Palladium-catalyzed alpha-arylation of carbonyl compounds and nitriles

Cyanomethylation of the Benzene Rings and Pyridine Rings via Direct Oxidative Cross-Dehydrogenative Coupling with Acetonitrile

A novel and efficient approach for the amine-directed dehydrogenative C(sp²)-C(sp³) coupling of arylamines with acetonitrile was reported by using FeCl₂ as the catalyst. Substituted anilines, aminopyridines, naphthylamines, and some nitrogen-containing heterocyclic arylamines react with inactive acetonitrile to form the corresponding arylacetonitriles in moderate to good yields. This protocol features nontoxic iron catalysis, efficient atom economy, nonprefunctionalized starting materials, good regioselectivity, and excellent compatibility of functional groups and aromatic rings, providing a novel, straightforward, and green approach toward arylacetonitriles.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Photoassisted Cross-Coupling Reaction of α-Chlorocarbonyl Compounds with Arylboronic Acids

A Suzuki-Miyaura cross-coupling reaction of α-chloroacetates or α-chloroacetamides with arylboronic acids is made possible by visible-light irradiation. This reaction provides a useful method for the synthesis of α-arylacetates and α-arylacetamides from chlorides under mild reaction conditions. An indole-3-acetic acid derivative that is the key intermediate of the plant hormone auxin can be synthesized from 1-Boc-indole in two steps by combining an iridium-catalyzed C-H borylation and a palladium-catalyzed cross-coupling reaction.

Nucleophilic Aromatic Substitution of 5-Bromo-1,2,3-triazines with Phenols

Nucleophilic aromatic substitution (S_NAr) reaction in classic textbook is a stepwise mechanism, and few examples of concerted reactions have been reported. Herein, we developed a concerted S_NAr reaction of 5-bromo-1,2,3-triazines with phenols in which the nonclassic mechanism of this reaction could be revealed by calculation. Furthermore, the resulting 5-aryloxy-1,2,3-triazines could be used as convenient precursors to access biologically important 3-aryloxy-pyridines in one-pot manner.

α-Functionalization of ketones promoted by sunlight and heterogeneous catalysis in the aqueous phase

Herein, a protocol that combines heterogeneous catalysis and solar photocatalysis for the regioselective α-substitution of asymmetric ketones with quinoxalinones has been reported. The result indicates that the reaction is more likely to occur on the α-carbon. This strategy provides a green and efficient way for the α-functionalization of ketones. A singlet oxygen involved mechanism is suggested for the transformation.

Electrochemical synthesis for α-arylation of ketones using enol acetates and aryl diazonium salts

In this article, an electrochemical method has been developed to achieve the α-arylation of ketones by reacting aryl diazonium salts with enol acetates. The broad scope of the reaction toward...

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.

Benzylic Aroylation of Toluenes Mediated by a LiN(SiMe3)2/Cs+ System

Chemoselective deprotonative functionalization of benzylic C-H bonds is challenging, because the arene ring contains multiple aromatic C(sp²)-H bonds, which can be competitively deprotonated and lead to selectivity issues. Recently it was found that bimetallic [MN(SiMe₃)₂ M = Li, Na]/Cs⁺ combinations exhibit excellent benzylic selectivity. Herein, is reported the first deprotonative addition of toluenes to Weinreb amides mediated by LiN(SiMe₃)₂/CsF for the synthesis of a diverse array of 2-arylacetophenones. Surprisingly, simple methyl benzoates also react with toluenes under similar conditions to form 2-arylacetophenones without double addition to give tertiary alcohol products. This finding greatly increases the practicality and impact of this chemistry. Some challenging substrates with respect to benzylic deprotonations, such as fluoro and methoxy substituted toluenes, are selectively transformed to 2-aryl acetophenones. The value of benzylic deprotonation of 3-fluorotoluene is demonstrated by the synthesis of a key intermediate in the preparation of Polmacoxib.

