Palladium-catalyzed R2(O)P directed C(sp2)-H acetoxylation

Tandem SN2 Nucleophilic Substitution/Phospho-Dieckmann Reaction: One-Step Synthesis of 2-Phosphonyl-3-hydroxybenzo[b]thiophenes

A novel and efficient tandem SN2 nucleophilic substitution/Dieckmann condensation reaction of α-iodomethyl phosphine oxide with methyl thiosalicylate derivatives has been developed by using NaOH as a base, which enables the expeditious synthesis of 2-phosphonyl-3-hydroxybenzo[b]thiophene derivatives in moderate to high yields under simple conditions. This research provides not only a convenient method for the functionalization of benzo[b]thiophenes at the 2-position and 3-position but also new organophosphorus molecules. Furthermore, several new phosphonyl-substituted benzo[b]thiophenes were obtained from the resultant 2-phosphonyl-3-hydroxybenzo[b]thiophenes.

Ruthenium(II)-Catalyzed Sterically Hindered C-H Acyloxylation to Synthesize Biaryl Isoquinoline Derivatives via Peresters

A novel C-H acyloxylation method of 1-(1-naphthalen-1-yl)isoquinoline derivatives with peresters in the presence of [Ru(p-cymene)Cl₂]₂ has been developed. The combination of ruthenium(II), AgBF₄, CoI₂, and 2,2,6,6-tetramethyl-1-piperidinyloxy is found to be an effective catalytic system to provide various biaryl compounds in satisfactory yields within minutes. Notably, steric hindrance is a very important determinant of the reaction.

Native carboxyl group-assisted C-H acetoxylation of hydrocinnamic and phenylacetic acids

The use of a native directing group to promote C-H activation is highly desirable. Herein, we report a method of native carboxyl-assisted, Pd(II)-catalyzed ortho-C-H acetoxylation of both hydrocinnamic and phenylacetic acids that can be found in many biologically active molecules as the key moieties. Based on the broad scope and the application potential showcased with drug molecules, it is anticipated that this C-H acetoxylation reaction will find attractive applicability in future synthetic endeavors.

The cobalt(ii)-catalyzed acyloxylation of picolinamides with bifunctional silver carboxylate via C–H bond activation

The cobalt(ii)-catalyzed C–H bond acyloxylation of picolinamides with bifunctional silver carboxylate has been developed. The mild and practical esterification provides an atom-economic route to access to polysubstituted naphthalene compounds.

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.

Synthesis of Indole-Fused Six-, Seven-, or Eight-Membered N,O-Heterocycles via Rhodium-Catalyzed NH-Indole-Directed C-H Acetoxylation/Hydrolysis/Annulation

We report herein the facile synthesis of indole-fused six-, seven-, or eight-membered N,O-heterocycles through rhodium-catalyzed C-H acetoxylation/hydrolysis/annulation. The notable features of this method include C-H acetoxylation using NH-indole as the intrinsic directing group, high functional group compatibility, and construction of indole-fused medium-sized rings.

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

Ir(III)-Catalyzed C-H Functionalization of Triphenylphosphine Oxide toward 3-Aryl Oxindoles

With triphenylphosphine oxide serving as both the directing group and the reagent, we have developed a Cp*Ir(III)-catalyzed direct C-H functionalization of triphenylphosphine oxide with 3-diazooxindoles to afford a range of 3-(2-(diphenylphosphoryl)phenyl)indolin-2-one derivatives in moderate to excellent yields. The title products are potentially important building blocks for organic synthesis through various chemical transformations. This protocol is simple and efficient and offers high atom economy with only N₂ as the byproduct.

Fast and Chemoselective Addition of Highly Polarized Lithium Phosphides Generated in Deep Eutectic Solvents to Aldehydes and Epoxides

Highly polarized lithium phosphides (LiPR₂ ) were synthesized, for the first time, in deep eutectic solvents as sustainable reaction media, at room temperature and in the absence of protecting atmosphere, through direct deprotonation of both aliphatic and aromatic secondary phosphines (HPR₂ ) by n-BuLi. The subsequent addition of in-situ generated LiPR₂ to aldehydes or epoxides proceeded quickly and chemoselectively, thereby allowing the straightforward access to the corresponding α- or β-hydroxy phosphine oxides, respectively, under air and at room temperature (bench conditions), which are traditionally considered as textbook-prohibited conditions in the field of polar organometallic chemistry of s-block elements.

