Palladium-catalyzed coupling reactions of aryl chlorides

Collectively, palladium-catalyzed coupling reactions represent some of the most powerful and versatile tools available to synthetic organic chemists. Their widespread popularity stems in part from the fact that they are generally tolerant to a large number of functional groups, which allows them to be employed in a wide range of applications. However, for many years a major limitation of palladium-catalyzed coupling processes has been the poor reactivity of aryl chlorides, which from the standpoints of cost and availability are more attractive substrates than the corresponding bromides, iodides, and triflates. Traditional palladium/triarylphosphane catalysts are only effective for the coupling of certain activated aryl chlorides (for example, heteroaryl chlorides and substrates that bear electron-withdrawing groups), but not for aryl chlorides in general. Since 1998, major advances have been described by a number of research groups addressing this challenge; catalysts based on bulky, electron-rich phosphanes and carbenes have proved to be particularly mild and versatile. This review summarizes both the seminal early work and the exciting recent developments in the area of palladium-catalyzed couplings of aryl chlorides.

The development of versatile methods for palladium-catalyzed coupling reactions of aryl electrophiles through the use of P(t-Bu)3 and PCy3 as ligands

Metal-catalyzed coupling reactions of aryl electrophiles with organometallics and with olefins serve as unusually effective tools for forming new carbon-carbon bonds. By 1998, researchers had developed catalysts that achieved reactions of aryl iodides, bromides, and triflates. Nevertheless, many noteworthy challenges remained; among them were couplings of aryl iodides, bromides, and triflates under mild conditions (at room temperature, for example), couplings of hindered reaction partners, and couplings of inexpensive aryl chlorides. This Account highlights some of the progress that has been made in our laboratory over the past decade, largely through the appropriate choice of ligand, in achieving these synthetic objectives. In particular, we have established that palladium in combination with a bulky trialkylphosphine accomplishes a broad spectrum of coupling processes, including Suzuki, Stille, Negishi, and Heck reactions. These methods have been applied in a wide array of settings, such as natural-product synthesis, materials science, and bioorganic chemistry.

A Convenient and General Method for Pd-Catalyzed Suzuki Cross-Couplings of Aryl Chlorides and Arylboronic Acids

From only commercially available reagents a wide array of Suzuki cross-couplings of aryl chlorides with arylboronic acids can be effected in excellent yield [Eq. (a)]. This development provides a general solution to a long-standing limitation of this extremely powerful process-the poor reactivity of inexpensive and readily accessible aryl chlorides. dba=dibenzylideneacetone.

Transition metal-catalyzed alkyl-alkyl bond formation: Another dimension in cross-coupling chemistry

Because the backbone of most organic molecules is composed primarily of carbon-carbon bonds, the development of efficient methods for their construction is one of the central challenges of organic synthesis. Transition metal-catalyzed cross-coupling reactions between organic electrophiles and nucleophiles serve as particularly powerful tools for achieving carbon-carbon bond formation. Until recently, the vast majority of cross-coupling processes had used either aryl or alkenyl electrophiles as one of the coupling partners. In the past 15 years, versatile new methods have been developed that effect cross-couplings of an array of alkyl electrophiles, thereby greatly expanding the diversity of target molecules that are readily accessible. The ability to couple alkyl electrophiles opens the door to a stereochemical dimension-specifically, enantioconvergent couplings of racemic electrophiles-that substantially enhances the already remarkable utility of cross-coupling processes.

A versatile catalyst for Heck reactions of aryl chlorides and aryl bromides under mild conditions

In the presence of Cy2NMe, Pd/P(t-Bu)3 serves as an exceptionally mild and versatile catalyst for Heck reactions of aryl chlorides and bromides. A sterically and electronically diverse array of aryl bromides, as well as activated aryl chlorides, couple with a range of mono- and disubstituted olefins at room temperature, furnishing the arylated product with high E/Z stereoselection. The corresponding reactions of a broad spectrum of electron-neutral and electron-rich aryl chlorides proceed at elevated temperature, also with high selectivity. In terms of scope and mildness, Pd/P(t-Bu)3/Cy2NMe represents an advance over previously reported catalysts for these Heck coupling processes.

Asymmetric copper-catalyzed C-N cross-couplings induced by visible light

Despite a well-developed and growing body of work in copper catalysis, the potential of copper to serve as a photocatalyst remains underexplored. Here we describe a photoinduced copper-catalyzed method for coupling readily available racemic tertiary alkyl chloride electrophiles with amines to generate fully substituted stereocenters with high enantioselectivity. The reaction proceeds at -40°C under excitation by a blue light-emitting diode and benefits from the use of a single, Earth-abundant transition metal acting as both the photocatalyst and the source of asymmetric induction. An enantioconvergent mechanism transforms the racemic starting material into a single product enantiomer.

Air-stable trialkylphosphonium salts: simple, practical, and versatile replacements for air-sensitive trialkylphosphines. Applications in stoichiometric and catalytic processes

Trialkylphosphines furnish unusual, sometimes unique, reactivity in a range of transformations. Unfortunately, their utility is compromised by their sensitivity to oxidation. We have examined a simple but powerful strategy for addressing this problem: convert air-sensitive trialkylphosphines into air-stable phosphonium salts via protonation on phosphorus. These robust salts serve as direct replacements for the corresponding phosphines (simple deprotonation under the reaction conditions by a Brønsted base liberates the trialkylphosphine) in a diverse set of applications. [reaction: see text]

Transition-Metal Catalysis of Nucleophilic Substitution Reactions: A Radical Alternative to SN1 and SN2 Processes

Classical methods for achieving nucleophilic substitutions of alkyl electrophiles (S_N1 and S_N2) have limited scope and are not generally amenable to enantioselective variants that employ readily available racemic electrophiles. Radical-based pathways catalyzed by chiral transition-metal complexes provide an attractive approach to addressing these limitations.

