Copper-catalyzed C-P bond construction via direct coupling of phenylboronic acids with H-phosphonate diesters

Mechanistic Insights into the Effect of Sodium Iodide on Copper-Mediated Iododeboronation

The copper-catalyzed Chan-Evans-Lam (CEL) coupling reaction advances carbon-heteroatom cross-coupling and has facilitated the development of radiohalogenation methodologies in radiochemistry. This study investigated the mechanisms and side reactions of CEL iodination under conditions relevant to radiosynthesis and typical organic synthesis, focusing on the effects of sodium iodide. The concentrations of copper and iodide, as well as the copper-to-iodide ratio, were identified as significant factors for successful copper-mediated CEL iodination, influencing the reaction mechanisms and side reactions. Excess iodide relative to the copper salt led to the formation of poorly soluble iodinated copper(I) complexes that competed with that of the desired aryl iodide. Additionally, the predominant copper complex involved in the catalytic cycle differed between the early and late stages of the reaction, depending on the copper-to-iodide ratio. The results of this study indicate that the specialized radiosynthesis conditions meet the requirements for efficient CEL iodination. In particular, an extremely low concentration of iodide is optimal for CEL iodination. These in-depth mechanistic insights not only provide a detailed comparison of CEL iodination across radiochemistry and synthetic organic chemistry but can also inspire the development of novel (radio)iodination methods.

Electrochemical Generation of Aryl Radicals from Organoboron Reagents Enabled by Pulsed Electrosynthesis

Aryl radicals play a pivotal role as reactive intermediates in chemical synthesis, commonly arising from aryl halides and aryl diazo compounds. Expanding the repertoire of sources for aryl radical generation to include abundant and stable organoboron reagents would significantly advance radical chemistry and broaden their reactivity profile. While traditional approaches utilize stoichiometric oxidants or photocatalysis to generate aryl radicals from these reagents, electrochemical conditions have been largely underexplored. Through rigorous mechanistic investigations, we identified fundamental challenges hindering aryl radical generation. In addition to the high oxidation potentials of aromatic organoboron compounds, electrode passivation through radical grafting, homocoupling of aryl radicals, and decomposition issues were identified. We demonstrate that pulsed electrosynthesis enables selective and efficient aryl radical generation by mitigating the fundamental challenges. Our discoveries facilitated the development of the first electrochemical conversion of aryl potassium trifluoroborate salts into aryl C-P bonds. This sustainable and straightforward oxidative electrochemical approach exhibited a broad substrate scope, accommodating various heterocycles and aryl chlorides, typical substrates in transition-metal catalyzed cross-coupling reactions. Furthermore, we extended this methodology to form aryl C-Se, C-Te, and C-S bonds, showcasing its versatility and potential in bond formation processes.

Direct Synthesis of Tertiary Phosphines via Alkoxide-Mediated Deborylative Phosphination of Organoboronates

The direct transformation of alkylboron has emerged as a versatile and powerful methodology for creating carbon-carbon and carbon-heteroatom bonds. However, its potential application in the formation of carbon and phosphorus remains unexplored. In this study, we present an alkoxide base-promoted reaction system that enables deborylative phosphination of benzylic organoboronates and geminal bis(boronates) via selective C-B bond cleavage. This approach allows for the synthesis of valuable tertiary phosphines in good yields under mild conditions. The practicality and industrial potential of this approach are underscored by the operational simplicity, broad substrate scope, and easy scalability.

Heteropolyacid-Catalyzed Phosphorylation of Secondary Aromatic Alcohols with H-Phosphine Oxides in DMC: A Simple Protocol for C-P Bond Formation

We successfully achieved the phosphorylation of secondary aromatic alcohols with H-phosphine oxides (less developed system) using phosphotungstic acid as a catalyst in dimethyl carbonate. The system was simple and environmentally friendly and showed better activity than traditional Lewis or Brønsted acids such as FeCl3, p-TsOH·H2O, etc., generating up to a 97% isolated yield. Control experiments indicated that the reaction did not occur through the radical pathway, and ethers and carbocation were the key intermediates in the pathway.

