Recent advances in transition metal (Pd, Ni)-catalyzed C(sp 3 ) H bond activation with bidentate directing groups

Metal-Carbon Bond Heterolysis Energy Scale for Model Palladium Catalysts

Thermodynamic studies of transition-metal intermediates are crucial for understanding of metal-catalyzed transformations. Herein, a series of arylpalladium cyanomethanides were synthesized and characterized. Their palladium-carbon bond heterolysis energies (ΔGhet(Pd-C)) were determined in DMSO for the first time by equilibrium methods. ΔGhet(Pd-C) values of 7.9-19.1 kcal/mol, located between the ΔGhet(Pd-O) and ΔGhet(Pd-N) scales previously established, are much smaller than the corresponding ΔGhet(C-H)s of phenylacetonitrile (30.0 kcal/mol). Linear free energy relationship (LEFR) analysis reveals insights into the structure-property relationship and the factor dictating the thermodynamics of metalation. These ΔGhet(Pd-X)s in combination with ΔGhet(X-H)s are successfully used to diagnose the reaction feasibility and selectivity of X-H bond activation.

Palladium-Catalyzed Arylation of C(sp2)-H Bonds and C(sp3)-H Bonds with 4-Amino-benzotriazole as the Bidentate Directing Group

The arylation of C(sp2)-H and C(sp3)-H bonds in carboxylic acids catalyzed by Pd(II) with 4-aminobentriazole as the directing group was investigated. In addition to activation of the C(sp2)-H bond, selective arylation of alkyl carboxylic acids and amino acids in the β position can also be achieved. This strategy involved a 5,5-bicyclic Pd intermediate complex whose structure was determined by X-ray single crystal diffraction analysis. Importantly, the DG (directing group) can be easily removed under mild conditions.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Pd(II)-catalyzed coupling of C-H bonds of carboxamides with iodoazobenzenes toward modified azobenzenes

In this paper, we report a synthetic protocol for the construction of biaryl motif-based or π-extended azobenzene and alkylated azobenzene derivatives via the Pd(II)-catalyzed bidentate directing group (DG)-aided C-H activation and functionalization strategy. In the past, the synthesis of biaryl motif-based azobenzenes was accomplished through the traditional cross-coupling reaction involving organometallic reagents and aryl halides or equivalent coupling partners. We have shown the direct coupling of C-H bonds of aromatic/aliphatic carboxamides (possessing a DG) with iodoazobenzenes as the coupling partners through the Pd(II)-catalyzed bidentate DG-aided, site-selective C-H functionalization method. Azobenzene-containing compounds are a versatile class of photo-responsive molecules that have found applications across branches of chemical, biological and materials sciences and are prevalent in medicinally relevant molecules. Accordingly, the synthesis of new and functionalized azobenzene-based scaffolds has been an attractive topic of research. Although the classical methods are efficient, they need pre-functionalized starting materials. This protocol involving the Pd(II)-catalyzed, directing group-aided site-selective C-H arylation of aromatic and aliphatic carboxamides using iodoazobenzene as the coupling partner affording azobenzene-based carboxamides is an additional route and also a contribution towards enriching the library of modified azobenzenes. We have also shown the photoswitching properties of representative compounds synthesized via the Pd(II)-catalyzed directing group-aided site-selective C-H functionalization method.

Pd(II)-catalyzed selective β-C-H functionalization of azobenzene carboxamides

We report a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided, site-selective β-C-H functionalization protocol for assembling modified azobenzene carboxamides. Considering the importance of azobenzenes in chemical sciences, this paper reports a new route for enriching the library of modified azobenzene motifs.

Overlooked potential of N,N-bidentate directing-groups in Ni-catalyzed C-H functionalization of benzamides

The Ni-catalyzed reactions of benzamides with bicyclic alkenes were explored using DFT calculations. An unprecedented "N-H deprotonation circumvented" catalytic mechanism was proposed, over the more common N-H/C-H activation mechanism, in which (i) the circumvention of N-H deprotonation ensures the presence of N-H⋯O hydrogen bond interaction, thereby stabilizing the critical ortho-C-H functionalization TS; and (ii) the N-H moiety retention results in a weak N⋯Ni σ-coordination, which is flexible to the configurational conversion during the key alkene insertion. These overlooked aspects of the functionalized N,N-bidentate directing groups will aid the design of new related catalytic reactions.

