Nickel-Catalyzed Formal Homocoupling of Methoxyarenes for the Synthesis of Symmetrical Biaryls via C–O Bond Cleavage

Cobalt-Catalyzed Formation of Grignard Reagents via C-O or C-S Bond Activation

C(aryl)-OMe bond functionalization catalyzed by cobalt(II) chloride in combination with a nacnac-type ligand and magnesium as a reductant is reported. Borylation and benzoylation of aryl methoxides are demonstrated, and C(aryl)-SMe bond borylation can be achieved under similar conditions. This is the first example of achieving these transformations using cobalt catalysis. Mechanistic studies suggest that a Grignard reagent is generated as an intermediate in a rare example of a magnesiation via a C-O bond activation reaction. Indeed, an organomagnesium species could be directly observed by electrospray ionization mass spectroscopic analysis. Kinetic experiments indicate that a heterogeneous cobalt catalyst performs the C-O bond activation.

Nickel-Catalyzed Enantioselective Arylative Activation of Aromatic C-O Bond

The pioneering nickel-catalyzed cross-coupling of C-O electrophiles was unlocked by Wenkert in the 1970s; however, the transition-metal-catalyzed asymmetric activation of aromatic C-O bonds has never been reported. Herein the first enantioselective activation of an aromatic C-O bond is demonstrated via the catalytic arylative ring-opening cross-coupling of diarylfurans. This transformation is facilitated via nickel catalysis in the presence of chiral N-heterocyclic carbene ligands, and chiral 2-aryl-2'-hydroxy-1,1'-binaphthyl (ArOBIN) skeletons are delivered axially in high yields with high ee. Moreover, this versatile skeleton can be transformed into various synthetic useful intermediates, chiral catalysts, and ligands by using the CH- and OH-based modifiable sites. This chemistry features mild conditions and good atom economy.

Biaryl Construction Based on Nickel-Catalyzed C–O Bond Activation

Nickel-catalyzed carbon–oxygen bond activation is one of the most powerful strategies for the direct construction of various biaryl compounds. Under nickel catalysis, efficiently produced and naturally abundant arenol-based electrophiles can be activated and coupled with different aryl nucleophiles, including nucleophiles containing magnesium, zinc, boron, etc., to produce biaryl structural units. This Account summarizes recent progress on biaryl synthesis via nickel-catalyzed C–O bond activation.

1 Introduction

2 Coupling of Arenols and Arenol Derivatives with Aryl Magnesium Reagents

3 Coupling of Arenols and Arenol Derivatives with Aryl Zinc Reagents

4 Coupling of Arenols and Arenol Derivatives with Aryl Boron Reagents

5 Others

6 Conclusion

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel-Catalyzed Direct Decarbonylative Borylation

The Ni-catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B₂ cat₂ has been achieved without recourse to any additives. This Ni-catalyzed method exhibits a broad substrate scope covering poorly reactive non-ortho-substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni⁰ catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B₂ cat₂ as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)-O bond to Ni⁰ catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron-promoted carboxylic acid activation is also applicable to other types of reactions.

Nickel-Catalyzed Homocoupling of Aryl Ethers with Magnesium Anthracene Reductant

Nickel-catalyzed reductive homocoupling of aryl ethers has been achieved with Mg(anthracene)(thf)3 as a readily available low-cost reductant. DFT calculations provided a rationale for the specific efficiency of the diorganomagnesium-type two-electron reducing agent. The calculations show that the dianionic anthracene-9,10-diyl ligand reduces the two aryl ether substrates, resulting in the homocoupling reaction through supply of electrons to the Ni-Mg bimetallic system to form organomagnesium nickel(0)-ate complexes, which cause two sequential C–O bond cleavage reactions. The calculations also showed cooperative actions of Lewis acidic magnesium atoms and electron-rich nickel atoms in the C–O cleavage reactions.

Selective C-O Bond Reduction and Borylation of Aryl Ethers Catalyzed by a Rhodium-Aluminum Heterobimetallic Complex

We report the catalytic reduction of a C-O bond and the borylation by a rhodium complex bearing an X-type PAlP pincer ligand. We have revealed the reaction mechanism based on the characterization of the reaction intermediate and deuterium-labeling experiments. Notably, this novel catalytic system shows steric-hindrance-dependent chemoselectivity that is distinct from conventional Ni-based catalysts and suggests a new strategy for selective C-O bond activation by heterobimetallic catalysis.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

An evaluation of palladium-based catalysts for the base-free borylation of alkenyl carboxylates

Palladium catalysis can achieve the base-free borylation of alkenyl carboxylates, enabling direct access to functionalized enones and heterocycles.

Advances in regioselective functionalization of 9-phenanthrenols

Various approaches are discussed for regioselective functionalization and transformation of 9-phenanthrenol and its ether derivatives, which are readily available from phenanthrene in coal tar.

Nickel(II)-Catalyzed Borylation of Alkenyl Methyl Ethers via C-O Bond Cleavage

A new protocol has been developed for the borylation of conjugated alkenyl methyl ethers using B₂Pin₂ via C-O bond cleavage catalyzed by Ni(II). In this cross-coupling reaction, both E/Z isomers of alkenyl ethers are converted into (E)-alkenyl boronic esters with good reactivity. This transformation exhibits high chemoselectivity in the presence of competitive C-O bonds such as aryl ether, ester, amide, and thioether groups, thus providing a new method for the construction of various alkenyl boronates.