Copper(II)-catalyzed cascade Csp2-P/C-C bond formation to construct benzo[d]thiazol-2-ylphosphonates

A novel, copper(II)-catalyzed cascade Csp2-P/C-C bond formation in o-haloaryl isothiocyanates with organophosphorus esters has been developed under mild conditions. A series of benzo[d]thiazol-2-ylphosphonates were synthesized in moderate to good yields. Different from the traditional method of obtaining these scaffolds with radical reactions, the method proposed allows accessing them via ionic reactions and has the advantages of easy access to raw materials and simple operation. Finally, we carried out a gram-scale experiment to further demonstrate the scalability of this strategy in the efficient synthesis of benzo[d]thiazol-2-ylphosphonates.

Tf2O/DMSO-mediated dual activation of aryl phosphinate to access various aryl phosphonates

A metal-free method for the dual activation of aryl phosphinate has been developed; the P-H and P-O bonds are sequentially activated by the Tf2O/DMSO system. Without the requirement of metals and unstable P-reagents, this one-pot procedure provides a convenient and practical access to a variety of aryl phosphonates. A mechanism involving twice generation of electrophilic P-species and two SN-processes is proposed on the basis of the control experiments.

Microwave-Assisted Palladium Acetate-Catalyzed C-P Cross-Coupling of Arylboronic Acids and >P(O)H Reagents in the Absence of the Usual Mono- and Bidentate P-Ligands: Mechanistic Insights

A less-studied halogen-free variation of the Hirao reaction involving the coupling of arylboronic acids with >P(O)H reagents, such as diarylphosphine oxides, diethyl phosphite, and ethyl phenyl-H-phosphinate, was investigated in detail using Pd(OAc)2 as the catalyst precursor and applying some excess of the P-reagent to supply the ligand via its trivalent tautomeric (>P-OH) form. The optimum conditions (1.2 equiv of the P-reagent, 135-150 °C, and air) were explored for the synthesis of diaryl-phenylphosphine oxides, aryl-diphenylphosphine oxides, diethyl arylphosphonates, ethyl diphenylphosphinate, and two bisphosphinoyl derivatives. In the reaction of 4-chlorophenyl- or 3-chlorophenylboronic acid with Ph2P(O)H, triphenylphosphine oxide was also formed as a byproduct. Theoretical calculations suggested that the catalytic cycle of the P-C coupling of PhB(OH)2 with Ph2P(O)H is different from that of the usual cross-coupling reactions. It comprises the addition of a phenyl anion and then the tautomeric form >P-OH of the >P(O)H reagent to the Pd2+ catalyst complex. This is then followed by reductive elimination affording Ph3PO that is accompanied with the conversion of Pd2+ to Pd0. There is a need for a subsequent stoichiometric oxidation of Pd(0) by molecular oxygen. The spontaneous formation of the self-assembling ligands around the Pd2+ center from the >P(O)H reactant plays a crucial role in the mechanism and promotes the efficiency of the catalyst.

Site-selective and metal-free C-H phosphonation of arenes via photoactivation of thianthrenium salts

Aryl phosphonates are prevalent moieties in medicinal chemistry and agrochemicals. Their chemical synthesis normally relies on the use of precious metals, harsh conditions or aryl halides as substrates. Herein, we describe a sustainable light-promoted and site-selective C-H phosphonation of arenes via thianthrenation and the formation of an electron donor-acceptor complex (EDA) as key steps. The method tolerates a wide range of functional groups including biomolecules. The use of sunlight also promotes this transformation and our mechanistic investigations support a radical chain mechanism.

