Palladium(II)-catalyzed direct alkoxylation of arenes: evidence for solvent-assisted concerted metalation deprotonation

Palladium-Catalyzed Direct C(sp2)-H Cyanomethylation of Arylamides using Chloroacetonitrile

In this study, we devised a palladium-catalyzed efficient and versatile method for C(sp2)-H ortho-cyanomethylation of arylamides with a broad substrate scope and moderate to excellent yields. An inexpensive and commercially available chloroacetonitrile was employed as a cyanomethylating source. This method is also compatible with the air atmosphere. Further, the synthetic feasibility of this technique is established by gram-scale synthesis and functional group transformation of the products.

Diastereoselective palladium-catalyzed C(sp3)-H cyanomethylation of amino acid and carboxylic acid derivatives

In this study, we report an efficient protocol for Pd-catalyzed methylene β-C(sp³)-H cyanomethylation of 8-aminoquinoline-directed α-amino acids using inexpensive chloroacetonitrile. Iodoacetonitrile generated in situ from chloroacetonitrile reacts with methylene C(sp³)-H bonds of α-amino acids with excellent diastereoselectivity, enabling access to a wide range of important γ-cyano-α-amino acids. Our protocol works well with different amino acid and carboxylic acid derivatives with good chemical yields and high functional group tolerance.

C-H Bond Activation Mechanism by a Pd(II)-(μ-O)-Au(0) Structure Unique to Heterogeneous Catalysts

We focused on identifying a catalytic active site structure at the atomic level and elucidating the mechanism at the elementary reaction level of liquid-phase organic reactions with a heterogeneous catalyst. In this study, we experimentally and computationally investigated efficient C-H bond activation for the selective aerobic α,β-dehydrogenation of saturated ketones by using a Pd-Au bimetallic nanoparticle catalyst supported on CeO₂ (Pd/Au/CeO₂) as a case study. Detailed characterization of the catalyst with various observation methods revealed that bimetallic nanoparticles formed on the CeO₂ support with an average size of about 2.5 nm and comprised a Au nanoparticle core and PdO nanospecies dispersed on the core. The formation mechanism of the nanoparticles was clarified through using several CeO₂-supported controlled catalysts. Activity tests and detailed characterizations demonstrated that the dehydrogenation activity increased with the coordination numbers of Pd-O species in the presence of Au(0) species. Such experimental evidence suggests that a Pd(II)-(μ-O)-Au(0) structure is the true active site for this reaction. Based on density functional theory calculations using a suitable Pd₁O₂Au₁₂ cluster model with the Pd(II)-(μ-O)-Au(0) structure, we propose a C-H bond activation mechanism via concerted catalysis in which the Pd atom acts as a Lewis acid and the adjacent μ-oxo species acts as a Brønsted base simultaneously. The calculated results reproduced the experimental results for the selective formation of 2-cyclohexen-1-one from cyclohexanone without forming phenol, the regioselectivity of the reaction, the turnover-limiting step, and the activation energy.

The annulation of N-hydroxyoximes and 1,3-diyne to synthesize alkynylated isoquinolines regioselectively catalyzed by ruthenium: a theoretical study

The mechanisms of the regioselective annulation of N-hydroxyoximes and 1,3-diyne to synthesize alkynylated isoquinolines by using catalyst [RuCl2(p-cymene)]2 have been theoretically investigated with the aid of density functional theory (DFT)...

Molecular Insights on Solvent Effects in Organic Reactions as Obtained through Computational Chemistry Tools

Molecular understanding of the role of protic solvents in a gamut of organic transformations can be developed using density functional and ab initio computational studies focused on the reaction mechanism. Inclusion of explicit solvent molecules in the vital TSs has been proven to be valuable toward improving the energetic estimates of organocatalytic as well as transition-metal-catalyzed organic reactions. Herein, we provide an overview of the importance of an explicit-implicit solvation model using a number of interesting examples.

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.

