Diastereoselective Radical Oxidative C–H Aminations toward Chiral Atropoisomeric (P, N) Ligand Precursors

Lewis-acid-catalyzed phosphorylation of alcohols

An efficient method has been developed for reacting dialkyl H-phosphonates or diarylphosphine oxides with alcohols for constructing C-P bonds. This reaction was catalyzed by Lewis acid and involved nucleophilic substitution. A series of diphenylphosphonates and diphenylphosphine oxides were obtained, from the phosphorylation of alcohols, with good-to-excellent yields.

Regioselective C(sp2)-H imidation of arenes by redox neutral visible-light photocatalysis

We report herein a redox neutral visible light-induced regioselective C(sp²)-H imidation of electron-rich arenes and heteroarenes using conceptually designed redox-active 1 as a source of the N-centered imidyl radical. Structurally diverse aromatic imides were obtained in moderate to good yields. This methodology has been successfully employed for the late stage imidation of complex molecules and has also been applied towards the formal total synthesis of the marine natural products carpatamides A, B and D. It has further been shown that the generated imides can easily be converted to the corresponding anilines in situ directly.

Copper(I)-Catalyzed Asymmetric Synthesis of P-Chiral Aminophosphinites

Herein, a copper(I)-catalyzed reaction of diarylphosphines and O-benzoyl hydroxylamines is developed. In the cases of symmetrical diarylphosphines, a series of aminophosphinites is prepared in high yields. In the cases of unsymmetrical diarylphosphines, an array of P-chiral aminophosphinites is synthesized in high yields with high enantioselectivity by using a copper(I)-(R,R_P )-Ph-FOXAP complex as a chiral catalyst. Based on several control experiments and ³¹ P NMR studies, a two-electron redox mechanism involving the dynamic kinetic asymmetric transformation of unsymmetrical diarylphosphines is proposed for the copper(I)-catalyzed asymmetric reaction. Finally, one representative P-chiral phosphoric amide generated through the oxidation with H₂ O₂ is transformed to a chiral diarylphosphinate in high yield with retained enantioselectivity, which allows further transformations towards various P-chiral tertiary phosphines.

Electrochemical Synthesis of Cyclic Diaryl Phosphinamides via Intramolecular sp2 C-H Phosphinamidation

We developed an oxidant- and transition-metal-free approach to construct six-membered cyclic phosphinamides via an intramolecular electrochemical C-H phosphinamidation process. With ⁿBu₄NBr as the catalyst and electrolyte, cyclic phosphinamides bearing a variety of functional groups (22 examples) were readily accessed under mild conditions. Meanwhile, this protocol provided an alternative route to organic electroluminescent materials and P-N ligands.

Synthesis of P-Substituted 5- and 6-Membered Benzo-Phostams: 2,3-Dihydro-1H-1,2-benzazaphosphole 2-Oxides and 2,3-Tetrahydro-1H-1,2-benzazaphosphinine 2-Oxides

Several approaches were developed for the preparation of phosphorus-substituted 5- and 6-membered benzophostams. Carbodiimide-promoted cyclization of zwitterionic aminophosphinates derived from a nitrobenzene precursor accomplished the cyclization in good yields. Alternatively, a novel copper-catalyzed cross-coupling between a phosphonamide and a bromobenzene precursor produced the heterocycles in moderate to good yields. Three different methods are compared for the synthesis of the P-ethoxy-substituted 5-membered benzophostam.

Synthesis of Six-Membered N-Heterocycle Frameworks Based on Intramolecular Metal-Free N-Centered Radical Chemistry

The formation of intramolecular C-N bond represents a powerful strategy in organic transformation as the great importance of N-heterocycles in the fields of natural products and bioactive molecules. This personal account describes the synthesis of cyclic phosphinamidation, benzosultam, benzosulfoximine, phenanthridine and their halogenated compounds through transition-metal-free intramolecular oxidative C-N bond formation. Mechanism study reveals that N-X bond is initially formed under the effect of hypervalent halogen, which is very unstable and easily undergoes thermal or light homolytic cleavage to generate nitrogen radical. Then the nitrogen radical is trapped by the arene to give aryl radical. Rearomatization of aryl radical under the oxidant to provide corresponding N-heterocycle. Under suitable conditions, the N-heterocycles can be further transformed to halogenated N-heterocycles.

Manganese-Catalyzed N-F Bond Activation for Hydroamination and Carboamination of Alkenes

A visible-light-promoted method for generating amidyl radicals from N-fluorosulfonamides via a manganese-catalyzed N-F bond activation strategy is reported. This protocol employs a simple manganese complex, Mn₂(CO)₁₀, as the precatalyst and a cheap silane, (MeO)₃SiH, as both the hydrogen-atom donor and the F-atom acceptor, enabling intramolecular/intermolecular hydroaminations of alkenes, two-component carboamination of alkenes, and even three-component carboamination of alkenes. A wide range of valuable aliphatic sulfonamides can be readily prepared using these practical reactions.