Thianthrenium-Enabled Phosphorylation of Aryl C-H Bonds via Electron Donor-Acceptor Complex Photoactivation

An efficient strategy for the preparation of aryl phosphonates via blue-light-promoted single electron transfer process of an EDA complex between phosphites and thianthrenium salts has been demonstrated. The corresponding substituted aryl phosphonates were obtained in good to excellent yields, and the byproduct thianthrene can be recovered and reused in quantity. This developed method realizes the construction of aryl phosphonates through the indirect C-H functionalization of arenes, which has potential application value in drug discovery and development.

Tandem addition of nucleophilic and electrophilic reagents to vinyl phosphinates: the stereoselective formation of organophosphorus compounds with congested tertiary carbons

Carbon anions formed via the addition of Grignard reagents to SP-vinyl phosphinates were modified with electrophilic reagents to afford organophosphorus compounds with diverse carbon skeletons. The electrophiles included acids, aldehydes, epoxy groups, chalcogens and alkyl halides. When alkyl halides were used, bis-alkylated products were afforded. Substitution reactions or polymerization occurred when the reaction was applied to vinyl phosphine oxides.

Ni-catalyzed C-F activation to construct C-P bond with P-P(O) and P(O)OR mediation

Here we developed an efficient Ni-catalyzed C-F bond phosphorylation of aryl fluorides via the crucial intermediates of P-P(O) and P(O)OR. P-P(O) mediated organophosphorus generation is observed for active aryl fluorides, whereas inactive aryl fluorides can also be activated and phosphorylated via a P(O)OR-mediated pathway, which is barely reported yet. Facile scale-up to the gram level and the upgrading of the bioactive molecule make this protocol to have promising applications in synthetic chemistry.

Recent advances in electrochemical C-H phosphorylation

The activation of C-H bond, and its direct one-step functionalization, is one of the key synthetic methodologies that provides direct access to a variety of practically significant compounds. Particular attention is focused on modifications obtained at the final stages of the synthesis of complicated molecules, which requires high tolerance to the presence of existing functional groups. Phosphorus is an indispensable element of life, and phosphorus chemistry is now experiencing a renaissance due to new emerging applications in medicinal chemistry, materials chemistry (polymers, flame retardants, organic electronics, and photonics), agricultural chemistry (herbicides, insecticides), catalysis (ligands) and other important areas of science and technology. In this regard, the search for new, more selective, low-waste synthetic routes become relevant. In this context, electrosynthesis has proven to be an eco-efficient and convenient approach in many respects, where the reagents are replaced by electrodes, where the reactants are replaced by electrodes, and the applied potential the applied potential determines their "oxidizing or reducing ability". An electrochemical approach to such processes is being developed rapidly and demonstrates some advantages over traditional classical methods of C-H phosphorylation. The main reasons for success are the exclusion of excess reagents from the reaction system: such as oxidants, reducing agents, and sometimes metal and/or other improvers, which challenge isolation, increase the wastes and reduce the yield due to frequent incompatibility with these functional groups. Ideal conditions include electron as a reactant (regulated by applied potential) and the by-products as hydrogen or hydrocarbon. The review summarizes and analyzes the achievements of electrochemical methods for the preparation of various phosphorus derivatives with carbon-phosphorus bonds, and collects data on the redox properties of the most commonly used phosphorus precursors. Electrochemically induced reactions both with and without catalyst metals, where competitive oxidation of precursors leads to either the activation of C-H bond or to the generation of phosphorus-centered radicals (radical cations) or metal high oxidation states will be examined. The review focuses on publications from the past 5 years.

