Palladium-catalyzed direct intermolecular α-arylation of amides with aryl chlorides

P,N-Coordinating Ylide-Functionalized Phosphines (NYPhos): A Ligand Platform for the Selective Monoarylation of Small Nucleophiles

Palladium-catalyzed coupling reactions of small nucleophiles are of great interest, but challenging due to difficulties in selectivity control. Herein, we report the development of a new platform of P,N-ligands consisting of ylide-functionalized phosphines with aminophosphonium groups (NYPhos) to address this challenge. These phosphine ligands are easily accessible in a wide structural diversity with highly modular electronic and steric properties. Based on a family of 14 ligands the selective monoarylation of acetone as well as other challenging ketones and amides was accomplished with record-setting activities even for aryl chlorides at room temperature including late-stage functionalizations of drug molecules. Moreover, ammonia and other small primary amines could be coupled at mild conditions. Isolation and structure analyses of palladium complexes within the catalytic cycle confirmed that the P,N-coordination mode is necessary to achieve the observed selectivities. It also demonstrated the facile adjustability of the N-donor strength, which is beneficial for the targeted design of tailored P,N-ligands for future applications.

Copper-Catalyzed Chemoselective O-Arylation of Oxindoles: Access to Cyclic Aryl Carboxyimidates

We have developed a highly efficient base- and additive-free chemoselective CuO-catalyzed strategy for the O-arylation of 2-oxindoles to synthesize 2-phenoxy-3H-indole and 2-phenoxy-1H-indole derivatives in the presence of diaryl iodonium salts. This method offers a variety of O-arylated oxindoles in good to excellent yields under relatively milder reaction conditions. Furthermore, this methodology was extended for the O-arylation of 2-pyridinone and isoindoline-1-one derivatives as well.

Development of a General Method for the Hiyama-Denmark Cross-Coupling of Tetrasubstituted Vinyl Silanes

General conditions for the Hiyama-Denmark cross-coupling of tetrasubstituted vinyl silanes and aryl halides are reported. Prior reports of Hiyama-Denmark reactions of tetrasubstituted vinyl silanes have required the use of vinyl silanols or silanolates, which are challenging to handle, or internally activated vinyl silanes, which lack structural generality. Now, unactivated tetrasubstituted vinyl silanes, bearing bench-stable tetraorganosilicon centers, and aryl halides can be coupled. The key to this discovery is the identification of dimethyl(5-methylfuryl)vinylsilanes as bench stable and easily prepared cross-coupling partners that are readily activated under mild conditions in Hiyama-Denmark couplings. These palladium-catalyzed cross-couplings proceed well with aryl chlorides, though aryl bromides and iodides are also tolerated, and the reactions display high stereospecificity in the formation of tetrasubstituted alkenes. In addition, only a mild base (KOSiMe3) and common solvents (THF/DMA) are required, and importantly toxic additives (such as 18-crown-6) are not needed. We also show that these conditions are equally applicable to Hiyama-Denamrk coupling of trisubstituted vinyl silanes.

Synthesis of α-Aryl Secondary Amides via Nickel-Catalyzed Reductive Coupling of Redox-Active Esters

The transition-metal-catalyzed α-arylation of secondary amides remains a synthetic challenge due to the presence of a free N-H bond. We report a strategy to synthesize secondary α-aryl amides via a Ni-catalyzed reductive arylation of redox-active N-hydroxyphthalimide (NHP) esters of malonic acid half amides. This transformation proceeds under mild conditions and displays excellent chemoselectivity for amide α-arylation in the presence of other enolizable carbonyls. The NHP ester substrates are readily prepared from Meldrum's acid.

Bimetallic PdII complexes with NHC/Py/PCy3 donor set ligands: applications in α-arylation, Suzuki–Miyaura and Sonogashira coupling reactions

Bimetallic PdII complexes bearing mixed NHC/Py/PCy3 donor set ligands are presented. A complex with a mixed NHC/PCy3 donor set ligands shows superior activity compared to the PEPPSI type complexes in α-arylation and Sonogashira coupling reactions.