From Academia to the Market - Air-stable Ni(ii)/Josiphos Catalysts

The design, synthesis, commercialization and application of air-stable Ni(II)/Josiphos complexes has been realized in a collaboration between Solvias and ICBMS (University Lyon 1). The Ni-complexes are utilized as versatile precatalysts for diverse cross-coupling reactions. Apart from being active in established C-C and C-N couplings at low catalyst loadings, the novel Ni-precatalysts enabled the development of the challenging monoarylation of ammonia, ammonia surrogates and even alkylammonium chlorides with aryl carbamates. Finally, the α-arylation of acetone with aryl chlorides, carbamates and pivalates was demonstrated using the Ni(II)/Josiphos precatalysts.

Selective divergent radical cyclization of 1,6-dienes with alkyl nitriles

An efficient, selective, and step economical radical cyclization of 1,6-dienes with alkyl nitriles initiated by α-C(sp³)-H functionalization under the Sc(OTf)₃ and Ag₂CO₃ system is described here. The selective divergent cyclization relies on the substitution effect at the α-position of the acrylamide moiety and nitriles, which is terminated by hydrogen abstraction, direct cyclization with the aryl ring, or further cyclization with the CN bond and hydrolysis, respectively.

Palladium-catalyzed synthesis of α-aryl acetophenones from styryl ethers and aryl diazonium salts via regioselective Heck arylation at room temperature

Preparation of α-aryl acetophenones from styryl ethers and aryldiazonium salts is described. The reaction is catalyzed by palladium acetate at room temperature in the absence of ligand and base. The developed method is highly attractive in terms of reaction conditions, substrate scope, functional group tolerance and yields. Synthetic applications of the present method are demonstrated by preparing α-aryl indoles and 3-aryl isocoumarin from styryl ethers.

Domino C-C/C-O bond formation: palladium-catalyzed regioselective synthesis of 7-iodobenzo[b]furans using 1,2,3-triiodobenzenes and benzylketones

A facile and efficient synthesis of 7-iodobenzo[b]furan derivatives via a highly regioselective tandem α-arylation/intramolecular O-arylation of 5-substituted-1,2,3-triiodobenzenes and benzylketones is described. Remarkably, the α-arylation coupling reactions initiate exclusively at the least sterically-hindered position of the triiodoarene, which results in a highly chemoselective transformation. The highest yields were observed in reactions between electron-poor 1,2,3-triiodoarenes and electron-rich benzylketones, yet the optimized reaction conditions were found to be tolerant to a wide range of different functional groups. This unprecedent synthesis of 7-iodobenzo[b]furans from 1,2,3-triiodobenzenes is scalable, general in scope, and provides easy access to valuable precursors for other chemical transformations.

Palladium-Catalyzed Direct α-Arylation of Indane-1,3-dione to 2-Substituted Indene-1,3-diones

A straightforward and feasible palladium-catalyzed direct α-arylation of indane-1,3-dione to 2-substituted aryl/heteroaryl indene-1,3-diones has been disclosed for the first time. Optimization of reaction conditions identified ^tBu-XPhos as a preferred ligand for the bis(acetonitrile)dichloropalladium(II) catalyst. A broad spectrum of aryl iodides and aryl triflates containing electron-donating, electron-withdrawing, and sterically hindered substituents gave an excellent yield for the quick access α-arylated 1,3-diones library.

Synthesis of α-Heteroaryl Propionic Esters by Palladium-Catalyzed α-Heteroarylation of Silyl Ketene Acetals

A practical and efficient synthesis of α-heteroaryl propionic esters is developed by employing palladium-catalyzed α-heteroarylation of silyl ketene acetals, forming a wide variety of α-heteroaryl propionic esters with various substituents and functionalities in high yields. The success of this transformation is credited to the development of the bulky P,P═O ligand. The method has provided an efficient synthesis of α-heteroaryl propionic acids.

