Nickel(II)-Mediated Regioselective CH Monoiodination of Arenes and Heteroarenes by using Molecular Iodine

Microwave-accelerated cross-dehydrogenative-coupling (CDC) of N-(quinolin-8-yl)amides with acetone/acetonitrile under metal-free conditions

A highly selective remote C(sp³)-H acetonation of N-(quinolin-8-yl)amide scaffolds at the C5-position under microwave irradiation has been developed. In the absence of a transition-metal-catalyst, benzoyl peroxide (BPO)-promoted cross-dehydrogenation coupling (CDC) of N-(quinolin-8-yl)amides with acetone/acetonitrile occurred smoothly to generate the corresponding 5-acetonated/acetonitriled N-(quinolin-8-yl)amides in good yields. The transformation is operationally simple, rapid, easily scaled-up to the gram scale, and shows a broad substrate scope.

Overlooked potential of N,N-bidentate directing-groups in Ni-catalyzed C-H functionalization of benzamides

The Ni-catalyzed reactions of benzamides with bicyclic alkenes were explored using DFT calculations. An unprecedented "N-H deprotonation circumvented" catalytic mechanism was proposed, over the more common N-H/C-H activation mechanism, in which (i) the circumvention of N-H deprotonation ensures the presence of N-H⋯O hydrogen bond interaction, thereby stabilizing the critical ortho-C-H functionalization TS; and (ii) the N-H moiety retention results in a weak N⋯Ni σ-coordination, which is flexible to the configurational conversion during the key alkene insertion. These overlooked aspects of the functionalized N,N-bidentate directing groups will aid the design of new related catalytic reactions.

Aerobic oxidation-functionalization of the aryl moiety in van Koten's pincer complex (NCN)Ni(ii)Br: relevance to carbon–heteroatom coupling reactions promoted by high-valent nickel species

Treating the pincer complex (NCN)NiBr with protic substrates HX (X = OH, OR, or NR2) under aerobic conditions leads to C–X functionalization of the pincer ligand. The crucial importance of aerobic conditions for the success of this coupling reaction implies the formation of high-valent intermediates during the course of the reaction.

Monodentate Transient Directing Group-Assisted Palladium-Catalyzed Direct ortho-C-H Iodination of Benzaldehydes for Total Synthesis of Hernandial

The first palladium-catalyzed direct o-C-H iodination of benzaldehydes was successfully developed with the assistance of commercially available 2,5-bis(trifluoromethyl)aniline as the optimal monodentate transient directing group (MonoTDG). Moderate to excellent yields and good selectivity were achieved for a broad substrate scope under mild conditions. More importantly, the synthetic application was demonstrated by a concise two-step total synthesis of the natural product hernandial, which was accomplished by merging this new MonoTDG-assisted C-H iodination and subsequent copper-catalyzed cross-coupling.

Recent Strategies for Carbon-Halogen Bond Formation Using Nickel

Nickel catalysis has demonstrated the capability of performing a broad range of synthetically challenging transformations over the last decade. Though recent literature has focused on the formation of C-C and C-N bonds, a variety of breakthroughs in the field of C-X bond generation have also been reported. A diverse range of strategies using nickel have been developed, in an effort to expand the scope and synthetic utility of these halogenation methods. This Minireview will cover six emerging strategies in this field including: oxidatively induced C-X reductive elimination, triflate-to-halogen exchange reactions, directed C-H halogenation, non-directed electrophilic C-H halogenation of arenes, enantioselective α-fluorination of carbonyl containing compounds, and 1,2-difunctionalization-halogenation reactions. The final section has been split into two parts: nickel-catalyzed hydrohalogenation and nickel-catalyzed carbohalogenation reactions.

A Guide to Directing Group Removal: 8-Aminoquinoline

The use of directing groups allows high levels of selectivity to be achieved in transition metal-catalyzed transformations. Efficient removal of these auxiliaries after successful functionalization, however, can be very challenging. This review provides a critical overview of strategies used for removal of Daugulis' 8-aminoquinoline (2005-2020), one of the most widely used N,N-bidentate directing groups. The limitations of these strategies are discussed and alternative approaches are suggested for challenging substrates. Our aim is to provide a comprehensive end-users' guide for chemists in academia and industry who want to harness the synthetic power of directing groups-and be able to remove them from their final products.

