A general strategy toward aromatic 1,2-ambiphilic synthons: palladium-catalyzed ortho-halogenation of PyDipSi-arenes

Palladium-Catalyzed Cascade Heck Coupling and Allylboration of Iododiboron Compounds via Diboryl Radicals

Geminal bis(boronates) are versatile synthetic building blocks in organic chemistry. The fact that they predominantly serve as nucleophiles in the previous reports, however, has restrained their synthetic potential. Herein we disclose the ambiphilic reactivity of α-halogenated geminal bis(boronates), of which the first catalytic utilization was accomplished by merging a formal Heck cross-coupling with a highly diastereoselective allylboration of aldehydes or imines, providing a new avenue for rapid assembly of polyfunctionalized boron-containing compounds. We demonstrated that this cascade reaction is highly efficient and compatible with various functional groups, and a wide range of heterocycles. In contrast to a classical Pd(0/II) scenario, mechanistic experiments and DFT calculations have provided strong evidence for a catalytic cycle involving Pd(I)/diboryl carbon radical intermediates.

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.

Nickel Meets Aryl Thianthrenium Salts: Ni(I)-Catalyzed Halogenation of Arenes

Herein, a regioselective, late-stage two-step arene halogenation method is reported. We propose how unusual Ni(I)/(III) catalysis is enabled by a combination of aryl thianthrenium and Ni redox properties that is hitherto unachieved with other (pseudo)halides. The catalyst is accessed in situ from inexpensive NiCl₂·6(H₂O) and zinc without the need of supporting ligands.

Meta-Selective C-H Functionalization of Arylsilanes Using a Silicon Tethered Directing Group

We describe meta-selective C-H functionalization of arylsilanes using a Si-tethered directing group. The current method enables a selective alkenylation of arenes bearing a variety of functional groups, and several electron-deficient olefins are also applicable as coupling partners. Further functional group transformations of the silicon-tethered directing group provide multisubstituted arenes efficiently.

Transition-Metal-Catalyzed Directing Group Assisted (Hetero)aryl C-H Functionalization: Construction of C-C/C-Heteroatom Bonds

Transition-metal-catalyzed C-H functionalization is one of the fascinating scientific fronts in organic synthesis for the formation of conjugated arenes and has emerged as a benchmark to revolutionize the synthetic enterprise since past decades. In this realm, chelation-guided functionalization of C-H bonds using an exogenous directing group has received considerable attention recently for the expedient regioselective construction of C-C and C-heteroatom bonds as an efficient and sustainable alternative. This article outlines our contribution towards a wide variety of transformations that have been achieved by the directed C-H functionalization through the fine tuning of catalytic systems.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

Differential formation of nitrogen-centered radicals leading to unprecedented, regioselective bromination of N,N′-(1,2-phenylene)bisamides and 2-amidophenols

A highly efficient, site-selective, visible light-accelerated, remote C–H halogenation of unsymmetrical aromatic bisamides/amidoesters has been developed.

Site-Selective Ni-Catalyzed Reductive Coupling of α-Haloboranes with Unactivated Olefins

A mild, chemo- and site-selective catalytic protocol that allows for incorporating an alkylboron fragment into unactivated olefins is described. The use of internal olefins enables C-C bond-formation at remote sp³ C-H sites, constituting a complementary and conceptually different approach to existing borylation techniques that are currently available at sp³ centers.

Palladium-Catalyzed Regioselective Olefination of O-Acetyl Cyanohydrins

A practical palladium-catalyzed ortho-olefination of O-acetyl cyanohydrins assisted by synergetic directing groups has been developed. Thus, a range of olefinated O-acetyl cyanohydrins were synthesized in moderate to good yields. The reaction occurs efficiently with high regioselectivity and with a satisfactory tolerance of functional groups.

Nickel-Catalyzed Umpolung Arylation of Ambiphilic α-Bromoalkyl Boronic Esters

A nickel-catalyzed reductive arylation of ambiphilic α-bromoalkyl boronic esters with aryl halides is described. This platform provides an unrecognized opportunity to promote the catalytic umpolung reactivity of ambiphilic reagents with aryl halides, thus unlocking a new cross-coupling strategy that complements existing methods for the preparation of densely functionalized alkyl-substituted organometallic reagents from simple and readily accessible precursors.

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.