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes

The Pd-catalyzed asymmetric α-arylation of carbonyl compounds is a valuable strategy to form benzylic stereocenters. However, the origin of the stereoselectivity of these reactions is poorly understood, and little is known about the reactivity of the putative diastereomeric arylpalladium enolate intermediates. To this end, we report the synthesis and characterization of a series of diphosphine-ligated arylpalladium fluoroenolate complexes, including complexes bearing a metal-bound, stereogenic carbon and an enantioenriched chiral diphosphine ligand. These complexes reductively eliminate to form chiral α-aryl-α-fluorooxindoles with enantioselectivities and rates that are relevant to those of the catalytic process with SEGPHOS as the ancillary ligand. Kinetic studies showed that the rate of reductive elimination is slightly slower than the rate of epimerization of the intermediate, causing the reductive elimination step to impart the greatest influence on the enantioselectivity. DFT calculations of these processes are consistent with these experimental rates and suggest that the minor diastereomer forms the major enantiomer of the product. The rates of reductive elimination from complexes containing a variety of electronically varied aryl ligands revealed the unusual trend that complexes bearing more electron-rich aryl ligands react faster than those bearing more electron-poor aryl ligands. Noncovalent Interaction (NCI) and Natural Bond Orbital (NBO) analyses of the transition-state structures for reductive elimination from the SEGPHOS-ligated complexes revealed key donor-acceptor interactions between the Pd center and the fluoroenolate fragment. These interactions stabilize the pathway to the major product enantiomer more strongly than they stabilize that to the minor enantiomer.

Electrochemical α-Arylation of Ketones via Anodic Oxidation of In Situ Generated Silyl Enol Ethers

An electrochemical procedure for the α-arylation of ketones has been developed. The method is based on the generation and one-pot anodic oxidation of silyl enol ethers in the presence of the arene. This strategy avoids isolation of the silyl enol intermediate and the utilization of external supporting electrolytes. Intermolecular arylations, which had not been reported so far, are possible when electron-rich arenes are utilized as coupling partners. The method has been demonstrated for a wide variety of aryl ketones and activated arenes, with moderate to good yields (up to 69%) obtained. Mechanistic insights and a theoretical rationale that explains the ketone α-arylation versus dimerization selectivity are also presented.

α-C(sp3)-H Arylation of Cyclic Carbonyl Compounds

α-C(sp3)-H arylation is an important type of C-H functionalization. Various biologically significant natural products, chemical intermediates, and drugs have been effectively prepared via C-H functionalization. Cyclic carbonyl compounds comprise of cyclic ketones, enones, lactones, and lactams. The α-C(sp3)-H arylation of these compounds have been exhibited high efficiency in forming C(sp3)-C(sp2) bonds, played a crucial role in organic synthesis, and attracted majority of interests from organic and medicinal communities. This review focused on the most significant advances including methods, mechanism, and applications in total synthesis of natural products in the field of α-C(sp3)-H arylations of cyclic carbonyl compounds in recent years.

Design of an Electron-Withdrawing Benzonitrile Ligand for Ni-Catalyzed Cross-Coupling Involving Tertiary Nucleophiles

The design of new ligands for cross-coupling is essential for developing new catalytic reactions that access valuable products such as pharmaceuticals. In this report, we exploit the reactivity of nitrile-containing additives in Ni catalysis to design a benzonitrile-containing ligand for cross-coupling involving tertiary nucleophiles. Kinetic and Hammett studies are used to elucidate the role of the optimized ligand, which demonstrate that the benzonitrile moiety acts as an electron-acceptor to promote reductive elimination over β-hydride elimination and stabilize low-valent Ni. With these conditions, a protocol for decyanation-metalation and Ni-catalyzed arylation is conducted, enabling access to quaternary α-arylnitriles from disubstituted malononitriles.

Aldehyde to Ketone Homologation Enabled by Improved Access to Thioalkyl Phosphonium Salts

Phosphines were previously unusable as Pummerer-type nucleophiles due to competing redox chemistry with sulfoxides. Here we circumvent this problem to achieve a formal phosphine Pummerer reaction that offers thioalkyl phosphonium salts that, in turn, give rise to diverse vinyl sulfides via Wittig olefinations. Thirty vinyl sulfides are thus prepared from (alkylthioalkyl)triphenyl phosphonium salts and aldehydes. The hydrolysis of these vinyl sulfides offers an efficient and versatile two-step one-carbon homologation of aldehydes to ketones.

The copper-catalyzed selective monoalkylation of active methylene compounds with alkylsilyl peroxides

A novel method for a mild copper-catalyzed selective monoalkylation of active methylene compounds with various alkylsilyl peroxides has been developed. The reaction has a broad substrate scope and our mechanistic studies suggest the participation of radical species in this alkylation reaction.