Solvent Anions Enable Photoinduced Borylation and Phosphonation of Aryl Halides via EDA Complexes

We report the synthesis of aryl boronic esters and aryl phosphonate esters promoted by visible-light in the absence of transition-metals or photoredox catalysts. The transformation proceeds at room temperature using sodium hydride, as a non-nucleophilic base, and exhibits functional group tolerance for anilines, amides, and esters. UV-vis spectroscopy, radical trapping experiments, and computational (TD-DFT) calculations suggest an electron-donor-acceptor (EDA) complex between solvent anions and aryl halides as the species responsible for this reactivity.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

Transition-Metal-Free Photoredox Phosphonation of Aryl C-N and C-X Bonds in Aqueous Solvent Mixtures

Herein, we present an efficient and mild methodology for the synthesis of aromatic phosphonate esters in good to excellent yields using 10H-phenothiazine, an inexpensive commodity chemical, as a photoredox catalyst. The reaction exhibits wide functional group compatibility enabling the transformation in the presence of ketone, amide, ester, amine, and alcohol moieties. Importantly, the reaction proceeds using a green solvent mixture primarily composed of water, thus lowering the environmental footprint of this transformation compared to current methods. The transformation also proceeds under atmospheric conditions, which further differentiates it from current methods that require inert atmosphere. Mechanistic work using fluorescence quenching experiments and radical trapping approaches support the proposed mechanism.

I2-mediated Csp2–P bond formation via tandem cyclization of o-alkynylphenyl isothiocyanates with organophosphorus esters

A highly efficient molecular-iodine-catalyzed cascade cyclization reaction has been developed, creating a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to excellent yields. This approach benefits from metal-free catalysts and available raw materials.

Silver(i)-catalyzed dehydrogenative cross-coupling of 2-aroylbenzofurans with phosphites

The silver(i)-catalyzed dehydrogenative cross-coupling reaction of 2-aroylbenzofurans with phosphites to afford 2-aroyl-3-phosphonylbenzofurans is reported.

Palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage

Herein we report a novel palladium-catalyzed phosphorylation of arylsulfonium salts with P(O)H compounds via C–S bond cleavage under mild conditions.

MW-Promoted Cu(I)-Catalyzed P–C Coupling Reactions without the Addition of Conventional Ligands; an Experimental and a Theoretical Study

An experimental and a theoretical study on the so far less investigated Cu(I) salt-catalyzed Hirao reaction of iodobenzene and diarylphosphine oxides (DAPOs) revealed that Cu(I)Br or Cu(I)Cl is the most efficient catalyst under microwave irradiation. The optimum conditions included 165 °C and a 1:2 molar ratio for DAPOs and triethylamine. The possible ligations of Cu(I) were studied in detail. Bisligated P---Cu(I)---P (A), P---Cu(I)---N (B) and N---Cu(I)---N (C) complexes were considered as the catalysts. Calculations on the mechanism suggested that complexes A and B may catalyze the P–C coupling, but the latter one is more advantageous both according to experiments and calculations pointing out the Cu(I) → Cu(III) conversion in the oxidative addition step. The P–C coupling cannot take place with PhBr, as in this case, the catalyst complex cannot be regenerated.

Palladium-Catalyzed C-P(III) Bond Formation by Coupling ArBr/ArOTf with Acylphosphines

Palladium-catalyzed C-P bond formation reaction of ArBr/ArOTf using acylphosphines as differential phosphination reagents is reported. The acylphosphines show practicable reactivity with ArBr and ArOTf as the phosphination reagents, though they are inert to the air and moisture. The reaction affords trivalent phosphines directly in good yields with a broad substrate scope and functional group tolerance. This reaction discloses the acylphosphines' capability as new phosphorus sources for the direct synthesis of trivalent phosphines.

The Synthesis of Biarylmonophosphonates via Palladium-Catalyzed Phosphonation, Iridium-Catalyzed C-H Borylation, Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling

Abstract

The iridium-catalyzed C-H borylation of diethyl phenylphosphonate results in nonselective mono and bisborylation to afford a near statistical mixture of 3-, 3,5- and 4-boryl substituted aryl phosphonates whereas 3-substituted aryl phosphonates undergo highly regioselective C-H borylation to afford the corresponding meta-phosphonate substituted arylboronic esters as the sole product; the resulting boronic esters were used as nucleophilic reagents in a subsequent palladium-catalyzed Suzuki–Miyaura cross-coupling to generate a range of biarylmonophosphonates. Gratifyingly, the Suzuki–Miyaura cross-coupling can be conducted without purifying the boronic ester which greatly simplifies the synthetic procedure.