Silicon-Tethered Strategies for C-H Functionalization Reactions

Selective and efficient functionalization of ubiquitous C-H bonds is the Holy Grail of organic synthesis. Most advances in this area rely on employment of strongly or weakly coordinating directing groups (DGs) which have proven effective for transition-metal-catalyzed functionalization of C(sp²)-H and C(sp³)-H bonds. Although most directing groups are important functionalities in their own right, in certain cases, the DGs become static entities that possess very little synthetic leverage. Moreover, some of the DGs employed are cumbersome or unpractical to remove, which precludes the use of this approach in synthesis. It is believed, that development of a set of easily installable and removable/modifiable DGs for C-H functionalization would add tremendous value to the growing area of directed functionalization, and hence would promote its use in synthesis and late-stage functionalization of complex molecules. In particular, silicon tethers have long provided leverage in organic synthesis as easily installable and removable/modifiable auxiliaries for a variety of processes, including radical transformations, cycloaddition reactions, and a number of TM-catalyzed methods, including ring-closing metathesis (RCM) and cross-coupling reactions. Employment of Si-tethers is highly attractive for several reasons: (1) they are easy to handle/synthesize and are relatively stable; (2) they utilize cheap and abundant silicon precursors; and (3) Si-tethers are easily installable and removable/modifiable. Hence, development of Si-tethers for C-H functionalization reactions is appealing not only from a practical but also from a synthetic standpoint, since the Si-tether can provide an additional handle for diversification of organic molecules post-C-H functionalization. Over the past few years, we developed a set of Si-tether approaches for C-H functionalization reactions. The developed Si-tethers can be categorized into four types: (Type-1) Si-tethers possessing a reacting group, where the reacting group is delivered to the site of functionalization; (Type-2) Si-tethers possessing a DG, designed for selective C(sp²)-H functionalization of arenes; (Type-3) reactive Si-tethers for C-H silylation of organic molecules; and finally, (Type-4) reactive Si-tethers containing a DG, developed for selective C-H silylation/hydroxylation of challenging C(sp³)-H bonds. In this Account, we outline our advances on the employment of silicon auxiliaries for directed C-H functionalization reactions. The discussion of the strategies for employment of different Si-tethers, functionalization/modification of silicon tethers, and the methodological developments on C-C, C-X, C-O, and C-Si bond forming reactions via silicon tethers will also be presented. While the work described herein presents a substantial advance for the area of C-H functionalization, challenges still remain. The use of noble metals are required for the C-H functionalization methods presented herein. Also, the need for stoichiometric use of high molecular weight silicon auxiliaries is a shortcoming of the presented concept.

Transition-Metal-Free Decarboxylative Iodination: New Routes for Decarboxylative Oxidative Cross-Couplings

Constructing products of high synthetic value from inexpensive and abundant starting materials is of great importance. Aryl iodides are essential building blocks for the synthesis of functional molecules, and efficient methods for their synthesis from chemical feedstocks are highly sought after. Here we report a low-cost decarboxylative iodination that occurs simply from readily available benzoic acids and I₂. The reaction is scalable and the scope and robustness of the reaction is thoroughly examined. Mechanistic studies suggest that this reaction does not proceed via a radical mechanism, which is in contrast to classical Hunsdiecker-type decarboxylative halogenations. In addition, DFT studies allow comparisons to be made between our procedure and current transition-metal-catalyzed decarboxylations. The utility of this procedure is demonstrated in its application to oxidative cross-couplings of aromatics via decarboxylative/C–H or double decarboxylative activations that use I₂ as the terminal oxidant. This strategy allows the preparation of biaryls previously inaccessible via decarboxylative methods and holds other advantages over existing decarboxylative oxidative couplings, as stoichiometric transition metals are avoided.

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.

An efficient and catalyst-free synthesis of N-arylidene-2-arylimidazo[1,2-a]pyridine-3-ylamine derivatives via Strecker reaction under controlled microwave heating

An efficient one-pot multicomponent reaction of 2-aminopyridine with aromatic aldehydes and either benzoyl cyanide or cyanamide in pyridine under controlled microwave heating afforded N-aryldene-2-arylimidazo[1,2-a]-pyridine-3-ylamine derivatives. The reaction is catalyst free and is of high atom economy.

Simple, Efficient and Controllable Synthesis of Iodo/Di-iodoarenes via Ipsoiododecarboxylation/Consecutive Iodination Strategy

A practical, efficient, and operationally simple strategy for the ipsoiododecarboxylation and di-iodination of aromatic carboxylic acids using the low-cost commercial reagent succinimide (NIS) as iodine source is reported. This iodination or di-iodination process can be easily controlled through reaction conditions, thereby providing corresponding iodination or di-iodination products with high yields. Furthermore, these two reactions can be easily scaled up to gram-scale by using palladium catalyst (0.66 mol%), which provides high isolated yield.

Regioselective C–H chlorination: towards the sequential difunctionalization of phenol derivatives and late-stage chlorination of bioactive compounds

Palladium-catalyzed C–H chlorination of phenol derivatives and late-stage chlorination of diflufenican and estrone.

Towards Uniform Iodine Catalysis: Intramolecular C-H Amination of Arenes under Visible Light

A photochemical catalytic amination of arenes is presented. The reaction proceeds under benign iodine catalysis in the presence of visible light as the initiator and provides access to a range of differently substituted arylamines. A total of 29 examples demonstrate the broad applicability of this mild oxidation method. The scope of the reaction could further be expanded to silyl-tethered derivatives, which undergo intramolecular amination upon formation of seven-membered heterocycles. Cleavage of the silicon tether provides access to the corresponding 3-substituted anilines.