Concise catalytic asymmetric synthesis of (R)-4-amino Uhle's ketone

A practical and asymmetric synthesis of (R)-4-amino-5-oxo-1,3,4,5-tetrahydrobenz[cd]indole, an enantiopure framework shared by most ergot alkaloids, was accomplished. Our method involves a Rh(i)-catalyzed 6-exo-trig intramolecular cyclization of an appropriate 4-pinacolboronic ester d-tryptophan aldehyde followed by the oxidation of the resulting secondary benzylic alcohol with a Cu(i)-ABNO catalyst and final deprotection under acidic conditions. This new procedure offers significant advantages over previous synthetic approaches, including brevity, mild reaction conditions, preservation of chiral integrity, and high overall yield and avoids the use of stoichiometric amounts of strongly basic and pyrophoric organometallic reagents.

NHC-Mediated Stetter-Aldol and Imino-Stetter-Aldol Domino Cyclization to Naphthalen-1(2H)-ones and Isoquinolines

N-Heterocyclic carbene-catalyzed tandem Stetter-aldol reaction of phthalaldehyde and α,β-unsaturated ketimines has been developed to afford functionalized naphthalen-1(2H)-one derivatives as the formal [4+2] annulation product. Interestingly, the reaction of aldimines led to the formation of isoquinoline derivatives instead of the expected indanone derivatives as a [4+1] annulation product.

Coupling of Reformatsky Reagents with Aryl Chlorides Enabled by Ylide-Functionalized Phosphine Ligands

The coupling of aryl chlorides with Reformatsky reagents is a desirable strategy for the construction of α‐aryl esters but has so far been substantially limited in the substrate scope due to many challenges posed by various possible side reactions. This limitation has now been overcome by the tailoring of ylide‐functionalized phosphines to fit the requirements of Negishi couplings. Record‐setting activities were achieved in palladium‐catalyzed arylations of organozinc reagents with aryl electrophiles using a cyclohexyl‐YPhos ligand bearing an ortho‐tolyl‐substituent in the backbone. This highly electron‐rich, bulky ligand enables the use of aryl chlorides in room temperature couplings of Reformatsky reagents. The reaction scope covers diversely functionalized arylacetic and arylpropionic acid derivatives. Aryl bromides and chlorides can be converted selectively over triflate electrophiles, which permits consecutive coupling strategies.

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.

Palladium-catalyzed functionalizations of acidic and non-acidic C(sp3)-H bonds - recent advances

A tremendous upsurge has been seen in the recent decade for the proximal and remote functionalization of activated and unactivated substrates via palladium redox pathways. This feature article discusses some of the recent reports on direct as well as indirect C(sp3)-H functionalization via cross-coupling reactions under palladium catalysis. Activated substrates (possessing acidic C(sp3)-H) including enones, ketones, aldehydes, silylenol ethers, esters, silyl ketene acetals, amides, cyano, α-amino esters, and O-carbamates, capable of undergoing cross-coupling reactions at the α-, β-, γ-, δ- and ε-positions, will be discussed. To overcome the challenging task of achieving regioselectivity, a variety of innovative modifications have been reported. The reports of C-H activations based on directing group, and as native functionality have been illustrated at the β-, γ- and δ-positions. Substrates such as α-amino esters, carbonyls, carboxylic acids and their derivatives, afford site-selective C(sp3)-H functionalization via varied-sized reactive metallacycles and are a unique class of substrates whose C(sp3)-H functionalizations were earlier considered as very difficult.

Cu-catalyzed cyanomethylation of imines and α,β-alkenes with acetonitrile and its derivatives

We describe copper-catalyzed cyanomethylation of imines and α,β-alkenes with a methylnitrile source and provide an efficient route to synthesize arylacrylonitriles and β,γ-unsaturated nitriles. This method tolerates aliphatic and aromatic alkenes substituted with a variety of functional groups such as F, Cl, Br, Me, OMe, tert-Bu, NO2, NH2 and CO2H with good to excellent yields (69-98%). These systems consist of inexpensive, simple copper catalyst and acetonitrile with its derivatives (α-bromo/α-iodo-acetonitrile) and are highly applicable in the industrial production of acrylonitriles.

The influence of additives on orthogonal reaction pathways in the Mizoroki–Heck arylation of vinyl ethers

Translating microscale high-throughput screening hits into preparative scale chemistry often requires an understanding of scale-dependent factors, such as the effect of additives on catalyst activation pathways.