Nickela-electrocatalyzed C-H Alkoxylation with Secondary Alcohols: Oxidation-Induced Reductive Elimination at Nickel(III)

Nickela-electrooxidative C-H alkoxylations with challenging secondary alcohols were accomplished in a fully dehydrogenative fashion, thereby avoiding stoichiometric chemical oxidants, with H2 as the only stoichiometric byproduct. The nickela-electrocatalyzed oxygenation proved viable with various (hetero)arenes, including naturally occurring secondary alcohols, without racemization. Detailed mechanistic investigation, including DFT calculations and cyclovoltammetric studies of a well-defined C-H activated nickel(III) intermediate, suggest an oxidation-induced reductive elimination at nickel(III).

A dual role for acetohydrazide in Pd-catalyzed controlled C(sp3)–H acetoxylation of aldehydes

The palladium catalyzed aldehyde directed acetoxylation of C(sp³)–H bonds was realized by a transient directing group approach for the first time.

New Developments on the Hirao Reactions, Especially from "Green" Point of View

BACKGROUND

The Hirao reaction discovered ca. 35 years ago is an important P-C coupling protocol between dialkyl phosphites and aryl halides in the presence of Pd(PPh3)4 as the catalyst and a base to provide aryl phosphonates. Then, the reaction was extended to other Preagents, such as secondary phosphine oxides and H-phosphinates and to other aryl and hetaryl derivatives to afford also phosphinic esters and tertiary phosphine oxides. Instead of the Pd(PPh3)4 catalyst, Pd(OAc)2 and Ni-salts were also applied as catalyst precursors together with a number of mono- and bidentate P-ligands.

OBJECTIVE

In our review, we undertook to summarize the target reaction with a special stress on the developments attained in the last 6 years, hence this paper is an update of our earlier reviews in a similar topic.

CONCLUSIONS

"Greener" syntheses aimed at utilizing phase transfer catalytic and microwave-assisted approaches, even under "P-ligand-free. or even solvent-free conditions are the up-to date versions of the classical Hirao reaction. The mechanism of the reaction is also in the focus these days.

Highly Selective Palladium-Catalyzed Arene C-H Acyloxylation with Benzothiadiazole as a Modifiable Directing Group

An efficient protocol for the palladium-catalyzed direct arene C-H acyloxylation of benzothiadiazole-arene derivatives is reported for the first time. The key strategy is the employment of benzothiadiazole as a modifiable directing group. The highly selective mono- or bis-acyloxylation can be achieved by tuning the reaction conditions, affording various acyloxylated benzothiadiazole derivatives, which could offer a rational tailoring of their electronic properties and be applied in organic electronic and optoelectronic materials. Finally, by diversity-oriented modification of the benzothiadiazole directing group, the acyloxylated products can be readily converted into various valuable functionalized (hetero)biaryls.

A versatile rhodium(iii) catalyst for direct acyloxylation of aryl and alkenyl C–H bonds with carboxylic acids

Rh(iii)-Catalyzed highly regioselective direct acyloxylation of aryl and alkenyl sp² C–H bonds with various carboxylic acids has been developed.

Late stage modifications of P-containing ligands using transition-metal-catalysed C–H bond functionalisation

Recent advances in C–H bond functionalisation of phosphorus derivatives are presented, with a special attention to preparation of phosphine ligands.

