Scope and selectivity in palladium-catalyzed directed C–H bond halogenation reactions

Experimental and Computational Development of a Conformationally Flexible Template for the meta-C-H Functionalization of Benzoic Acids

A conformationally flexible template for the meta-C-H olefination of benzoic acids was designed through both experimental and computational efforts. The newly designed template favors a silver-palladium heterodimer low barrier transition state, and demonstrates that it is feasible to lengthen templates so as to achieve meta-selectivity when the distance between the functional handle of the native substrate and target C-H bond decreases. Analysis of the ortho-, meta-, and para-C-H cleavage transition states determined that the new template conformation optimizes the interaction between the nitrile and palladium-silver dimer in the meta-transition state, enabling palladium to cleave meta-C-H bonds with moderate-to-good yields and generally high regioselectivity. Regioselectivity is governed exclusively by the template, and kinetic experiments reveal that there is a 4-fold increase in rate in the presence of monoprotected amino acid ligands. Using a Boltzmann distribution of all accessible C-H activation transition states, it is possible to computationally predict meta-selectivity in a number of investigated templates with reasonable accuracy. Structural and distortion energies reported may be used for the further development of templates for meta-C-H activation of hitherto unexplored arene substrates.

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.

Ketone-catalyzed photochemical C(sp3)-H chlorination

Photoexcited arylketones catalyze the direct chlorination of C(sp3)-H groups by N-chlorosuccinimide. Acetophenone is the most effective catalyst for functionalization of unactivated C-H groups while benzophenone provides better yields for benzylic C-H functionalization. Activation of both acetophenone and benzophenone can be achieved by irradiation with a household compact fluorescent lamp. This light-dependent reaction provides a better control of the reaction as compared to the traditional chlorination methods that proceed through a free radical chain propagation mechanism.

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.

Ligand-Enabled Pd(II)-Catalyzed Bromination and Iodination of C(sp3)-H Bonds

We herein report the palladium(II)-catalyzed bromination and iodination of a variety of α-hydrogen-containing carboxylic acid and amino acid-derived amides. These reactions are exclusively enabled by quinoline-type ligands. The halogenated products obtained in this reaction are highly versatile and rapidly undergo further diversification. Further, we report the first example of a free carboxylic acid-directed Pd(II)-catalyzed C(sp³)-H bromination, enabled by quinoline ligands.

Silver-Catalyzed C(sp3)-H Chlorination

A silver-catalyzed chlorination of benzylic, tertiary, and secondary C(sp³)-H bonds was developed. The reaction proceeded with as low as 0.2 mol % catalyst loading at room temperature under air atmosphere with synthetically useful functional group compatibility. The regioselectivity and reactivity tendencies suggest that the chlorination proceeded through a radical pathway, but an intermediate alkylsilver species cannot be ruled out.

Palladium-Catalyzed, ortho-Selective C-H Halogenation of Benzyl Nitriles, Aryl Weinreb Amides, and Anilides

A palladium-catalyzed, ortho-selective C-H halogenation methodology is reported herein. The highlight of the work is the highly selective C(sp²)-H functionalization of benzyl nitriles in the presence of activated C(sp³)-H bond, which results in good yields of the halogenated products with excellent regioselectivity. Along with benzyl nitriles, aryl Weinreb amides and anilides have been evaluated for the transformation using aprotic conditions. Mechanistic studies yield interesting aspects with respect to the pathway of the reaction and the directing group abilities.

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.

Three-step synthesis of 2,5,7-trisubstituted indoles from N-acetyl-2,4,6-trichloroaniline using Pd-catalyzed site-selective cross-coupling

A facile three-step synthesis of 2,5,7-trisubstituted indoles was realized by using Cy-DHTP as an effective ligand for site-selective cross couplings.

Pd(OAc)2-catalysed regioselective alkoxylation of aryl (β-carbolin-1-yl)methanones via β-carboline directed ortho-C(sp(2))-H activation of an aryl ring

Synthesis of (2-alkoxyphenyl)(9H-pyrido[3,4-b]indol-1-yl)methanone via Pd(OAc)2-catalyzed regioselective alkoxylation of aryl (β-carbolin-1-yl) methanones employing β-carboline directed ortho-C(sp(2))-H activation of an aryl ring under oxidative conditions is described.

Palladium-catalyzed picolinamide-directed iodination of remote ortho-C-H bonds of arenes: Synthesis of tetrahydroquinolines

A new palladium-catalyzed picolinamide (PA)-directed ortho-iodination reaction of ε-C(sp²)−H bonds of γ-arylpropylamine substrates is reported. This reaction proceeds selectively with a variety of γ-arylpropylamines bearing strongly electron-donating or withdrawing substituents, complementing our previously reported PA-directed electrophilic aromatic substitution approach to this transformation. As demonstrated herein, a three step sequence of Pd-catalyzed γ-C(sp³)−H arylation, Pd-catalyzed ε-C(sp²)−H iodination, and Cu-catalyzed C−N cyclization enables a streamlined synthesis of tetrahydroquinolines bearing diverse substitution patterns.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Palladium-Catalyzed Carbamate-Directed Regioselective Halogenation: A Route to Halogenated Anilines

This study describes an efficient method for ortho-selective halogenation of N-arylcarbamates under mild conditions for the first time. Although being weakly coordinating, N-arylcarbamates act very well as a removable directing group for activation of C-H bonds. The developed procedure results in extremely valuable halogenated N-arylcarbmates that can further be hydrolyzed to halogenated anilines. The obtained reaction conditions showed broad scope and wide functional group tolerance. All the products were formed in good yields with extremely high selectivity.

