Mild CH Halogenation of Anilides and the Isolation of an Unusual Palladium(I)-Palladium(II) Species

Branch-Selective Alkene Hydroarylation by Cooperative Destabilization: Iridium-Catalyzed ortho-Alkylation of Acetanilides

An iridium(I) catalyst system, modified with the wide-bite-angle and electron-deficient bisphosphine d^Fppb (1,4-bis(di(pentafluorophenyl)phosphino)butane) promotes highly branch-selective hydroarylation reactions between diverse acetanilides and aryl- or alkyl-substituted alkenes. This provides direct and ortho-selective access to synthetically challenging anilines, and addresses long-standing issues associated with related Friedel–Crafts alkylations.

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.

Chlorination of (Phebox)Ir(mesityl)(OAc) by Thionyl Chloride

Pincer (Phebox)Ir(mesityl)(OAc) (2) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)) complex, formed by benzylic C-H activation of mesitylene (1,3,5-trimethylbenzene) using (Phebox)Ir(OAc)2OH2 (1), was treated with thionyl chloride to rapidly form 1-(chloromethyl)-3,5-dimethylbenzene in 50% yield at 23 °C. A green species was obtained at the end of reaction, which decomposed during flash column chromatography to form (Phebox)IrCl2OH2 in 87% yield.

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.

Palladium catalyzed ortho-halogenation of 2-arylbenzothiazole and 2,3-diarylquinoxaline

A Pd-catalysed mono and di-o-halogenation strategy has been demonstrated selectively using benzothiazoles and quinoxalines as the directing substrates.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

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.

Palladium N-heterocyclic carbene catalyzed regioselective C–H halogenation of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones using N-halosuccinimides (NXS)

A novel palladium N-heterocyclic carbene complex of vitamin B₁ was prepared and employed for regioselective C–H halogenation of 1-aryl-3-methyl-1H-pyrazol-5(4H)-ones using N-halosuccinimides.

Synthesis of ortho-haloaminoarenes by aryne insertion of nitrogen-halide bonds

A rapid and general access to ortho-haloaminoarenes has been developed by aryne insertion into N-chloramine, N-bromoamine, and N-iodoamine bonds via two complementary protocols harnessing fluoride-promoted 1,2-elimination of ortho-trimethylsilyl aryltriflates. Typically, electron-deficient N-chloramines effectively react with aryne intermediates generated at elevated temperature with CsF, while less stable N-haloamines are found more efficient under milder, TBAF-mediated aryne formation at room temperature. Both protocols demonstrate a good level of regioselectivity and functional group tolerance. Efforts to elucidate the mechanism of N-X insertion are also discussed. The practical value of this transformation is highlighted by rapid synthesis of novel analogues of the antipsychotic cariprazine.

Co(III)-catalyzed C-H activation/formal SN-type reactions: selective and efficient cyanation, halogenation, and allylation

The first cobalt-catalyzed cyanation, halogenation, and allylation via C-H activation have been realized. These formal SN-type reactions generate valuable (hetero)aryl/alkenyl nitriles, iodides, and bromides as well as allylated indoles using a bench-stable Co(III) catalyst. High regio- and mono-selectivity were achieved for these reactions. Additionally, allylation proceeded efficiently with a turnover number of 2200 at room temperature, which is unprecedented for this Co(III) catalyst. Alkenyl substrates and amides have been successfully utilized in Cp*Co(III)-catalyzed C-H activation for the first time.

Ruthenium-catalyzed ortho-C-H halogenations of benzamides

[Ru3(CO)12] and AgO2C(1-Ad) enabled the first ruthenium-catalyzed intermolecular halogenations of arenes via C-H activation. Thereby, brominations and iodinations of electron-rich and electron-deficient benzamides were achieved in a highly selective fashion.