Pseudo-Tris(heteroleptic) Red Phosphorescent Iridium(III) Complexes Bearing a Dianionic C,N,C',N'-Tetradentate Ligand

1-Phenyl-3-(1-phenyl-1-(pyridin-2-yl)ethyl)isoquinoline (H₂MeL) has been prepared by Pd(N-XantPhos)-catalyzed “deprotonative cross-coupling processes” to synthesize new phosphorescent red iridium(III) emitters (601–732 nm), including the carbonyl derivative Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)Cl(CO) and the acetylacetonate compound Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)(acac). The tetradentate 6e-donor ligand (6tt′) of these complexes is formed by two different bidentate units, namely, an orthometalated 2-phenylisoquinoline and an orthometalated 2-benzylpyridine. The link between the bidentate units reduces the number of possible stereoisomers of the structures [6tt′ + 3b] (3b = bidentate 3e-donor ligand), with respect to a [3b + 3b′ + 3b″] emitter containing three free bidentate units, and it permits a noticeable stereocontrol. Thus, the isomers fac-Ir(κ⁴-cis-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆H₄-py)}, mer-Ir(κ⁴-cis-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆H₃R-py)}, and mer-Ir(κ⁴-trans-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆HR-py)} (R = H, Me) have also been selectively obtained. The new emitters display short lifetimes (0.7–4.6 μs) and quantum yields in a doped poly(methyl methacrylate) film at 5 wt % and 2-methyltetrahydrofuran at room temperature between 0.08 and 0.58. The acetylacetonate complex Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)(acac) has been used as a dopant for a red PhOLED device with an electroluminescence λ_max of 672 nm and an external quantum efficiency of 3.4% at 10 mA/cm².

The proligand 1-phenyl-3-(1-phenyl-1-(pyridine-2-yl)ethyl)isoquinoline is used to generate a new family of neutral phosphorescent red iridium(III) emitters containing a tetradentate ligand, formed by two different bidentate units, and a third bidentate ligand with a good stereocontrol of the resulting [6tt′ + 3b] products. One of the new emitters has been used in the fabrication of an OLED device.

Pd-catalysed β-selective C(sp3)-H arylation of simple amides

An efficient Pd-catalysed β-C(sp³)-H arylation of diverse native amides with aryl iodides was developed. This protocol overcomes the necessity of the Thorpe-Ingold effect and features broad substrate scope and good functional group tolerance. The potential application of this protocol is collectively demonstrated by gram-scale synthesis and the synthesis of several bioactive molecules.

N-Heterocyclic carbene complexes enabling the α-arylation of carbonyl compounds

The considerable importance of α-arylated carbonyl compounds, which are widely used as final products or as key intermediates in the pharmaceutical industry, has prompted numerous research groups to develop efficient synthetic strategies for their preparation in recent decades. In this context, the α-arylation of carbonyl compounds catalyzed by transition-metal complexes have been particularly helpful in constructing this motif. As illustrated in this contribution, tremendous advances have taken place using palladium- and nickel-NHC (NHC = N-heterocyclic carbene) complexes as pre-catalysts for the arylation of a wide range of ketones, aldehydes, esters and amides with electron-rich, electron-neutral, electron-poor, and sterically hindered aryl halides or pseudo-halides. Despite significant progress, especially in asymmetric α-arylations promoted by chiral NHC ligands, there are numerous challenges which have and continue to encourage further studies on this topic. Some of these are presented in this report.

Cobalt-Catalyzed 1,4-Aryl Migration/Desulfonylation Cascade: Synthesis of α-Aryl Amides

A cobalt-catalyzed 1,4-aryl migration/disulfonylation cascade applied to α-bromo N-sulfonyl amides was developed. The reaction was highly chemoselective, allowing the preparation of α-aryl amides possessing a variety of functional groups. The method was used as the key step to synthesize an alkaloid, (±)-deoxyeseroline. Mechanistic investigations suggest a radical process.

Nickel-Catalyzed Amination of Aryl Chlorides with Amides

A nickel-catalyzed amination of aryl chlorides with diverse amides via C-N bond cleavage has been realized under mild conditions. A broad substrate scope with excellent functional group tolerance at a low catalyst loading makes the protocol powerful for synthesizing various aromatic amines. The aryl chlorides could selectively couple to the amino fragments rather than the carbonyl moieties of amides. Our protocol complements the conventional amination of aryl chlorides and expands the usage of inactive amides.

Cyclopentyl Methyl Ether: An Elective Ecofriendly Ethereal Solvent in Classical and Modern Organic Chemistry

Solvents represent one of the major contributions to the environmental impact of fine-chemical synthesis. As a result, the use of environmentally friendly solvents in widely employed reactions is a challenge of vast real interest in contemporary organic chemistry. Within this Review, a great variety of examples showing how cyclopentyl methyl ether has been established as particularly useful for this purpose are reported. Indeed, its low toxicity, high boiling point, low melting point, hydrophobicity, chemical stability towards a wide range of conditions, exceptional stability towards the abstraction of hydrogen atoms, relatively low latent heat of vaporization, and the ease with which it can be recovered and recycled enable its successful employment as a solvent in a wide range of synthetic applications, including organometallic chemistry, catalysis, biphasic reactions, oxidations, and radical reactions.