Silver(I)-Catalyzed Iodination of Arenes: Tuning the Lewis Acidity of N-Iodosuccinimide Activation

A mild and rapid method for the iodination of arenes that utilizes silver(I) triflimide as a catalyst for activation of N-iodosuccinimide has been developed. The transformation was found to be general for a wide range of anisole, aniline, acetanilide, and phenol derivatives and allowed the late-stage iodination of biologically active compounds such as PIMBA, a SPECT imaging agent of breast cancer, and (-)-IBZM, a dopamine D2 receptor antagonist. The method was also modified for the radioiodination of arenes using a one-pot procedure involving the in situ generation of [(125)I]-N-iodosuccinimide followed by the silver(I)-catalyzed iodination.

Amino acid-promoted C–H alkylation with alkylboronic acids using a removable directing group

Palladium-catalyzed C–H alkylation reaction with alkylboronic acids has been developed using a removable pyridyldiisopropylsilyl directing group.

Cobalt(iii)-catalyzed C–H halogenation of 6-arylpurines: facile entry into arylated, sulfenylated and alkoxylated 6-arylpurines

Cp*Co(iii)-catalyzed C–H halogenation of 6-arylpurines has been reported under mild conditions and this protocol was further applied for the synthesis of arylated, sulfenylated and alkoxylated purine analogues.

Copper(ii)-catalyzed C5 and C7 halogenation of quinolines using sodium halides under mild conditions

A simple and mild protocol for copper catalyzed halogenation of quinoline at C5 and C7 positions was developed, affording the desired remote C–H activation products in moderate to good yields.

Arylation with Diaryliodonium Salts

This chapter focuses on recent developments in metal-free and metal-catalyzed arylations with diaryliodonium salts (diaryl-λ3-iodanes). Synthetic routes to diaryliodonium salts are briefly described, and chemoselectivity trends with unsymmetric iodonium salts are discussed.

Ruthenium-Catalyzed meta-Selective C-H Bromination

The first example of a transition-metal-catalyzed, meta-selective C-H bromination procedure is reported. In the presence of catalytic [{Ru(p-cymene)Cl2 }2 ], tetrabutylammonium tribromide can be used to functionalize the meta C-H bond of 2-phenylpyridine derivatives, thus affording difficult to access products which are highly predisposed to further derivatization. We demonstrate this utility with one-pot bromination/arylation and bromination/alkenylation procedures to deliver meta-arylated and meta-alkenylated products, respectively, in a single step.

Mechanistic Details of Pd(II)-Catalyzed C-H Iodination with Molecular I2: Oxidative Addition vs Electrophilic Cleavage

Transition metal-catalyzed C-H bond halogenation is an important alternative to the highly utilized directed-lithiation methods and increases the accessibility of the synthetically valuable aryl halide compounds. However, this approach often requires impractical reagents, such as IOAc, or strong co-oxidants. Therefore, the development of methodology utilizing inexpensive oxidants and catalyst containing earth-abundant transition metals under mild experimental conditions would represent a significant advance in the field. Success in this endeavor requires a full understanding of the mechanisms and reactivity governing principles of this process. Here, we report intimate mechanistic details of the Pd(II)-catalyzed C-H iodination with molecular I2 as the sole oxidant. Namely, we elucidate the impact of the: (a) Pd-directing group (DG) interaction, (b) nature of oxidant, and (c) nature of the functionalized C-H bond [C(sp(2))-H vs C(sp(3))-H] on the Pd(II)/Pd(IV) redox and Pd(II)/Pd(II) redox-neutral mechanisms of this reaction. We find that both monomeric and dimeric Pd(II) species may act as an active catalyst during the reaction, which preferentially proceeds via the Pd(II)/Pd(II) redox-neutral electrophilic cleavage (EC) pathway for all studied substrates with a functionalized C(sp(2))-H bond. In general, a strong Pd-DG interaction increases the EC iodination barrier and reduces the I-I oxidative addition (OA) barrier. However, the increase in Pd-DG interaction alone is not enough to make the mechanistic switch from EC to OA: This occurs only upon changing to substrates with a functionalized C(sp(3))-H bond. We also investigated the impact of the nature of the electrophile on the C(sp(2))-H bond halogenation. We predicted molecular bromine (Br2) to be more effective electrophile for the C(sp(2))-H halogenation than I2. Subsequent experiments on the stoichiometric C(sp(2))-H bromination by Pd(OAc)2 and Br2 confirmed this prediction.The findings of this study advance our ability to design more efficient reactions with inexpensive oxidants under mild experimental conditions.

Photo-induced Metal-Catalyst-Free Aromatic Finkelstein Reaction

The facile iodination of aromatic compounds under mild conditions is a great challenge for both organic and medicinal chemistry. Particularly, the synthesis of functionalized aryl iodides by light has long been considered impossible due to their photo-lability, which actually makes aryl iodides popular starting materials in many photo-substitution reactions. Herein, a photo-induced halogen exchange in aryl or vinyl halides has been discovered for the first time. A broad scope of aryl iodides can be prepared in high yields at room temperature under exceptionally mild conditions without any metal or photo-redox catalysts. The presence of a catalytic amount of elemental iodine could promote the reaction significantly.