New Approaches for Biaryl-Based Phosphine Ligand Synthesis via P═O Directed C-H Functionalizations

Given the important influence of phosphine ligands in transition metal-catalyzed reactions, chemists have searched for straightforward and efficient methodologies for the synthesis of diverse phosphine ligands. Although significant progress has been made in this aspect over the past decades, the development of new phosphorus-containing ligands with properties superior to their predecessors remains a central task for chemists. Recently, researchers have demonstrated that biphenyl monophosphine ligands function as highly efficient ligands for transition-metal-catalyzed organic transformations, especially for reactions where chelating bisphosphine ligands cannot be used. In 1998, Buchwald introduced a new class of air-stable phosphine ligands based on the dialkylbiaryl phosphine backbone. These ligands have been successfully used for a wide variety of palladium-catalyzed carbon-carbon, carbon-nitrogen, and carbon-oxygen construction processes as well as serving as supporting ligands for a number of other reactions. At the same time, the use of the biphenyl monophosphine ligands often allows reactions to proceed with short reaction times and low catalyst loadings and under mild reaction conditions. However, the synthesis of chiral biphenyl monophosphine ligands, especially those the chirality of which is due to biaryl axial chirality, is very limited. In this Account, we summarize our methodologies for the synthesis of this kind of biphenyl monophosphine ligands including the P═O directed C-H functionalization, P═O directed diastereoselective C-H functionalization, P═O directed enantioselective C-H functionalization, and metal-free diastereoselective radical oxidative C-H amination under mild reaction conditions. With these methods, a series of biphenyl phosphine ligand precursors containing achiral or axially chiral centers and precursors possessing both axial chirality and a chirogenic phosphorus center with different electronic properties and steric effect have been obtained under different reaction conditions. For the preparation of chiral biphenyl monophosphine ligands, which not only possess axial chirality but in many cases also possess chirality at phosphorus, the primary means of introducing chirality is through the use of the menthyl phenylphosphinate. As a chiral auxiliary group, the menthyl phenylphosphinate has some unique features: (i) it is easy to prepare; (ii) the products contain both axial chirality and central chirality on the phosphorus atom; (iii) the menthyl group could easily be transformed into other functional groups, which is crucial for the diversity of the corresponding biphenyl ligands. In our reaction, the P═O group not only acts as the directing group but also facilitates the construction of the phosphine ligands. In addition, the application of these products in asymmetric catalysis has also been studied with good results obtained in some reactions. The further application of these ligands, especially the chiral biphenyl monophosphine ligands in catalysis reactions is underway in our laboratory, and we hope different kinds of reactions will be achieved with these ligands.

Palladium-Catalyzed C(sp2)-H Acetoxylation via Electrochemical Oxidation

Palladium-catalyzed arene C(sp²)-H acetoxylation has emerged as a powerful tool to construct a carbon-oxygen (C-O) bond. However, the requirement of stoichiometric chemical oxidants for this transformation possesses a significant disadvantage. To solve this fundamental problem, we now report an anodic oxidation strategy to achieve arene C(sp²)-H acetoxylation.

A theoretical study on the mechanism of ruthenium(ii)-catalyzed phosphoryl-directed ortho-selective C–H bond activations: the phosphoryl hydroxy group triggered Ru(ii)/Ru(0) catalytic cycle

A theoretical study on the mechanism of ruthenium(ii)-catalyzed phosphoryl-directed ortho-selective C–H bond activations has been reported.

Palladium catalyzed C–H bond acetoxylation: isoxazolinyl as a directing group

Pd(OAc)₂-catalyzed acetoxylations of 3-aryl or 3-alkyl groups mounted on a 2-isoxazoline ring were studied. Substituent effect on reactivity and regioselectivity was examined. The acetoxylation was applied to the synthesis of useful intermediates.

Metal-free regioselective C-3 acetoxylation of N-substituted indoles: crucial impact of nitrogen-substituent

Regioselective C-3 acetoxylation of the N-substituted indoles with PhI(OAc)₂ is described under metal-free conditions. The π-electron-deficient aryl-substituents on the N-atom of indoles remarkably favor the C-3 acetoxylation.

Copper-catalyzed aerobic cascade cycloamination and acyloxylation: a direct approach to 4-acyloxy-1H-pyrazoles

An efficient copper-catalyzed regioselective olefinic C(sp²)–H bond cycloamination and acyloxylation was developed to give acyloxylated pyrazoles under mild conditions, which combines the formation of the pyrazole skeleton and installation of an acyloxyl group in a single step.