Pd(PhCN)(2)Cl(2)/P(t-Bu)(3): a versatile catalyst for Sonogashira reactions of aryl bromides at room temperature

[reaction: see text] Pd(PhCN)(2)Cl(2)/P(t-Bu)(3) serves as an efficient and a versatile catalyst for room-temperature Sonogashira reactions of aryl bromides.

Enantioselective Decarboxylative Arylation of α-Amino Acids via the Merger of Photoredox and Nickel Catalysis

An asymmetric decarboxylative Csp(3)-Csp(2) cross-coupling has been achieved via the synergistic merger of photoredox and nickel catalysis. This mild, operationally simple protocol transforms a wide variety of naturally abundant α-amino acids and readily available aryl halides into valuable chiral benzylic amines in high enantiomeric excess, thereby producing motifs found in pharmacologically active agents.

Asymmetric catalysis with "planar-chiral" derivatives of 4-(dimethylamino)pyridine

Whereas chiral Lewis acid catalysis has been intensively investigated, chiral Lewis base (nucleophilic) catalysis has been comparatively neglected. We have developed "planar-chiral" derivatives of 4-(dimethylamino)pyridine (DMAP), a highly versatile nucleophilic catalyst, that are effective in a diverse array of processes, including the Staudinger synthesis of beta-lactams, the acylation of silyl ketene acetals, and the kinetic resolution of amines.

Pd/P(t-Bu)(3): a mild and general catalyst for Stille reactions of aryl chlorides and aryl bromides

Pd/P(t-Bu)(3) serves as an unusually reactive catalyst for Stille reactions of aryl chlorides and bromides, providing solutions to a number of long-standing challenges. An unprecedented array of aryl chlorides can be cross-coupled with a range of organotin reagents, including SnBu(4). Very hindered biaryls (e.g., tetra-ortho-substituted) can be synthesized, and aryl chlorides can be coupled in the presence of aryl triflates. The method is user-friendly, since a commercially available complex, Pd(P(t-Bu)(3))(2), is effective. Pd/P(t-Bu)(3) also functions as an active catalyst for Stille reactions of aryl bromides, furnishing the first general method for room-temperature cross-couplings.

The first general method for palladium-catalyzed Negishi cross-coupling of aryl and vinyl chlorides: use of commercially available Pd(P(t-Bu)(3))(2) as a catalyst

With a single protocol, commercially available Pd(P(t-Bu)(3))(2) can effect the Negishi cross-coupling of a wide range of aryl and vinyl chlorides with aryl- and alkylzinc reagents. The process tolerates nitro groups, and it efficiently generates sterically hindered biaryls. In addition, a high turnover number (>3000) can be achieved.

Enantioselective nucleophilic catalysis with "Planar-Chiral" heterocycles

Although Lewis bases (e.g., tertiary phosphines, tertiary amines, and pyridines) serve as nucleophilic catalysts for a wide array of reactions, there have been relatively few reports of enantioselective nucleophilic catalysts. In this Account, we describe the design and synthesis of a new family of chiral nucleophilic catalysts, specifically, planar-chiral heterocycles. These complexes provide good levels of enantiomeric excess in the addition of alcohols to ketenes, the rearrangement of O-acylated azlactones, and the kinetic resolution of secondary alcohols.

Nickel-catalyzed Negishi arylations of propargylic bromides: a mechanistic investigation

Although nickel-catalyzed stereoconvergent couplings of racemic alkyl electrophiles are emerging as a powerful tool in organic chemistry, to date there have been no systematic mechanistic studies of such processes. Herein, we examine the pathway for enantioselective Negishi arylations of secondary propargylic bromides, and we provide evidence for an unanticipated radical chain pathway wherein oxidative addition of the C–Br bond occurs through a bimetallic mechanism. In particular, we have crystallographically characterized a diamagnetic arylnickel(II) complex, [(i-Pr-pybox)Ni^IIPh]BAr^F₄, and furnished support for [(i-Pr-pybox)Ni^IIPh]⁺ being the predominant nickel-containing species formed under the catalyzed conditions as well as a key player in the cross-coupling mechanism. On the other hand, our observations do not require a role for an organonickel(I) intermediate (e.g., (i-Pr-pybox)Ni^IPh), which has previously been suggested to be an intermediate in nickel-catalyzed cross-couplings, oxidatively adding alkyl electrophiles through a monometallic pathway.

Catalytic Asymmetric Synthesis of Piperidine Derivatives through the [4 + 2] Annulation of Imines with Allenes

Although tertiary phosphines have emerged as remarkably versatile nucleophilic catalysts, there has been only very limited progress in achieving asymmetric catalysis with chiral phosphines. In this report, the first highly enantioselective variant of the Kwon annulation of imines with allenes is described. Thus, C2-symmetric chiral phosphepine 1 serves as an effective catalyst for this powerful process, furnishing an array of functionalized piperidine derivatives with very good stereoselectivity.

The first applications of carbene ligands in cross-couplings of alkyl electrophiles: sonogashira reactions of unactivated alkyl bromides and iodides

A Pd/N-heterocyclic carbene-based catalyst achieves the Sonogashira coupling of an array of functionalized, beta-hydrogen-containing alkyl bromides and iodides under mild conditions. By furnishing the first example of a nonphosphine-based palladium catalyst for cross-coupling unactivated alkyl electrophiles, this study provides an impetus for future efforts at catalyst development that extend beyond phosphine ligands.