Redox-neutral rhodium(III)-catalyzed divergent synthesis of tetrasubstituted 1,3-enynes and alkynylated benzofurans

With the assistance of the acetamido directing group (DG), a rhodium-catalyzed C-H alkenylation/DG migration cascade for the synthesis of tetrasubstituted 1,3-enynes from N-phenoxyacetamides and 1,3-diynes has been achieved in this work. Alternatively, a rhodium-catalyzed [3 + 2] annulation for the synthesis of alkynylated benzofurans from the same set of substrates has also been achieved by simply changing the reaction conditions. This work highlights the tunable divergent synthesis of valuable compounds triggered by C-H activation.

Construction of carbazole-based unnatural amino acid scaffolds via Pd(II)-catalyzed C(sp3)-H functionalization

We report the synthesis of carbazole-based unnatural α-amino acid and non-α-amino acid derivatives via a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided β-C(sp³)-H activation/functionalization method. Various N-phthaloyl, DL-, L- and D-carboxamides derived from their corresponding α-amino acids, non-α-amino acids and aliphatic carboxamides were subjected to the β-C(sp³)-H functionalization with 3-iodocarbazoles in the presence of a Pd(II) catalyst to afford the corresponding carbazole moiety installed unnatural amino acid derivatives and aliphatic carboxamides. Carbazole motif-containing racemic (DL) and enantiopure (L and D) amino acid derivatives including phenylalanine, norvaline, leucine, norleucine and 2-aminooctanoic acid with anti-stereochemistry and various non-α-amino acid derivatives including GABA have been synthesized. Removal of the 8-aminoquinoline directing group, deprotection of the phthalimide moiety and the preparation of carbazole amino acid derivatives containing free amino- and carboxylate groups are shown. The carbazole motif is prevalent in alkaloids and biologically active molecules and functional materials. Thus, this work on the synthesis of carbazole-based unnatural amino acid derivatives would enrich the libraries of unnatural amino acid derivatives and carbazoles.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

Computational insights into Ir(iii)-catalyzed allylic C-H amination of terminal alkenes: mechanism, regioselectivity, and catalytic activity

Computational studies on Ir(iii)-catalyzed intermolecular branch-selective allylic C–H amination of terminal olefins with methyl dioxazolone have been carried out to investigate the mechanism, including the origins of regioselectivity and catalytic activity difference. The result suggests that the reaction proceeds through generation of active species, alkene coordination, allylic C–H activation, decarboxylation, migratory insertion, and protodemetalation. The presence of AgNTf₂ could thermodynamically promote the formation of catalytically active species [Cp*Ir(OAc)]⁺. Both the weaker Ir–C(internal) bond and the closer interatomic distance of N⋯C(internal) in the key allyl-Ir(v)-nitrenoid intermediate make the migratory insertion into Ir–C(internal) bond easier than into the Ir–C(terminal) bond, leading to branch-selective allylic C–H amidation. The high energy barrier for allylic C–H activation in the Co system could account for the observed sluggishness, which is mainly ascribed to the weaker coordination capacity of alkenes to the triplet Cp*Co(OAc)⁺ and the deficient metal⋯H interaction to assist hydrogen transfer.

DFT studies on Ir(iii)-catalyzed branch-selective allylic C–H amination of terminal olefins with methyl dioxazolone have been carried out to investigate the mechanism, including the origins of regioselectivity and catalytic activity difference.

The role of hypervalent iodine(iii) reagents in promoting alkoxylation of unactivated C(sp3)-H bonds catalyzed by palladium(ii) complexes