IrIII -Catalyzed Selective ortho-Monoiodination of Benzoic Acids with Unbiased C-H Bonds

An iridium-catalyzed selective ortho-monoiodination of benzoic acids with two equivalent C-H bonds is presented. A wide range of electron-rich and electron-poor substrates undergo the reaction under mild conditions, with >20:1 mono/di selectivity. Importantly, the C-H iodination occurs selectively ortho to the carboxylic acid moiety in substrates bearing competing coordinating directing groups. The reaction is performed at room temperature and no inert atmosphere or exclusion of moisture is required. Mechanistic investigations revealed a substrate-dependent reversible C-H activation/protodemetalation step, a substrate-dependent turnover-limiting step, and the crucial role of the AgI additive in the deactivation of the iodination product towards further reaction.

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.

Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp2)-H and C(sp3)-H Functionalization of Aminoquinoline Substrates

This Article describes the synthesis and characterization of cyclometalated aminoquinoline Ni^II σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These Ni^II complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I₂. They also react stoichiometrically with I₂ to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with Ag^I salts at -30 °C to afford isolable five-coordinate Ni^III species. The Ni^III σ-aryl complexes proved inert toward C(sp²)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a Ni^III σ-alkyl analogue underwent C(sp³)-N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter Ni^III species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of Ni^III species in these C-H functionalization reactions.

Carbon-Carbon Bond Formation of Trifluoroacetyl Amides with Grignard Reagents via C(O)-CF3 Bond Cleavage

The reaction of trifluoroacetyl amides with Grignard reagent for the substitution of CF₃ group with various alkyl or aryl groups is described. A variety of aryl, quinolin-8-yl, and (hetero)alkyl functional groups as well as F, Cl, and Br atoms are well tolerated. These moisture-stable and easily available trifluoroacetyl amides can be conveniently obtained and used as new versatile precursors for isocyanates. The control experiments show that the reaction proceeds via an isocyanate intermediate and/or alkoxide/amide dual anionic intermediate.

Rhodium-Catalyzed C(sp2 )- or C(sp3 )-H Bond Functionalization Assisted by Removable Directing Groups

In recent years, transition-metal-catalyzed C-H activation has become a key strategy in the field of organic synthesis. Rhodium complexes are widely used as catalysts in a variety of C-H functionalization reactions because of their high reactivity and selectivity. The availability of a number of rhodium complexes in various oxidation states enables diverse reaction patterns to be obtained. Regioselectivity, an important issue in C-H activation chemistry, can be accomplished by using a directing group to assist the reaction. However, to obtain the target functionalized compounds, it is also necessary to use a directing group that can be easily removed. A wide range of directed C-H functionalization reactions catalyzed by rhodium complexes have been reported to date. In this Review, we discuss Rh-catalyzed C-H functionalization reactions that are aided by the use of a removable directing group such as phenol, amine, aldehyde, ketones, ester, acid, sulfonic acid, and N-heteroaromatic derivatives.

Nickel-catalyzed oxidative C-H/N-H annulation of N-heteroaromatic compounds with alkynes

The reaction of N-heteroaromatic compounds, such as 2-aryl-pyrrole, benzimidazole, imidazole, indole, and pyrazole derivatives, with alkynes in the presence of a catalytic amount of a nickel complex results in C-H/N-H oxidative annulation. The reaction shows a high functional group compatibility. While both Ni(0) and Ni(ii) complexes show a high catalytic activity, Ni(0) is proposed as a key catalytic species in the main catalytic cycle. In the case of the Ni(ii) system, the presence of a catalytic amount of a strong base, such as KOBu t , is required for the reaction to proceed. In sharp contrast, a base is not required in the case of the Ni(0) system. The proposed mechanism is supported by DFT studies.

Nickel-catalyzed regioselective C–H oxygenation: new routes for versatile C–O bond formation

Nickel-catalyzed regioselective C–H oxygenation reactions of chelating arenes using iodobenzene diacetate, alcohols, and benzoic acids respectively as attacking reagents have been developed for the first time.