Advances in Transition-Metal-Catalyzed C–H Bond Oxygenation of Amides

C–O bond formation represents a fundamental chemical transformation in organic synthesis to develop valuably oxygenated (hetero)arenes. Particularly, the direct and regioselective C–H bond oxygenation of privileged amides, using a transition metal catalyst and a mild oxygenating source, is a step-economy and attractive approach. During the last decade, considerable progress has been realized in the direct C–H oxygenation of primary, secondary, and tertiary amides. This Short Review compiles the advances in transition-metal-catalyzed oxygenation of C(sp2)–H and C(sp3)–H bonds on various amides with diverse oxygenation sources. The review is categorized into two different major sections: (i) C(sp2)–H oxygenation and (ii) C(sp3)–H oxygenation. Each section is discussed based on the directing group (monodentate and bidentate) attached to the amide derivatives.

1 Introduction

2 C(sp2)–H Oxygenation

2.1 Monodentate Directed

2.2 Bidentate Directed

3 C(sp3)–H Oxygenation

3.1 Monodentate Directed

3.2 Bidentate Directed

4 Conclusion and Outlook

Scaling Relationships and Volcano Plots in Homogeneous Catalysis

Scaling relations and volcano plots are widely used in heterogeneous catalysis. In this Perspective, we discuss the prospects and challenges associated with the application of similar concepts in homogeneous catalysis using examples from the literature that have appeared recently.

Computational Study on the Fate of Oxidative Directing Groups in Ru(II), Rh(III), and Pd(II) Catalyzed C-H Functionalization

Activation of C-H bonds assisted by a directing group is indispensable in organic synthesis. Among them, utilizing oxidative directing groups that can serve as an internal oxidant to drive the Mⁿ/Mⁿ⁺² catalytic cycle has recently become a promising strategy. A survey of published reactions involving N-alkoxyamides or N-acyloxyamides reveals that not all N-O bonds act as an internal oxidant. We have therefore systematically investigated the effect of the oxidative groups on a model reaction catalyzed by Ru(II), Rh(III), and Pd(II) complexes. DFT calculations show that N-methoxy and N-acyloxy groups oxidize Ru(II) to Ru(IV) and Rh(III) to Rh(V), but cannot oxidize a cyclo-Pd(II) intermediate to Pd(IV). The stability of the metal imido intermediate 7-M (M = Ru, Rh, and Pd) controls whether the oxidation occurs or not. N-Acyloxy groups show a more pronounced selectivity than N-methoxy to oxidize Ru(II) and Rh(III) species, while no distinctive effect is observed for Pd(II).

Theoretical studies on the N–X (X = Cl, O) bond activation mechanism in catalytic C–H amination

A favorable S_N2-type N–Cl bond cleavage mechanism are proposed for Rh-catalysed C–H amination, which also works for N–O bond cleavage in Rh, Ru, and Pd analogous systems. These results could provide new understanding of C–H amination.

Silver-Mediated Methoxylation of Aryl C(sp2)–H Bonds Directing by DMEDA

The first example of silver-mediated phosphine-promoted methoxylation of aryl C(sp2)–H bonds with the commercially available reagent for the preparation of alkyl aryl ethers has been developed. This protocol is characterized by mild reaction conditions, broad substrate scope, and high regioselectivity.

Multiple pathways for C–H cleavage in cationic Cp*Rh(iii)-catalyzed C–H activation without carboxylate assistance: a computational study

Multiple pathways for C–H cleavage was uncovered in cationic Cp*Rh(iii)-catalyzed C–H functionalization with different heteroatom-containing species as possible proton acceptors.

Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: a DFT insight into the reaction mechanism

The reaction of N-ortho-chlorophenyl-substituted acetylpyridine hydrazones (a and d) with K₂[PtCl₄] (n-butanol/water, 100 °C) gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. In contrast, the reaction of N-phenyl or N-meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated species 2b and 2c with the ligand as [C,N,N] terdentate. The treatment of the mononuclear complexes 1a and 1d with NaOAc (n-butanol, 100 °C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle 2e was prepared in a similar fashion and its reaction with tertiary mono- and triphosphines gave mono- or trinuclear species depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed an aromatic character. With an aim to rationalize the different reactivities shown by acetylpyridine hydrazones and the precise role of the acetate anion, the energy profiles for the three main steps of cycloplatination (iminoplatinum complex formation, chelation and cyclometallation) have been determined by using the DFT (M06) methods. Calculations indicate that the cycloplatination of 1b proceeds via electrophilic substitution, involving the direct replacement of the chloride anion at the Pt(ii) centre with the N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b+ that, subsequently, leads to the elimination of a proton as hydrogen chloride. When present as an "external" base, acetate enters the coordination sphere around the Pt(ii) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Copper-promoted direct C–H alkoxylation of S,S-functionalized internal olefins with alcohols

Efficient copper-promoted direct C–H alkoxylation of internal olefins, α-oxo ketene dithioacetals, was achieved with alcohols as the alkoxylating reagents.