Graphical Abstract

Advances in Carbon–Element Bond Construction under Chan–Lam Cross-Coupling Conditions: A Second Decade

Copper-mediated carbon–heteroatom bond-forming reactions involving a wide range of substrates have been in the spotlight for many organic chemists. This review highlights developments between 2010 and 2019 in both stoichiometric and catalytic copper-mediated reactions, and also examples of nickel-mediated reactions, under modified Chan–Lam cross-coupling conditions using various nucleophiles; examples include chemo- and regioselective N-arylations or O-arylations. The utilization of various nucleophiles as coupling partners together with reaction optimization (including the choice of copper source, ligands, base, and other additives), limitations, scope, and mechanisms are examined; these have benefitted the development of efficient and milder methods. The synthesis of medicinally valuable or pharmaceutically important nitrogen heterocycles, including isotope-labeled compounds, is also included. Chan–Lam coupling reaction can now form twelve different C–element bonds, making it one of the most diverse and mild reactions known in organic chemistry.

1 Introduction

2 Construction of C–N and C–O Bonds

2.1 C–N Bond Formation

2.1.1 Original Discovery via Stoichiometric Copper-Mediated C–N Bond Formation

2.1.2 Copper-Catalyzed C–N Bond Formation

2.1.3 Coupling with Azides, Sulfoximines, and Sulfonediimines as Nitrogen­ Nucleophiles

2.1.4 Coupling with N,N-Dialkylhydroxylamines

2.1.5 Enolate Coupling with sp3-Carbon Nucleophiles

2.1.6 Nickel-Catalyzed Chan–Lam Coupling

2.1.7 Coupling with Amino Acids

2.1.8 Coupling with Alkylboron Reagents

2.1.9 Coupling with Electron-Deficient Heteroarylamines

2.1.10 Selective C–N Bond Formation for the Synthesis of Heterocycle-Containing Compounds

2.1.11 Using Sulfonato-imino Copper(II) Complexes

2.2 C–O Bond Formation

2.2.1 Coupling with (Hetero)arylboron Reagents

2.2.2 Coupling with Alkyl- and Alkenylboron Reagents

3 C–Element (Element = S, P, C, F, Cl, Br, I, Se, Te, At) Bond Forma tion under Modified Chan–Lam Conditions

4 Conclusions

Shining Light on the Light-Bearing Element: A Brief Review of Photomediated C–H Phosphorylation Reactions

Organophosphorus compounds have numerous useful applications, from versatile ligands and nucleophiles in the case of trivalent organophosphorus species to therapeutics, agrochemicals and material additives for pentavalent species. Although phosphorus chemistry is a fairly mature field, the construction of C–P(V) bonds relies heavily on either prefunctionalized substrates such as alkyl or aryl halides, or requires previously oxidized bonds such as C=N or C=O, leading to potential sustainability issues when looking at the overall synthetic route. In light of the recent advances in photochemistry, using photons as a reagent can provide better alternatives for phosphorylations by unlocking radical mechanisms and providing interesting redox pathways. This review will showcase the different photomediated phosphorylation procedures available for converting C–H bonds into C–P(V) bonds.

1 Introduction

1.1 Organophosphorus Compounds

1.2 Phosphorylation: Construction of C–P(V) Bonds

1.3 Photochemistry as an Alternative to Classical Phosphorylations

2 Ionic Mechanisms Involving Nucleophilic Additions

3 Mechanisms Involving Radical Intermediates

3.1 Mechanisms Involving Reactive Carbon Radicals

3.2 Mechanisms Involving Phosphorus Radicals

3.2.1 Photoredox: Direct Creation of Phosphorus Radicals

3.2.2 Photoredox: Indirect Creation of Phosphorus Radicals

3.2.3 Dual Catalysis

3.3 Photolytic Cleavage

4 Conclusion and Outlook

Radical Arylation of Triphenyl Phosphite Catalyzed by Salicylic Acid: Mechanistic Investigations and Synthetic Applications