Room-temperature ortho-alkoxylation and -halogenation of N-tosylbenzamides by using palladium(II)-catalyzed C-H activation

The N-tosylcarboxamide group can direct the room-temperature palladium-catalyzed C-H alkoxylation and halogenation of substituted arenes in a simple and mild procedure. The room-temperature stoichiometric cyclopalladation of N-tosylbenzamide was first studied, and the ability of the palladacycle to react with oxidants to form C-X and C-O bonds under mild conditions was demonstrated. The reaction conditions were then adapted to promote room-temperature ortho-alkoxylations and ortho-halogenations of N-tosylbenzamides using palladium as catalyst. The scope and limitation of both alkoxylations and halogenations was studied and the subsequent functional transformation of the N-tosylcarboxamide group through nucleophilic additions was evaluated. This methodology offers a simple and mild route to diversely functionalized arenes.

Insight into the efficiency of cinnamyl-supported precatalysts for the Suzuki-Miyaura reaction: observation of Pd(I) dimers with bridging allyl ligands during catalysis

Despite widespread use of complexes of the type Pd(L)(η(3)-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd(I) dimers of the form (μ-allyl)(μ-Cl)Pd2(L)2 has been underexplored. Here, the relationship between the monomeric and the dimeric compounds is investigated using both experiment and theory. We report an efficient synthesis of the Pd(I) dimers (μ-allyl)(μ-Cl)Pd2(IPr)2 (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) through activation of Pd(IPr)(η(3)-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd(II) monomers and their Pd(I) μ-allyl dimer congeners for the Suzuki-Miyaura reaction is compared. We propose that the (μ-allyl)(μ-Cl)Pd2(IPr)2-type dimers are activated for catalysis through disproportionation to Pd(IPr)(η(3)-allyl)Cl and monoligated IPr-Pd(0). The microscopic reverse comproportionation reaction of monomers of the type Pd(IPr)(η(3)-allyl)Cl with IPr-Pd(0) to form Pd(I) dimers is also studied. It is demonstrated that this is a facile process, and Pd(I) dimers are directly observed during catalysis in reactions using Pd(II) precatalysts. In these catalytic reactions, Pd(I) μ-allyl dimer formation is a deleterious process which removes the IPr-Pd(0) active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd(IPr)(η(3)-crotyl)Cl and Pd(IPr)(η(3)-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr-Pd(0) catalyst to enter the catalytic cycle when these substituted precatalysts are used. Furthermore, we have developed reaction conditions for the Suzuki-Miyaura reaction using Pd(IPr)(η(3)-cinnamyl)Cl which are compatible with mild bases.

Storable palladacycles for selective functionalization of alkyne-containing proteins

We report the facile preparation of palladacycles as storable arylpalladium(II) reagents from acetanilides via cyclopalladation. The palladacycles exhibit good stability in PBS buffer and are capable of functionalizing a metabolically encoded HPG-containing protein, thus providing a new type of biocompatible organometallic reagent for selectively functionalizing the alkyne-encoded proteins.

DDQ: the chlorinating reagent and oxidant for the ligand-directed ortho-chlorination of 2-arylpyridines

Dual roles: DDQ behaved as both the chlorinating reagent and oxidant in palladium-catalyzed ligand-directed ortho-chlorination of 2-arylpyridines.

Twofold unsymmetrical C-H functionalization of PyrDipSi-substituted arenes: a general method for the synthesis of substituted meta-halophenols

Two-fold unsymmetrical functionalization of arenes, which includes sequential halogenation/oxygenation reactions, has been developed. This transformation allows for a general and efficient synthesis of substituted meta -halophenols from simple aryl iodides. Importantly, the PyrDipSi (2-diisopropylsilylpyrimidine) group can be easily removed or efficiently converted into valuable functionalities.

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

Pd-catalyzed ortho-C-H iodination directed by a weakly coordinating amide auxiliary using I2 as the sole oxidant was developed. This reaction is compatible with a wide range of heterocycles including pyridines, imidazoles, oxazoles, thiazoles, isoxazoles, and pyrazoles.

Insertion of arynes into N-halo bonds: a direct approach to o-haloaminoarenes

A new approach to access o-haloaminoarenes has been achieved by insertion of arynes into a nitrogen-halide bond (N-X). This transition-metal-free transformation displays a broad substrate scope of arynes, good compatibility with functional groups, and high regioselectivity. Representative transformations of the o-haloaminoarenes are described to highlight their utility for rapid access to diversely functionalized aminoarene derivatives.