Cu-Mediated Direct Aryl CH Halogenation: a Strategy to Control Mono- and Di-Selectivity

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF₂ complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF₂ in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

The Influence of Copper on Halogenation/Dehalogenation Reactions of Aromatic Compounds and Its Role in the Destruction of Polyhalogenated Aromatic Contaminants

The effect of copper and its compounds on halogenation and dehalogenation of aromatic compounds will be discussed in the proposed article. Cu oxidized to appropriate halides is an effective halogenation catalyst not only for the synthesis of halogenated benzenes or their derivatives as desired organic fine chemicals, but is also an effective catalyst for the undesirable formation of thermodynamically stable and very toxic polychlorinated and polybrominated aromatic compounds such as polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans accompanied incineration of waste contaminated with halogenated compounds or even inorganic halides. With appropriate change in reaction conditions, copper and its alloys or oxides are also able to effectively catalyze dehalogenation reactions, as will be presented in this review.

Pd-Catalyzed C-H Halogenation of Indolines and Tetrahydroquinolines with Removable Directing Group

Pd-catalyzed directing-group-assisted regioselective halogenations to C7 of indolines and C8 of tetrahydroquinolines were achieved in good to excellent yields. The practicality and utility of the developed method have been illustrated by various functional group transformations such as arylation, alkenylation, cyanation, and silylation utilizing the installed synthetic handle. The concise synthesis of primaquine, an antimalarial drug, and formal syntheses of two bioactive natural products, hippadine and pratosine, have also been demonstrated.

Rhodium-Catalyzed ortho-Bromination of O-Phenyl Carbamates Accelerated by a Secondary Amide-Pendant Cyclopentadienyl Ligand

It has been established that a newly developed cyclopentadienyl rhodium(III) [Cp^A Rh^III ] complex, bearing an acidic secondary amide moiety on the Cp ring, is able to catalyze the ortho-bromination of O-phenyl carbamates with N-bromosuccinimide (NBS) at room temperature. The presence of the acidic secondary amide moiety on the Cp^A ligand accelerates the bromination by the hydrogen bond between the acidic NH group of the Cp^A ligand and the carbonyl group of NBS.

Directed C-H Halogenation Reactions Catalysed by PdII Supported on Polymers under Batch and Continuous Flow Conditions

A new generation of N-heterocyclic carbene palladium(II) complexes containing vinyl groups in different positions in the backbone of the N-heterocycle have been developed. The fully characterised monomers were copolymerised with divinylbenzene to fabricate robust polymer supported NHC-Pd^II complexes and these polymers were applied as heterogeneous catalysts in directed C-H halogenation of arenes with a pyridine-type directing group. The catalysts demonstrated medium-high catalytic activity with up to 90 % conversion and 100 % selectivity in chlorination. They are heterogeneous and recyclable (at least six times) with no significant leaching of palladium in batch mode catalysis. The best catalyst was also applied under continuous flow conditions where it disclosed an exceptional activity (90 % conversion) and 100 % selectivity for the mono-halogenated product for at least six days, with no leaching of palladium, no loss of activity and an ability to maintain the original oxidation state of Pd^II .

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Palladium-Catalyzed Decarboxylative ortho-Acylation of Anilines with Carbamate as a Removable Directing Group

An efficient palladium-catalyzed decarboxylative ortho-acylation of anilines with α-oxocarboxylic acids has been realized by using carbamate as a directing group (DG). The reaction proceeds smoothly with high regioselectivity to afford diverse acylation products of aniline derivatives in moderate to good yields under mild conditions. This transformation exhibits broad substrate scope and highly functional group tolerance. In addition, the employed DG can be easily removed to give the corresponding 2-amino aromatic ketones. Importantly, several transformations of the synthesized ortho-acylated anilines into several synthetically valuable products have been demonstrated for their utilities.

Palladium-Catalyzed Directed Halogenation of Bipyridine N-Oxides

The palladium-catalyzed directed C-H halogenation of bipyridine N-oxides was investigated. Using NCS or NBS (N-chloro- or N-bromosuccinimide) and 5 mol % Pd(OAc)₂ in chlorobenzene (0.10 molar) at 110 °C, pyridine-directed functionalization took place and 3-chloro- or 3-bromobipyridine N-oxides were obtained in high yields. The reaction is sensitive to steric hindrance by 4- and 6'-substituents. Only in the latter case, where coordination of palladium by the pyridine is hindered, 3'-halogenation directed by the N-oxide function was observed. The halogenated products were deoxygenated by PCl₃ or PBr₃.

Transition metal-catalyzed site- and regio-divergent C–H bond functionalization

The regioselectivity of C–H functionalization reactions can be redirected to obtain regioisomeric products form the same starting materials.

Recent advances in copper-mediated chelation-assisted functionalization of unactivated C–H bonds

Recent advances in copper-mediated (both stoichiometric and catalytic) chelation-assisted functionalization of unactivated C–H bonds are reviewed.

Nickel-catalyzed ortho-halogenation of unactivated (hetero)aryl C–H bonds with lithium halides using a removable auxiliary

The first example of Ni-catalyzed halogenation of (hetero)aryl C–H bonds with lithium halides using PIP as a removable directing group is described.

C-H bond halogenation catalyzed or mediated by copper: an overview

Carbon-halogen (C-X) bonds are amongst the most fundamental groups in organic synthesis, they are frequently and widely employed in the synthesis of numerous organic products. The generation of a C-X bond, therefore, constitutes an issue of universal interest. Herein, the research advances on the copper-catalyzed and mediated C-X (X = F, Cl, Br, I) bond formation via direct C-H bond transformation is reviewed.

Pd(II)-catalyzed enantioselective synthesis of P-stereogenic phosphinamides via desymmetric C-H arylation

We present the enantioselective synthesis of P-stereogenic phosphinamides through Pd-catalyzed desymmetric ortho C-H arylation of diarylphosphinamides with boronic esters. The method represents the first example of the synthesis of P-stereogenic phosphorus compounds via the desymmetric C-H functionalization strategy. The reaction proceeded efficiently with a wide array of reaction partners to afford the P-stereogenic phosphinamides in up to 74% yield and 98% ee. The efficiency was further demonstrated by gram scale syntheses. Moreover, the flexible conversion of the P-stereogenic phosphinamides into various types of P-stereogenic phosphorus derivatives was also elaborated. Thus, the protocol provides a novel tool for the efficient and versatile synthesis of P-stereogenic compounds.

Copper-catalyzed ortho-halogenation of arenes and heteroarenes directed by a removable auxiliary

Copper-catalyzed ortho-halogenation of C(sp²)–H bonds directed by a PIP directing group with NXS (X = Cl, Br, I) has been developed. The reaction is scalable and tolerates a broad range of functional groups and heteroarenes, providing an efficient access to halogenated arenes and heteroarenes.

Cu-mediated direct regioselective C-2 chlorination of indoles

Cu-mediated C-2 chlorination of indoles was accomplished with copper(ii) chloride through the use of a directing pyrimidyl protection group. A highly regioselective manner can be achieved on a range of indole substrates with excellent functional group tolerance.

Crystal structures of benzoxazolyl–copper(iii,ii,i) complexes and investigation of Cu(ii)-mediated aryl carbon–hydrogen bromination

We report three new copper complexes with different oxidation states based on the phenylbenzoxazole system.