A straightforward and scalable methodology to synthesize diphenyl arylphosphonates at 20 °C within 1-2 h is reported using inexpensive SA as the catalytic promoter of the reaction. Mechanistic investigations suggest that the reaction proceeds via radical-radical coupling, consistent with the so-called persistent radical effect. The reaction tolerated a wide range of functional groups and heteroaromatic moieties. The synthetic usefulness and the unique reactivity of the obtained phosphonates were demonstrated in different one-step transformations.

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Arylphosphonate-Directed Ortho C-H Borylation: Rapid Entry into Highly-Substituted Phosphoarenes

Phosphonate-directed ortho C-H borylation of aromatic phosphonates is reported. Using simple starting materials and commercially accessible catalysts, this method provides steady access to o-phosphonate arylboronic esters bearing pendant functionality and flexible substitution patterns. These products serve as flexible precursors for a variety of highly substituted phosphoarenes, and in situ downstream functionalization of the products is described.

Phosphine Oxides from a Medicinal Chemist's Perspective: Physicochemical and in Vitro Parameters Relevant for Drug Discovery

Phosphine oxides and related phosphorus-containing functional groups such as phosphonates and phosphinates are established structural motifs that are still underrepresented in today's drug discovery projects, and only few examples can be found among approved drugs. In this account, the physicochemical and in vitro properties of phosphine oxides and related phosphorus-containing functional groups are reported and compared to more commonly used structural motifs in drug discovery. Furthermore, the impact on the physicochemical properties of a real drug scaffold is exemplified by a series of phosphorus-containing analogs of imatinib. We demonstrate that phosphine oxides are highly polar functional groups leading to high solubility and metabolic stability but occasionally at the cost of reduced permeability. We conclude that phosphine oxides and related phosphorus-containing functional groups are valuable polar structural elements and that they deserve to be considered as a routine part of every medicinal chemist's toolbox.

Recent developments in photoredox-catalyzed remote ortho and para C-H bond functionalizations

In recent years, the research area of direct C-H bond functionalizations was growing exponentially not only due to the ubiquity of inert C-H bonds in diverse organic compounds, including bioactive natural and nonnatural products, but also due to its impact on the discovery of pharmaceutical candidates and the total synthesis of intricate natural products. On the other hand, more recently, the field of photoredox catalysis has become an indispensable and unparalleled research topic in modern synthetic organic chemistry for the constructions of challenging bonds, having the foremost scope in academia, pharmacy, and industry. Therefore, the development of green, simpler, and effective methodologies to accomplish direct C-H bond functionalization is well overdue and highly desirable to the scientific community. In this review, we mainly highlight the impact on, and the utility of, photoredox catalysts in inert ortho and para C-H bond functionalizations. Although a surge of research papers, including reviews, demonstrating C-H functionalizations have been published in this vital area of research, to our best knowledge, this is the first review that focuses on ortho and para C-H functionalizations by photoredox catalysis to provide atom- and step-economic organic transformations. We are certain that this review will act as a promoter to highlight the application of photoredox catalysts for the functionalization of inert bonds in the domain of synthetic organic chemistry.

Cobalt Catalyzed C-P Bond Formation by Cross-Coupling of Boronic Acids with P(O)H Compounds in Presence of Zinc

In our current work, we have reported the first cobalt-catalyzed cross-coupling of arylboronic acid with alkyl/aryl phosphites under mild conditions. The reaction was carried out in the presence of zinc powder as an additive and ter-pyridine as a ligand. The use of non-precious cobalt salt makes the protocol advantageous, as it is inexpensive and more abundant than the previously used methods where precious metal salts (Pd and Pt) were used. The reaction has a wide substrate scope and the products were obtained in good yields.