Palladium-Catalyzed Serendipitous Synthesis of Arylglyoxylic Amides from Arylglyoxylates and N,N-Dialkylamides in the Presence of Halopyridines

A palladium-catalyzed synthesis of arylglyoxylic amides by the reaction of arylglyoxylates and N,N-dialkylamides in the presence of a 2,3-dihalopyridine has been realized for the first time. An anticipated 2,3-diaroylpyridine did not form in this reaction. The current study unveils an unprecedented role of 2,3-dihalopyridine toward this amidation. Our mechanistic study reveals that the arylglyoxylate could react with halopyridine to form a traceless activated pyridyl ester of arylglyoxylic acid, which upon subsequent reaction with amino surrogate, N,N-dialkylamides could form the arylglyoxylic amides.

Palladium(II)/N-Heterocyclic Carbene Catalyzed One-Pot Sequential α-Arylation/Alkylation: Access to 3,3-Disubstituted Oxindoles

Rationally designed fluorene-based mono- and bimetallic Pd-PEPPSI complexes were synthesized and demonstrated to be effective for the one-pot sequential α-arylation/alkylation of oxindoles. This streamlined approach offers efficient access to functionalized 3,3-disubstituted oxindoles in excellent yields (up to 89%) under mild reaction conditions.

Recent progress in the synthesis of phosphorus-containing indole derivatives

Phosphorus-containing indole derivatives represent a special class of phosphorus-containing nitrogen heterocycles. This review summarizes the recent progress in the synthesis of such compounds, briefly discusses the reaction mechanisms and challenges, and outlines the synthetic opportunities still open.

Cobalt-Catalyzed Cross-Coupling of α-Bromo Amides with Grignard Reagents

A cobalt-catalyzed cross-coupling between α-bromo amides and Grignard reagents is disclosed. The reaction is general and allows access to a large variety of α-aryl and β,γ-unsaturated amides. Some mechanistic investigations have been undertaken to determine the nature of the intermediate species.

Cascade Amination and Acetone Monoarylation with Aryl Iodides by Palladium/Norbornene Cooperative Catalysis

A palladium/norbornene cocatalyzed three-component reaction of aryl iodides, O-benzoylhydroxylamines, and acetone is reported. o'-Aminoaryl acetones or o,o'-diaminoaryl acetones are efficiently prepared via tandem ortho-C-H amination/ipso-C-I α-arylation sequence, and the regiospecificity has been confirmed by X-ray analysis. The proposed method addresses the condensation/amination of free-N-H-bearing substrates in acetone monoarylations and the synthesis of extremely congested 2,6-disubstituted aryl acetones.

Tandem C(sp3)-H Arylation/Oxidation and Arylation/Allylic Substitution of Isoindolinones

Isoindolinones comprise an important class of medicinally active compounds. Herein we report a straightforward functionalization of the isoindolinones with aryl bromides (22 examples) using a Pd(OAc)2/NIXANTPHOS-based catalyst system. Additionally 3-aryl 3-hydroxy isoindolinone derivatives, which exhibit anti-tumor activity, can be accessed via a tandem reaction. Thus, when the arylation product is exposed to air under basic conditions, in situ oxidation takes place to install the 3-hydroxyl group. Furthermore, a tandem arylation/allylic substitution reaction is advanced in which both the arylation and allylic substitution are catalyzed by the same palladium catalyst. Finally, a tandem arylation/alkylation procedure is presented. These tandem reactions enable the synthesis of a variety of structurally diverse isoindolinone derivatives from common starting materials.

Palladium-Catalyzed α-Arylation of Methyl Sulfonamides with Aryl Chlorides

A Palladium-catalyzed α-arylation of sulfonamides with aryl chlorides is presented. A Buchwald type precatalyst formed with Kwong's indole-based ligand enabled this transformation to be compatible with a large variety of methyl sulfonamides and aryl chlorides in good to excellent yields. Importantly, under the optimized reaction conditions, only mono-arylated products were observed. This method has been applied to the efficient synthesis of sumatriptan, which is used to treat migraines.

