Pd(II)-catalyzed C-H iodination using molecular I2 as the sole oxidant

Metal-free iodination of arylaldehydes for total synthesis of aristogins A-F and hernandial

Iodine is one of the most effective sources for iodination of aromatic compounds; however, its electrophilicity is insufficient for direct iodination. The selection of appropriate environmentally friendly and cost-effective oxidants in combination with iodine for the iodination of aromatic rings, along with its application in the synthesis of natural products, holds significant importance. A highly efficient method utilizing I(III) as the initiator has been successfully developed for monoiodination of arylaldehydes. The method demonstrates good compatibility with a wide range of (hetero)aromatic aldehydes, resulting in moderate to excellent yields, without the need for any toxic, volatile or explosive reagents. The synthesis of seven natural products, namely aristogins A-F and hernandial, was achieved through this iodination followed by Ullmann-type coupling.

I2-Mediated Site-Selective C-H Functionalization: Access to p-Amino-Substituted Unsymmetrical Benzils and Quinoxalines from Sulfoxonium Ylides

An I2-mediated approach for selective C-H functionalization of unprotected aniline derivatives for synthesizing benzils and quinoxaline derivatives from sulfoxonium ylides has been described. Aniline derivatives and sulfoxonium ylides ornamented with different functional groups showed good compatibility. They afforded the corresponding products with moderate to high yields via a mild and simple procedure. Finally, we validated the practicality of this method by scaling up the reaction and further conversion of the synthesized derivatives into other valuable molecules.

Selective C-H Iodination of Weinreb Amides and Benzamides through Iridium Catalysis in Solution and under Mechanochemical Conditions

The acid mediated ortho-iodination of Weinreb amides using a readily available catalyst is described. The selective ortho-iodination of Weinreb amides, challenging substrates in directed C-H activations, and also of benzamides is achieved. The process works under mild conditions and tolerates air and moisture, having a great potential for industrial applications. The methodology can be applied under mechanochemical conditions maintaining the reaction outcome and selectivity.

Regio- and Stereoselective 1,2-Oxyhalogenation of Non-Conjugated Alkynes via Directed Nucleopalladation: Catalytic Access to Tetrasubstituted Alkenes

A catalytic 1,2-oxyhalogenation method that converts non-conjugated internal alkynes into tetrasubstituted alkenes with high regio- and stereoselectivity is described. Mechanistically, the reaction involves a Pd^II /Pd^IV catalytic cycle that begins with a directed oxypalladation step. The origin of regioselectivity is the preference for formation of a six-membered palladacycle intermediate, which is facilitated by an N,N-bidentate 2-(pyridin-2-yl)isopropyl (PIP) amide directing group. Selectivity for C(alkenyl)-X versus -N (X=halide) reductive elimination from the Pd^IV center depends on the identity of the halide anion; bromide and iodide engage in C(alkenyl)-X formation, while intramolecular C(alkenyl)-N reductive elimination occurs with chloride to furnish a lactam product. DFT calculations shed light on the origins of this phenomenon.

Alkali halides as nucleophilic reagent sources for N-directed palladium-catalysed ortho-C-H halogenation of s-tetrazines and other heteroaromatics

A general palladium-catalysed selective C-H halogenation reaction is reported, which was successfully achieved for a large variety of functionalized aromatic rings incorporating diverse N-directing groups. By using simple alkali halides of MX type as the nucleophilic reagent source (M = Li, Na, K, Cs and X = I, Br and Cl), and phenyliodanediacetate oxidant, clean C-H-iodination, bromination and chlorination reactions were performed. This general protocol of selective ortho-monohalogenation, which complements but contrasts with the classical methods using electrophilic reagents, is achievable in a short time (30 min) with microwave irradiation assistance. The reaction was extended to substrates bearing N-directing pyridine, pyrimidine, pyrazole, oxazoline, naphtho[1,2-d]thiazole, and azobenzene groups. Notably, the topical and selectivity-challenging s-tetrazine, as a nitrogen-rich heteroaromatic, was successfully halogenated by this protocol.

Formal C-H/C-I Metathesis: Site-Selective C-H Iodination of 2-Aryl Benzoic Acid Derivatives Using Aryl Iodide

C-H functionalization via functional group metathesis is extremely rare. A protocol of remote site-selective C-H iodination of 2-aryl benzoic acid derivatives via formal C(sp²)-H/C(sp²)-I metathesis using readily available 1-iodo-4-methoxy-2-nitrobenzene as the mild iodinating reagent was reported herein. A range of 2-aryl benzoic acid derivatives including 2-(naphthalen-1-yl)benzoic acids and [1,1'-binaphthalene]-2-carboxylic acids were iodinated under mild conditions to give valuable iodinated products in a site- and chemo-selective fashion.

Formal C-H/C-I Metathesis: Site-Selective C-H Iodination of Anilines Using Aryl Iodides

Functional group metathesis has the potential to render mild reaction conditions for C-H functionalization. Protocols for the meta- and ortho-C-H iodination of aniline derivatives via formal C(sp²)-H/C(sp²)-I metathesis using 2-nitrophenyl iodides as mild iodinating reagents are reported herein. These protocols led to the production of a range of valuable iodinated aniline derivatives. These results demonstrate the potential of developing novel site-selective C-H activation reactions with electron-rich compounds, since mild reagents can often been utilized in functional group metathesis reactions.

Visible-Light-Induced Catalytic Selective Halogenation with Photocatalyst

Halide moieties are essential structures of compounds in organic chemistry due to their popularity and wide applications in many fields such as natural compounds, agrochemicals, and pharmaceuticals. Thus, many methods have been developed to introduce halides into various organic molecules. Recently, visible-light-driven reactions have emerged as useful methods of organic synthesis. Particularly, halogenation strategies using visible light have significantly improved the reaction efficiency and reduced toxicity, as well as promoted reactions under mild conditions. In this review, we have summarized recent studies in visible-light-mediated halogenation (chlorination, bromination, and iodination) with photocatalysts.