Palladium catalyzed C–C and C–N bond forming reactions: an update on the synthesis of pharmaceuticals from 2015–2020

Some of the most prominent and promising catalysts in organic synthesis for the requisite construction of C–C and C–N bonds are palladium (Pd) catalysts, which play a pivotal role in pharmaceutical and medicinal chemistry.

Different effects of metal-NHC bond cleavage on the Pd/NHC and Ni/NHC catalyzed α-arylation of ketones with aryl halides

Fundamental differences in the behavior of Pd/NHC and Ni/NHC catalytic systems in ketones α-arylation were elucidated and exploited.

Introducing deep eutectic solvents in enolate chemistry: synthesis of 1-arylpropan-2-ones under aerobic conditions

An environmentally friendly procedure for the generation of enolates from 1-arylpropan-2-ones, followed by functionalization with electrophiles and (hetero)aryl halides in deep eutectic solvents under aerobic conditions, is disclosed.

The Mizoroki–Heck reaction initiated formal C(sp3)–H arylation of carbonyl compounds

This work described a Mizoroki–Heck reaction initiated formal C(sp3)–H arylation of α-methylcinnamaldehydes under phosphane-free conditions.

Ligand Control of Palladium-Catalyzed Site-Selective α- and γ-Arylation of α,β-Unsaturated Ketones with (Hetero)aryl Halides

This study describes the first palladium-catalyzed, site-selective α- and γ-arylation of α,β-unsaturated ketones with (hetero)aryl halides. A wide range of hetero(aryl)halides coupled with α,β-unsaturated ketones, and transformation into the arylated products proceeded with excellent to good yields. The site selectivity of the reaction is switchable by simply changing the phosphine ligand to access either α-arylated or γ-arylated products in good to excellent yields by using a low catalyst loading, and the method demonstrates good functional-group compatibility.

The site-selectivity and mechanism of Pd-catalyzed C(sp2)-H arylation of simple arenes

Control over site-selectivity is a critical challenge for practical application of catalytic C–H functionalization reactions in organic synthesis. Despite the seminal breakthrough of the Pd-catalyzed C(sp²)–H arylation of simple arenes via a concerted metalation–deprotonation (CMD) pathway in 2006, understanding the site-selectivity of the reaction still remains elusive. Here, we have comprehensively investigated the scope, site-selectivity, and mechanism of the Pd-catalyzed direct C–H arylation reaction of simple arenes. Counterintuitively, electron-rich arenes preferably undergo meta-arylation without the need for a specifically designed directing group, whereas electron-deficient arenes bearing fluoro or cyano groups exhibit high ortho-selectivity and electron-deficient arenes bearing bulky electron-withdrawing groups favor the meta-product. Comprehensive mechanistic investigations through a combination of kinetic measurements and stoichiometric experiments using arylpalladium complexes have revealed that the Pd-based catalytic system works via a cooperative bimetallic mechanism, not the originally proposed monometallic CMD mechanism, regardless of the presence of a strongly coordinating L-type ligand. Notably, the transmetalation step, which is influenced by a potassium cation, is suggested as the selectivity-determining step.

The transmetalation step, not the C–H activation step, is suggested as the selectivity-determining step in Pd-catalyzed C–H arylation of simple arenes.

Pd-catalyzed arylation/aza-Michael addition cascade to C2-spiroindolines and azabicyclo[3.2.2]nonanones

A palladium-catalyzed arylation/aza-Michael addition cascade reaction of β-substituted cyclic enones and 2-haloanilines has been reported. Using 1 mol% Pd(PPh₃)₄ as a catalyst, C2-spiroindolines are accessed via an intermolecular vinylogous arylation of β-alkyl cyclic enones and 2-haloanilines followed by an intramolecular aza-Michael addition. The functional group tolerance of this transformation is examined by 18 examples in up to 93% yield. In the second part, we developed an α'-arylation/aza-Michael addition cascade strategy to construct azabicyclo[3.2.2]nonanones catalyzed by Pd(MeCN)₂Cl₂·PPh₃. This study provides a quick route to complex and useful spiro- and bridged-heterocycles from readily available starting materials in good yields with high regioselectivity.