Role of Lewis acid additives in a palladium catalyzed directed C-H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl₂ and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl₂ and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl₂ are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl₂·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to C_aryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl₂ and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the C_aryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.

Palladium-catalysed directed C–H activation by anilides and ureas; water participation in a general base mechanism

A general-base pathway for CH-activation in anilides and ureas en route to palladacycles was developed through DFT calculations and fits available evidence.

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.

Cross-dehydrogenative coupling for the intermolecular C-O bond formation

The present review summarizes primary publications on the cross-dehydrogenative C-O coupling, with special emphasis on the studies published after 2000. The starting compound, which donates a carbon atom for the formation of a new C-O bond, is called the CH-reagent or the C-reagent, and the compound, an oxygen atom of which is involved in the new bond, is called the OH-reagent or the O-reagent. Alcohols and carboxylic acids are most commonly used as O-reagents; hydroxylamine derivatives, hydroperoxides, and sulfonic acids are employed less often. The cross-dehydrogenative C-O coupling reactions are carried out using different C-reagents, such as compounds containing directing functional groups (amide, heteroaromatic, oxime, and so on) and compounds with activated C-H bonds (aldehydes, alcohols, ketones, ethers, amines, amides, compounds containing the benzyl, allyl, or propargyl moiety). An analysis of the published data showed that the principles at the basis of a particular cross-dehydrogenative C-O coupling reaction are dictated mainly by the nature of the C-reagent. Hence, in the present review the data are classified according to the structures of C-reagents, and, in the second place, according to the type of oxidative systems. Besides the typical cross-dehydrogenative coupling reactions of CH- and OH-reagents, closely related C-H activation processes involving intermolecular C-O bond formation are discussed: acyloxylation reactions with ArI(O2CR)2 reagents and generation of O-reagents in situ from C-reagents (methylarenes, aldehydes, etc.).

Copper(ii)-catalyzed methoxylation of unactivated (hetero)aryl C–H bonds using a removable bidentate auxiliary

A copper-catalyzed methoxylation of unactivated (hetero)aryl C–H bonds has been developed.

Pd-catalysed ortho-alkoxylation of benzamides N-protected with an iminophosphorane functionality

The mild Pd-catalysed ortho-alkoxylation of benzamides, protected as keto-stabilised iminophosphoranes, with alcohols, is regioselective and tolerates different substituents and alcohols.

Palladium-catalyzed ortho-alkoxylation of 2-aryl-1,2,3-triazoles

Palladium-catalyzed alkoxylation of 2-aryl-1,2,3-triazoles was described in the presence of various groups in the aromatic rings. In addition, some other directing groups of heterocycles containing nitrogen were explored.

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.

Non-innocent additives in a palladium(II)-catalyzed C-H bond activation reaction: insights into multimetallic active catalysts

The role of a widely employed additive (AgOAc) in a palladium acetate-catalyzed ortho-C-H bond activation reaction has been examined using the M06 density functional theory. A new hetero-bimetallic Pd-(μ-OAc)3-Ag is identified as the most likely active species. This finding could have far-reaching implications with respect to the notion of the active species in palladium catalysis in the presence of other metal salt additives.

Palladium-catalyzed direct ortho alkoxylation of aromatic azo compounds with alcohols

An efficient and highly regioselective synthesis of 2-alkoxy aromatic azo compounds via palladium(II)-catalyzed alkoxylation of azobenzene derivatives directed by the azo group using alcohols as the alkoxylation reagents and PhI(OAc)2 as the oxidant has been developed. The method is applicable to both primary and secondary alcohols and affords moderate to good yields.

Palladium-catalyzed ortho-alkoxylation of anilides via C-H activation

A palladium-catalyzed ortho-alkoxylation of anilides with both primary and secondary alcohols via ligand-directed C-H activation has been explored. This alkoxylation promoted by catalytic methanesulfonic acid proceeds well at room temperature in most cases and affords aryl alkyl ethers in moderate to good yields.