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.

The Activating Effect of Strong Acid for Pd-Catalyzed Directed C-H Activation by Concerted Metalation-Deprotonation Mechanism

A computational study on the origin of the activating effect for Pd-catalyzed directed C–H activation by the concerted metalation-deprotonation (CMD) mechanism is conducted. DFT calculations indicate that strong acids can make Pd catalysts coordinate with directing groups (DGs) of the substrates more strongly and lower the C–H activation energy barrier. For the CMD mechanism, the electrophilicity of the Pd center and the basicity of the corresponding acid ligand for deprotonating the C–H bond are vital to the overall C–H activation energy barrier. Furthermore, this rule might disclose the role of some additives for C–H activation.

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.

Transition-Metal-Free Site-Selective γ-C(sp2)-H Monoiodination of Arenes Directed by an Aliphatic Keto Group

A general γ-C(sp²)-H iodination method directed by an aliphatic keto group has been developed under transition-metal-free conditions for the first time, generating iodoarenes in good to excellent yields with excellent site selectivity. This protocol features a wide range of aryl-substituted ketones, short reaction times, mild reaction conditions, and scalable synthetic procedures. A possible reaction mechanism was also proposed based on several control experiments.

Palladium-Catalyzed Direct Mono- or Polyhalogenation of Benzothiadiazole Derivatives

An unprecedented method for the palladium (II)-catalyzed direct halogenation of benzothiadiazole derivatives is reported here, which would allow the rational tuning of their electronic properties by further transformations. The key strategy is the combination of the double functions of benzothiadiazole as both an important moiety of functional materials and modifiable directing group. Various mono/polychlorinated, brominated, or iodinated benzothiadiazole products could be prepared efficiently, which can be converted into a wide range of valuable polyaromatic compounds with potential applications in material chemistry.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp³)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp³)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Visible light irradiation of 8-aminoquinoline Pd(ii) complexes initiates photoinduced electron transfer with alkyl halides, affording C–H halogenation over C–C bond adducts. A method for inert C(sp³)–H bond halogenation (Br, Cl and I) is reported.

Regioselective C-C cross-coupling of 1,2,4-thiadiazoles with maleimides through iridium-catalyzed C-H activation

A regioselective C-C cross-coupling of 1,2,4-thiadiazoles with maleimides through iridium catalysis was developed. This transformation tactically linked the 1,2,4-thiadiazoles and succinimides together, and the novel molecules formed may have potential biological activity.

The palladium(ii)-catalyzed regioselective ortho-C-H bromination/iodination of arylacetamides with in situ generated imidic acid as the directing group: mechanistic exploration

In the present study, we report the palladium(ii)-catalyzed regioselective ortho-C-H bromination/iodination of challenging arylacetamide derivatives using N-halosuccinimides as halogenating agents. Diverse arylacetamides underwent the regioselective ortho-bromination and iodination of aromatic C-H bonds in the presence of a reactive benzylic C(sp3)-H bond without installing any bulky auxiliaries via unfavorable six-membered metallacycles. Weak coordination, the use of ubiquitous primary amides for challenging C-H functionalization, the simple catalytic system and the wide substrate scope are the key features of this transformation. Further, the halogenated amide derivatives were transformed into a variety of valuable synthons. Detailed mechanistic studies revealed some interesting aspects concerning the reaction pathway. We present for the first time strong evidence for the formation of imidic acid (in situ) from primary amides under Brønsted acid conditions that eventually aids in the stabilization of palladacycles of amide derivatives and drives regioselective C-X bond formation.

Utilizing Native Directing Groups: Mechanistic Understanding of a Direct Arylation Leads to Formation of Tetracyclic Heterocycles via Tandem Intermolecular, Intramolecular C-H Activation

A mechanistic study on a direct arylation using a native picolylamine directing group is reported. Kinetic studies determined the concentration dependence of substrates and catalysts, as well as catalyst degradation, which led to the development of a new set of reaction conditions capable of affording a robust kinetic profile. During reaction optimization, a small impurity was observed, which was determined to be a dual C-H activation product. A second set of conditions were found to flip the selectivity of the C-H activation to form this tetracycle in high yield. A catalytic cycle is proposed for the intermolecular/intramolecular C-H activation pathway.

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.

Innate pharmacophore assisted selective C–H functionalization to therapeutically important nicotinamides

Tunable and regioselective C–H functionalization of nicotinamides was accomplished assisted by the innate pharmacophore NHOxa.

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.

Synthesis of C4-alkynylisoxazoles via a Pd-catalyzed Sonogashira cross-coupling reaction

A Pd-catalyzed Sonogashira cross-coupling reaction for the synthesis of C4-alkynylisoxazoles from 3,5-disubsitituted-4-iodoisoxazoles and terminal alkynes was described, which could afford the corresponding products with high yield (up to 98%). The results indicated that the steric effect from the group at the C3 position of the isoxazole had greater influence on the cross-coupling reaction than that from the group at the C5 position. In addition, the group at the C3 position of the isoxazole showed negligible electronic effects on the cross-coupling reaction. Furthermore, a gram-scale reaction of the Sonogashira coupling reaction was also investigated. Finally, a plausible mechanism for the Sonogashira coupling reaction was proposed.

A Pd-catalyzed Sonogashira cross-coupling reaction for the synthesis of C4-alkynylisoxazoles from 3,5-disubsitituted-4-iodoisoxazoles and terminal alkynes was described, which could afford the corresponding products with high yield (up to 98%).