coupling partners to obtain moderate to excellent yields. The amount of solvent is critical for the transformation: both increasing and decreasing the amount of solvent decreased the yield. Notably, the amidation of bimolecular 2‐phenylpyridine with the dual N H bonds of a primary amide proceeded smoothly in one‐pot to afford a good yield under the same conditions. The amidation can be performed with a good yield at the gram scale.

     

One‐Pot Palladium‐Catalyzed Synthesis of Selectively Substituted Phenanthridines by Sequential Aryl‐Aryl and Heck Couplings,Aza‐Michael and Retro‐Mannich Reactions

A catalytic synthesis of selectively substituted phenanthridines is achieved through a reaction sequence involving palladium/norbornene‐catalyzed unsymmetrical aryl‐aryl and Heck couplings followed by aza‐Michael and retro‐Mannich reactions. In spite of the many steps involved the method is very simple and allows the formation of selectively substituted phenanthridines under mild conditions in a straightforward one‐pot reaction starting from readily available aryl iodides and bromides.     

Direct ortho‐Hydroxylation of 2‐Phenylpyridines using Palladium(II) Chloride and Hydrogen Peroxide

Direct functionalization of the ubiquitous C H bond is receiving much attention because complex structures can be formed from simple precursors. This paper reports a useful method for the direct hydroxylation of 2‐phenylpyridines using palladium(II) chloride and aqueous hydrogen peroxide. In this method, hydrogen peroxide, which has high atom efficiency, is employed as the oxidant and phenol derivatives are generated via C H activation.

     

Catalytic Dehydrogenation of o‐Alkylated or o‐Alkoxylated Iodoarenes with Concomitant Hydrogenolysis

Palladium‐catalyzed dehydrogenation of suitable chains bonded to an ortho position of an iodoarene has been achieved by two methods both involving oxidative addition of the iodoarene to palladium(0) and palladacycle formation under mild conditions.     

Copper‐Catalyzed Coupling of Indoles with Dimethylformamide as a Methylenating Reagent

By using N,N‐dimethylformamide (DMF) as a methylenating reagent, the copper‐catalyzed C H activation of indole was demonstrated as an efficient and facile protocol for synthesizing 3,3′‐diindolylmethane (DIM) and its derivatives. The results indicate that copper chloride was the best catalyst among the investigated transition metal salts, which affords an excellent regioselectivity and good yield when tert‐butyl hydroperoxide (TBHP) was used as an oxidant.

     

Synthesis of [60]Fullerene‐Fused Tetralones via Palladium‐ Catalyzed Ketone‐Directed sp2 C?H Activation and sp3 C?H Functionalization

The palladium‐catalyzed ketone‐directed dual sp² C H activation and sp³ C H functionalization has been applied for fullerene functionalization for the first time. The sec‐alkyl aryl ketones have been exploited to react with [60]fullerene (C₆₀) to provide the novel and scarce C₆₀‐fused tetralones. The combined use of a highly active cationic palladium(II) catalyst and trifluoromethanesulfonic acid is crucial for the improvement of the reaction yield. A plausible reaction mechanism leading to the observed products has been proposed, and the electrochemistry of the fullerene products has also been investigated.

     

Palladium‐Catalyzed/Lewis Acid‐Promoted Alkene Dimerization and Cross‐Coupling with Alcohols via C?H Bond Activation

The cross‐couplings of alcohols to alkenes with the palladium/Lewis acid system are reported. This reaction occurs in a successive alkene dimerization, direct C H activation of alcohol and sp³ sp³ bond forming sequence via an interesting domino process.     

Copper‐Catalyzed Cross‐Dehydrogenative Coupling (CDC) of Alkynes and Benzylic C?H Bonds

The activation of benzylic C H bonds and subsequent coupling with terminal alkynes in the presence of 2,3‐dichloro‐5,6‐dicyanoquinone (DDQ) and a catalytic amount of copper(I) triflate is presented. Good to moderate yields of disubstituted alkynes are obtained for this cross‐dehydrogenative coupling (CDC) reaction between a variety of aromatic alkynes and diphenylmethane derivatives.     