Copper-catalyzed arylation of polycyclic aromatic hydrocarbons by the P[double bond, length as m-dash]O group

The first example of a directed and regioselective arylation of polycyclic aromatic hydrocarbons (PAHs) by using a P[double bond, length as m-dash]O directing group is reported herein. The protocol uses a cheap copper catalyst, and results in a breakthrough meta-selective C-H functionalization of arylphosphine oxide compounds. Substrates with potential fluorescence properties, for example, pyrene and fluoranthene, were successfully arylated under the system, thus achieving an efficient modification of fluorescent molecules containing the P[double bond, length as m-dash]O functional group.

Scalable Synthesis of Esp and Rhodium(II) Carboxylates from Acetylacetone and RhCl3·xH2O

Rhodium(II) carboxylates are privileged catalysts for the most challenging carbene-, nitrene-, and oxo-transfer reactions. In this work, we address the strategic challenges of current organic and inorganic synthesis methods to access these rhodium(II) complexes through an oxidative rearrangement strategy and a reductive ligation reaction. These studies illustrate the multiple benefits of oxidative rearrangement in the process-scale synthesis of congested carboxylates over nitrile anion alkylation reactions, and the impressive effect of inorganic additives in the reductive ligation of rhodium(III) salts.

NIS-mediated oxidative arene C(sp2)-H amidation toward 3,4-dihydro-2(1H)-quinolinone, phenanthridone, and N-fused spirolactam derivatives

A new radical-mediated intramolecular arene C(sp2)-H amidation of 3-phenylpropanamides or [1,1'-biphenyl]-2-carboxamides was developed to prepare a series of 3,4-dihydro-2(1H)-quinolinone and phenanthridone derivatives in moderate to excellent yields (33-94%). Spirolactams could also be obtained using this protocol.

Radical-mediated direct C-H amination of arenes with secondary amines

Direct radical-based substitution of aryl C–H bonds allows one-pot access to valuable polycyclic aryl amines from simple secondary amines.

Aryl dialkyl amines, valuable subunits of a wide range of effect chemicals, are accessed by intramolecular amination of aromatic C–H bonds employing UV photolysis of N-chloroamines. The reactions show good functional group tolerance and allow access to a range of fused and bridged polycyclic structures. The homogeneous reaction conditions allow for the one-pot conversion of secondary amines to their arylated derivatives. Experimental and theoretical evidence supports the involvement of electrophilic aminium radicals which react via direct ortho-attack on the arene.

Oxidation of active sp3C–H bonds initiated consecutive intermolecular/intramolecular cyclization between glycine derivatives ando-vinylphenols: construction of a polycyclic benzofuroquinoline skeleton

The construction of the polycyclic benzofuroquinoline skeleton was realized through a consecutive cyclization.

NIS Mediated Cross-Coupling of C(sp2)-H and N-H Bonds: A Transition-Metal-Free Approach toward Indolo[1,2-a]quinazolinones

A metal- and base-free protocol for intramolecular cross-coupling of C(sp²)-H and N-H bonds using N-iodosuccinimide (NIS) has been demonstrated. This environmentally benign approach furnishes a series of substituted indolo[1,2-a]quinazolinones from the suitably fabricated indoles via C-N bond forming cyclization in 28-82% yield. A plausible mechanism is proposed for this cyclization based on the results of a control experiment. This methodology requires no additional metal catalyst, oxidant, or base. Furthermore, the synthetic utility of the protocol is demonstrated by performing a gram scale reaction.

o-Phthalaldehyde catalyzed hydrolysis of organophosphinic amides and other P([double bond, length as m-dash]O)-NH containing compounds

Over 50 years ago, Jencks and Gilchrist showed that formaldehyde catalyses the hydrolysis of phosphoramidate through electrophilic activation, induced by covalent attachment to its nitrogen atom. Given our interest in the use of aldehydes as catalysts, this work was revisited to identify a superior catalyst, o-phthalaldehyde, which facilitates hydrolyses of various organophosphorus compounds bearing P([double bond, length as m-dash]O)-NH subunits under mild conditions. Interestingly, chemoselective hydrolysis of the P([double bond, length as m-dash]O)-N bonds could be accomplished in the presence of P([double bond, length as m-dash]O)-OR bonds.

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.

Cu/Pd cooperatively catalyzed tandem C–N and C–P bond formation: access to phosphorated 2H-indazoles

An unprecedented Cu/Pd cooperatively catalyzed tandem C–N/C–P bond formation reaction for the synthesis of phosphorated 2H-indazoles has been developed.

Efficient synthesis of cyclic P-stereogenic phosphinamides from acyclic chiral precursors via radical oxidative intramolecular aryl C–H phosphinamidation

A highly efficient method for the synthesis of cyclic P-stereogenic phosphinamides from acyclic chiral precursors via radical oxidative C–H phosphinamidation, followed by the versatile synthesis of various P-stereogenic compounds, is presented.