Dual Nickel-/Palladium-Catalyzed Reductive Cross-Coupling Reactions between Two Phenol Derivatives

Cross-coupling between substrates that can be easily derived from phenols is highly attractive due to the abundance of phenols. Here, we report a dual nickel-/palladium-catalyzed reductive cross-coupling between aryl tosylates and aryl triflates; both substrates can be accessed in just one step from readily available phenols. The reaction has a broad functional group tolerance and substrate scope (>60 examples). Furthermore, it displays low sensitivity to steric effects demonstrated by the synthesis of a 2,2'-disubstituted biaryl and a fully substituted aryl product. The widespread presence of phenols in natural products and pharmaceuticals allows for straightforward late-stage functionalization, illustrated with examples such as ezetimibe and tyrosine.

Methoxylation of Acyl Fluorides with Tris(2,4,6-trimethoxyphenyl)phosphine via C-OMe Bond Cleavage under Metal-Free Conditions

Acyl fluorides are subjected to methoxylation with tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) to afford the corresponding methyl esters in good to excellent yields. This transformation is featured by C(sp²)-OMe bond cleavage under metal-free conditions. Unprecedented utilization of TMPP as a methoxylating agent realized the installation of an OMe group into the desired products.

Nickel-Catalyzed Homo- and Cross-Coupling of Allyl Alcohols via Allyl Boronates

A nickel-catalyzed homo- and cross-coupling of allylic alcohols to 1,5-dienes in the presence of B₂pin₂ with excellent regioselectivity has been developed. Mechanistic studies indicate that the reaction proceeds via sequential nickel-catalyzed borylation of allyl alcohols followed by cross-coupling of the resulting allyl boronates with allyl alcohols. The method was effectively applied to nickel-catalyzed allylation of aldehydes using allylic alcohols directly.

Visible Light-Induced Borylation of C-O, C-N, and C-X Bonds

Boronic acids are centrally important functional motifs and synthetic precursors. Visible light-induced borylation may provide access to structurally diverse boronates, but a broadly efficient photocatalytic borylation method that can effect borylation of a wide range of substrates, including strong C-O bonds, remains elusive. Herein, we report a general, metal-free visible light-induced photocatalytic borylation platform that enables borylation of electron-rich derivatives of phenols and anilines, chloroarenes, as well as other haloarenes. The reaction exhibits excellent functional group tolerance, as demonstrated by the borylation of a range of structurally complex substrates. Remarkably, the reaction is catalyzed by phenothiazine, a simple organic photocatalyst with MW < 200 that mediates the previously unachievable visible light-induced single electron reduction of phenol derivatives with reduction potentials as negative as approximately - 3 V versus SCE by a proton-coupled electron transfer mechanism. Mechanistic studies point to the crucial role of the photocatalyst-base interaction.

Sterically controlled C-H/C-H homocoupling of arenes via C-H borylation

A mild one-pot protocol for the synthesis of symmetrical biaryls by sequential Ir-catalyzed C-H borylation and Cu-catalyzed homocoupling of arenes is described. The regiochemistry of the biaryl formed is sterically controlled as dictated by the C-H borylation step. The methodology is also successfully extended to heteroarenes. Some of the products obtained by this approach are impossible to obtain via the Ullmann or the Suzuki coupling protocols. Finally, we have shown a one-pot sequence describing C-H borylation/Cu-catalyzed homocoupling/Pd-catalyzed Suzuki coupling to obtain π-extended arene frameworks.

Ni-Catalyzed Reductive Liebeskind-Srogl Alkylation of Heterocycles

Herein we present a Ni-catalyzed alkylation of C–SMe with alkyl bromides for the decoration of heterocyclic frameworks. The protocol, reminiscent to the Liebeskind–Srogl coupling, makes use of simple C(sp²)–SMe to be engaged in a reductive coupling. The reaction is suitable for a preponderance of highly valuable heterocyclic motifs. In addition to cyclic bromides, noncyclic alkyl bromides are well accommodated with exquisite levels of retention over isomerization. The protocol is scalable and permits orthogonal couplings in the presence of other functionalization handles.

Nickel-Catalyzed Csp2-Csp3 Bond Formation via C-F Bond Activation

A nickel-catalyzed cross coupling of aryl fluorides via C-F bond activation has been developed. The alkylation method allows selective replacement of aryl fluorides by alkyl groups and enables the synthesis of diverse and otherwise difficult to access scaffolds in good yields.

Nickel-catalysed decarbonylative borylation of aroyl fluorides

A transformation of aroyl fluorides with diboron via nickel-catalysed carbon–fluorine bond cleavage and a sequential decarbonylation, which provides an efficient protocol to functionalize arylboronates, has been reported.

Rhodium-Catalyzed Decarbonylative Borylation of Aromatic Thioesters for Facile Diversification of Aromatic Carboxylic Acids

Transformation of aromatic thioesters into arylboronic esters was achieved efficiently using a rhodium catalyst. The broad functional-group tolerance and mild conditions of the method have allowed for the two-step decarboxylative borylation of a wide range of aromatic carboxylic acids, including commercially available drugs.

A new route to π-extended polycyclic aromatic hydrocarbons via cross-dehydrogenative coupling

A rare example of fruitful construction of highly π-extended PAHs expressly via palladium-catalyzed CDC reaction is demonstrated.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.