Transition metal catalysed C7 and ortho-selective halogenation of 2-arylbenzo[d]oxazoles

An approach to construct C–X bonds on the C7 or ortho-position of 2-arylbenzo[d]oxazoles by Rh and Ru catalysts has been reported.

Nickel-catalyzed regioselective C–H halogenation of electron-deficient arenes

A straightforward Ni(ii)-catalyzed general strategy was developed for the ortho-halogenation of electron-deficient arenes with easily available halogenating reagents N-halosuccinimides (NXS; X = Br, Cl and I).

A computational study of cobalt-catalyzed C–H iodination reactions using a bidentate directing group with molecular iodine

A computational methodology was used to collect detailed mechanistic information on the cobalt-catalyzed C–H iodination of aromatic amides with molecular iodine using an N,N′-bidentate directing group.

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.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Nickel-catalyzed direct C-H trifluoroethylation of heteroarenes with trifluoroethyl iodide

A highly selective nickel-catalyzed C-H trifluoroethylation of heteroarenes was developed with the assistance of a monodentate directing group. This protocol provides efficient access to various trifluoroethyl-substituted heteroarenes, including indoles, pyrroles, furans, and thiophenes, with commercially available CF₃CH₂I as an alkylation reagent. This robust catalytic procedure is scalable and tolerates a broad range of functional groups. Moreover, multifluoroalkylation of indoles is also viable.

Cobalt(ii)-catalyzed chelation-assisted C–H iodination of aromatic amides with I2

An air stable and inexpensive cobalt-metal, mild and efficient catalytic system.

An efficient nickel/silver co-catalyzed remote C–H amination of 8-aminoquinolines with azodicarboxylates at room temperature

A highly efficient nickel/silver co-catalyzed C–H amination at the C5 position of 8-aminoquinolines with azodicarboxylates at room temperature is reported. The reaction undergoes a self-redox process without the necessity of external oxidant. It proceeded under simple and mild conditions without any additional ligand, base or oxidant and provided the desired products in good to excellent yields. This method also possessed the merits of good functional group compatibility and air and moisture tolerance. It provides an efficient strategy for the synthesis of useful quinoline derivatives.

C–H amination at the C5 position of 8-aminoquinolines with azodicarboxylates proceeded efficiently using a nickel/silver co-catalyst at room temperature without any additional ligand, base or oxidant.

Rh(iii)-Catalyzed regioselective mono- and di-iodination of azobenzenes using alkyl iodide

We developed a highly chemoselective oxidative access to ortho-iodinated azobenzenes by Rh(iii)-catalysis with alkyl iodide as the iodinating reagent.

Selective remote C–H trifluoromethylation of aminoquinolines with CF3SO2Na under visible light irradiation in the absence of an external photocatalyst

A selective remote C–H trifluoromethylation of aminoquinolines with sodium trifluoromethanesulfinate (CF₃SO₂Na) under visible-light irradiation without an external photocatalyst was developed.

The mechanism of directed Ni(ii)-catalyzed C-H iodination with molecular iodine

This computational study reveals electrophilic cleavage pathways for substrates with N,N-bidentate directing centers in Ni(ii)-catalyzed C–H iodination with molecular iodine.

The density functional theory method is used to elucidate the elementary steps of Ni(ii)-catalyzed C(sp²)–H iodination with I₂ and substrates bearing N,N′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na₂CO₃ base to the reaction mixture facilitates C–H activation. The presence of I₂ in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni(ii)/Ni(iv) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation.

Expeditious and Solvent-Free Nickel-Catalyzed C-H Arylation of Arenes and Indoles

An efficient solvent-free nickel-catalyzed method for C-H bond arylation of arenes and indoles has been developed, which proceeds expeditiously through chelation assistance. The reaction is highly selective for mono-arylation and tolerates sensitive and structurally diverse functionalities, such as halides, ethers, amines, indole, pyrrole and carbazole. This reaction represents the first example of a nickel-catalyzed C-H arylation by monochelate assistance and symbolizes a rare precedent in solvent-free C-H arylation. Mechanistic investigations by various controlled reactions, kinetic studies, and deuterium labeling experiments suggest that the arylation follows a single electron transfer (SET) pathway involving the turnover-limiting C-H nickelation process.