Regioselectivity in C-H activation: reagent control in cyclometallation of 2-(1-naphthyl)-pyridine

2-(1-Naphthyl)-pyridine () possesses sp(2) C-H bonds in both the γ- and δ-positions and is therefore a suitable substrate for studying the cyclometallation selectivity with different reagents and conditions. Such selectivity studies are reported. Based on deuterium-exchange experiments it is concluded that cycloruthenation with RuCl2(p-cymene) dimer is reversible with kinetic and thermodynamic preference for γ-substitution. Electrophilic cycloborylation, on the other hand, shows unusual δ-substitution. The previously published cyclopalladation and cycloauration of the substrate was studied in detail and was shown to be irreversible; they proceed under kinetic control and give γ- and δ-substitution for palladium and gold, respectively.

Pd(II) catalyzed ortho C-H iodination of phenylcarbamates at room temperature using cyclic hypervalent iodine reagents

A novel approach to access ortho iodinated phenols using cyclic hypervalent iodine reagents through palladium(II) catalyzed C-H activation has been developed through weak coordination. The reaction showed excellent regioselectivity, reactivity and good functional group tolerance. A unique mechanism was proposed.

Synthesis of Redshifted Azobenzene Photoswitches by Late-Stage Functionalization

Azobenzenes are versatile photoswitches that can be cycled between their trans- and cis-configuration with light. The wavelengths required for this isomerization are substantially shifted from the UV to the visible range through tetra-ortho-chlorination. These halogenated azobenzenes display unique photoswitching characteristics, but their syntheses remain limited and inefficient. A new general method for the synthesis of tetra-ortho-chloro azobenzenes has been developed, which relies on direct palladium(II)-catalyzed C-H activation of pre-existing standard azobenzenes. This late-stage functionalization has a broad substrate scope and can be used to create a variety of useful building blocks for the construction of more elaborate redshifted photopharmaceuticals. This method is used to prepare red-AzCA-4, a photoswitchable vanilloid that enables optical control of the cation channel TRPV1 with visible light.

Auxiliary-assisted palladium-catalyzed halogenation of unactivated C(sp3)–H bonds at room temperature

The direct transformation of unactivated C(sp³)–H bonds into C-halogen bonds was achieved by palladium catalysis at room temperature with good functional group tolerance.

Synthesis of esters via sp3 C–H functionalisation

This review describes various methods for the synthesis of eleven classes of esters involving sp³ C–H functionalisation.

The medicinal chemist's toolbox for late stage functionalization of drug-like molecules

The advent of modern C–H functionalization chemistries has enabled medicinal chemists to consider a synthetic strategy, late stage functionalization (LSF), which utilizes the C–H bonds of drug leads as points of diversification for generating new analogs.

Efficient, versatile and practical palladium-catalyzed highly regioselective ortho-halogenation of azoxybenzenes

A highly efficient and practical strategy of regio-selective ortho-halogenation of azoxybenzenes with NXS in the presence of palladium catalysts has been developed in good to excellent yields.

Palladium-catalyzed enolate arylation as a key C-C bond-forming reaction for the synthesis of isoquinolines

The palladium-catalyzed coupling of an enolate with an ortho-functionalized aryl halide (an α-arylation) furnishes a protected 1,5-dicarbonyl moiety that can be cyclized to an isoquinoline with a source of ammonia. This fully regioselective synthetic route tolerates a wide range of substituents, including those that give rise to the traditionally difficult to access electron-deficient isoquinoline skeletons. These two synthetic operations can be combined to give a three-component, one-pot isoquinoline synthesis. Alternatively, cyclization of the intermediates with hydroxylamine hydrochloride engenders direct access to isoquinoline N-oxides; and cyclization with methylamine, gives isoquinolinium salts. Significant diversity is available in the substituents at the C4 position in four-component, one-pot couplings, by either trapping the in situ intermediate after α-arylation with carbon or heteroatom-based electrophiles, or by performing an α,α-heterodiarylation to install aryl groups at this position. The α-arylation of nitrile and ester enolates gives access to 3-amino and 3-hydroxyisoquinolines and the α-arylation of tert-butyl cyanoacetate followed by electrophile trapping, decarboxylation and cyclization, C4-functionalized 3-aminoisoquinolines. An oxime directing group can be used to direct a C-H functionalization/bromination, which allows monofunctionalized rather than difunctionalized aryl precursors to be brought through this synthetic route.

Pd(II)-catalyzed, controllable C-H mono-/diarylation of aryl tetrazoles: concise synthesis of Losartan

A palladium(II)-catalyzed C-H arylation directed by tetrazole, a metabolically stable surrogate for the carboxylic acid group in drug design, has been developed. Excellent mono-/di-selectivity was achieved through adjustment of the protecting site on the tetrazole ring. The synthetic utility of this new transformation was demonstrated in the concise total synthesis of Losartan.

Rhenium-catalyzed C-H aminocarbonylation of azobenzenes with isocyanates

The first C-H aminocarbonylation of azobenzenes with isocyanates is achieved by using rhenium-catalysis, which provides an expedient and atom-economical access to varied o-azobenzamides from readily available starting materials. The reaction efficiency can be enhanced by the catalytic use of sodium acetate via accelerated C-H bond activation.