Catalytic α-Arylation of Imines Leading to N-Unprotected Indoles and Azaindoles

A palladium N-heterocyclic carbene catalyzed methodology for the synthesis of substituted, N-unprotected indoles and azaindoles is reported. The protocol permits access to various, highly substituted members of these classes of compounds. Although two possible reaction pathways (deprotonative and Heck-like) can be proposed, control experiments, supported by computational studies, point toward a deprotonative mechanism being operative.

Palladium-Catalyzed Benzylic Arylation of Pyridylmethyl Silyl Ethers: One-Pot Synthesis of Aryl(pyridyl)methanols

An efficient palladium-catalyzed direct arylation of pyridylmethyl silyl ethers with aryl bromides is described. A Pd(OAc)2/NIXANTPHOS-based catalyst provides aryl(pyridyl)methyl alcohol derivatives in good to excellent yields (33 examples, 57-100% yield). This protocol is compatible with different silyl ether protecting groups, affording either the protected or the free alcohols in an effective one-pot process. The scalability of the reaction is demonstrated.

Chemoselective Palladium-Catalyzed Deprotonative Arylation/[1,2]-Wittig Rearrangement of Pyridylmethyl Ethers

Control of chemoselectivity is one of the most challenging problems facing chemists and is particularly important in the synthesis of bioactive compounds and medications. Herein, the first highly chemoselective tandem C(sp3)-H arylation/[1,2]-Wittig rearrangement of pyridylmethyl ethers is presented. The efficient and operationally simple protocols enable generation of either arylation products or tandem arylation/[1,2]-Wittig rearrangement products with remarkable selectivity and good to excellent yields (60-99%). Choice of base, solvent, and reaction temperature play a pivotal role in tuning the reactivity of intermediates and controlling the relative rates of competing processes. The novel arylation step is catalyzed by a Pd(OAc)2/NIXANTPHOS-based system via a deprotonative cross-coupling process. The method provides rapid access to skeletally diverse aryl(pyridyl)methanol core structures, which are central components of several medications.

Metal catalyst-free substitution of allylic and propargylic phosphates with diarylmethyl anions

Substitution of secondary allylic and propargylic phosphates with the anions derived from Ar₂CH₂ with BuLi or LDA proceeded regioselectively and stereoselectively without a metal catalyst, affording inversion products in good yields.

Palladium-Catalyzed Benzylic C-H Arylation of Azaarylmethylamines

A direct C-H functionalization approach to produce aryl(azaaryl)methylamines from azaarylmethylamines without directing groups is described. Under conditions where the azaarylmethylamines' C-H is reversibly deprotonated, a Pd(OAc)(2)/NIXANTPHOS-based catalyst couples the resulting carbanions with various aryl halides to provide aryl(azaaryl)methylamines. This umpolung strategy directly provides tertiary amines without protecting or activating groups.

Generating Active "L-Pd(0)" via Neutral or Cationic π-Allylpalladium Complexes Featuring Biaryl/Bipyrazolylphosphines: Synthetic, Mechanistic, and Structure-Activity Studies in Challenging Cross-Coupling Reactions

Two new classes of highly active yet air- and moisture-stable π-R-allylpalladium complexes containing bulky biaryl- and bipyrazolylphosphines with extremely broad ligand scope have been developed. Neutral π-allylpalladium complexes incorporated a range of biaryl/bipyrazolylphosphine ligands, while extremely bulky ligands were accommodated by a cationic scaffold. These complexes are easily activated under mild conditions and are efficient for a wide array of challenging C-C and C-X (X = heteroatom) cross-coupling reactions. Their high activity is correlated to their facile activation to a 12-electron-based "L-Pd(0)" catalyst under commonly employed conditions for cross-coupling reactions, noninhibitory byproduct release upon activation, and suppression of the off-cycle pathway to form dinuclear (μ-allyl)(μ-Cl)Pd2(L)2 species, supported by structural (single crystal X-ray) and kinetic studies. A broad scope of C-C and C-X coupling reactions with low catalyst loadings and short reaction times highlight the versatility and practicality of these catalysts in organic synthesis.

Synthesis of triarylmethanols via tandem arylation/oxidation of diarylmethanes

A tandem arylation/oxidation of diarylmethanes for the convenient synthesis of unsymmetrical triarylmethanols bearing different aryl and heteroaryl groups is described. A Pd(OAc)2-NiXantphos catalyst system efficiently catalyzed arylation of weakly acidic sp3-hybridized C-H bonds of diarylmethanes with aryl bromides, and the arylation products were then oxidized in situ to carbinols by simply opening the reaction flasks to air. The triarylmethanol products were obtained in 35-98% yield.