Transition Metal‐Catalyzed C?H Activation of Indoles

The last decades have seen a tremendous expanse in the application of C H activation of many different substrate classes, including the invaluable indole scaffold. Following the exciting emergence of C H activation as a multi‐faceted platform for functionalization, a versatile tool box has been developed for the preparation of structurally diverse indoles. This review article discusses recent advances and strategies for transition metal‐catalyzed C H activation of indoles.

     

Ligand‐Assisted Palladium(II)/(IV) Oxidation for sp3 C?H Fluorination

The direct functionalization of inert sp³ C H bonds is limited to a few bond types. Although the activation of sp³ C H bonds can be accomplished under mild conditions using palladium catalysts, the subsequent functionalization is not trivial due to the high energy required to convert palladium(II) to palladium(IV). We have systematically studied the palladium oxidation using computation‐guided experiments for reactions involving strong chelation control. We find that a mild external ligand could significantly accelerate the oxidation of palladium(II) to palladium(IV) for strong bidentate directing groups. The acceleration is believed to be a result of ligand stabilization of both the palladium(II) and palladium(IV) intermediates.

     

Direct C‐2 Alkylation of Quinoline N‐Oxides with Ethers via Palladium‐Catalyzed Dehydrogenative Cross‐Coupling Reaction

An efficient and concise one‐pot strategy for the direct alkylation of quinoline N‐oxides via palladium‐catalyzed dual C H bonds activation has been developed. This methodology provides quinoline‐containing heterocyclic molecules in moderate to excellent yields.

     

An Efficient Synthesis of Polysubstituted Pyridines via C?H Oxidation and C?S Cleavage of Dimethyl Sulfoxide

An expedient cleavage of the C S bond of dimethyl sulfoxide (DMSO) has been developed for the preparation of substituted pyridines from ketones. In this transformation, the co‐product formic acid was formed from ammonium formate, which acted as an important catalyst for the reaction. Notably, this transformation exhibited a broad substrate scope towards a wide variety of different ketones to give the corresponding substituted pyridines in high yields. Mechanistic studies suggested that dimethyl sulfoxide delivered a methylene fragment, which was subsequently captured in situ to give a pyridine.

     

Efficient Palladium‐Catalyzed Oxidative Indolation of Allylic Compounds with DDQ via sp3 C?H Bond Activation and Carbon‐Carbon Bond Formation Under Mild Conditions

An efficient oxidative cross‐coupling reaction between 1,3‐diarylpropenes and indoles in the presence of palladium chloride was achieved with 2,3‐dichloro‐5,6‐dicyanoquinone (DDQ) as oxidant. The reaction afforded 1,3‐diphenylallylindoles in moderate to high yields under mild conditions, thus providing a novel methodology to synthesize the respective products.     

Base‐Mediated Direct Arylation of Pyrrole Derivatives

It appears that transition metal catalysts are not necessary to perform the direct arylation of electron‐rich heterocycles with aryl iodides and bromides. Lithium tert‐butoxide in DMF promotes this reaction for a variety of N‐alkyl‐ and N‐arylpyrroles as well as for benzofuran and some other electron‐rich aromatic compounds and provides the desired products in moderate to high yields. In contrast to all previous reports on the Pd‐catalyzed direct arylation of indolizine, the reaction mediated by lithium tert‐butoxide proceeds selectively at position 5.     

Palladium‐Catalysed Intramolecular Direct Arylation of 2‐Bromobenzenesulfonic Acid Derivatives

A palladium‐catalysed intramolecular direct arylation of 2‐bromobenzenesulfonic acid derivatives was found to proceed using 1 mol% of palladium acetate as the catalyst. The influence of the substituents on the phenol moiety of 2‐bromobenzenesulfonic acid phenyl esters reveals that electron‐donating substituents favour the reaction while electron‐withdrawing ones are unfavourable. The reactivity of sulfonamides was also studied and, in all cases, a selective activation at sp² C H vs. sp³ C H was observed. A sulfonamide bearing both phenyl and benzyl substituents on nitrogen gave selectively the six‐membered ring product.     

Palladium‐Catalyzed Oxidative Annulation via C?H/N?H Functionalization: Access to Substituted Pyrroles

Pyrroles, ubiquitous bioactive heterocycles in nature, are readily prepared via a palladium‐catalyzed oxidative annulation of cyclic trans‐enamines to various internal alkynes in the absence of a directing group.