Although Pd(OAc)2-catalysed alkoxylation of the C(sp3)-H bonds mediated by hypervalent iodine(iii) reagents (ArIX2) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations. In this study, we shed light on this important issue with the aid of the density functional theory (DFT) calculations for alkoxylation of butyramide derivatives. We found that the previously proposed mechanism in the literature is not consistent with the experimental observations and thus cannot be operating. The calculations allowed us to discover an unprecedented mechanism composed of four main steps as follows: (i) activation of the C(sp3)-H bond, (ii) oxidative addition, (iii) reductive elimination and (iv) regeneration of the active catalyst. After completion of step (i) via the CMD mechanism, the oxidative addition commences with an X ligand transfer from the iodine(iii) reagent (ArIX2) to Pd(ii) to form a square pyramidal complex in which an iodonium occupies the apical position. Interestingly, a simple isomerization of the resultant five-coordinate complex triggers the Pd(ii) oxidation. Accordingly, the movement of the ligand trans to the Pd-C(sp3) bond to the apical position promotes the electron transfer from Pd(ii) to iodine(iii), resulting in the reduction of iodine(iii) concomitant with the ejection of the second X ligand as a free anion. The ensuing Pd(iv) complex then undergoes the C-O reductive elimination by nucleophilic attack of the solvent (alcohol) on the sp3 carbon via an outer-sphere SN2 mechanism assisted by the X- anion. Noteworthy, starting from the five coordinate complex, the oxidative addition and reductive elimination processes occur with a very low activation barrier (ΔG ‡ 0-6 kcal mol-1). The strong coordination of the alkoxylated product to the Pd(ii) centre causes the regeneration of the active catalyst, i.e. step (iv), to be considerably endergonic, leading to subsequent catalytic cycles to proceed with a much higher activation barrier than the first cycle. We also found that although, in most cases, the alkoxylation reactions proceed via a Pd(ii)-Pd(iv)-Pd(ii) catalytic cycle, the other alternative in which the oxidation state of the Pd(ii) centre remains unchanged during the catalysis could be operative, depending on the nature of the organic substrate.

Palladium-catalyzed functionalizations of acidic and non-acidic C(sp3)-H bonds - recent advances

A tremendous upsurge has been seen in the recent decade for the proximal and remote functionalization of activated and unactivated substrates via palladium redox pathways. This feature article discusses some of the recent reports on direct as well as indirect C(sp3)-H functionalization via cross-coupling reactions under palladium catalysis. Activated substrates (possessing acidic C(sp3)-H) including enones, ketones, aldehydes, silylenol ethers, esters, silyl ketene acetals, amides, cyano, α-amino esters, and O-carbamates, capable of undergoing cross-coupling reactions at the α-, β-, γ-, δ- and ε-positions, will be discussed. To overcome the challenging task of achieving regioselectivity, a variety of innovative modifications have been reported. The reports of C-H activations based on directing group, and as native functionality have been illustrated at the β-, γ- and δ-positions. Substrates such as α-amino esters, carbonyls, carboxylic acids and their derivatives, afford site-selective C(sp3)-H functionalization via varied-sized reactive metallacycles and are a unique class of substrates whose C(sp3)-H functionalizations were earlier considered as very difficult.

Palladium-catalyzed directed synthesis of ortho-deuterated phenylacetic acid and analogues

The synthesis of deuterium-labeled organic compounds is of increased interest, especially after the approval of deutetrabenazine by the Food and Drug Administration in 2014. The selective incorporation of deuterium in the place of hydrogen not only represents uniqueness in terms of a novel chemical class, but it also can improve the pharmacokinetic profiles of drug molecules while retaining potency and other parameters; thus, hydrogen-deuterium (H/D) exchange methods have been proven to be powerful additions in different areas of chemical science. In that regard, metal-catalyzed deuterium labeling via C-H activation mediated by a unique inbuilt directing group (DG) can play a significant role in the synthesis of novel deuterated chemical entities. In this context, herein, we divulge our results relating to Pd(ii)-catalyzed deuterium incorporation (>97%) at the γ C(sp2)-position of pyridone-containing phenylacetic acid derivatives, where 3-amino-1-methyl-1H-pyridin-2-one (AMP) not only acts as an efficient N,O-directing group, but it also constitutes a part of the target molecules of medicinal importance. Our methodology, which has been optimized based on the effects of temperature, catalyst, time, and substrate scope, shows advantages over existing protocols, with non-selectivity or meager deuteration or the use of an expensive metal (catalytic or super stoichiometric) and a deuterated solvent, reported previously for the deuteration of phenylacetic acid and its derivatives. Moreover, towards our aim of synthesizing deuterium-labeled biologically relevant compounds, the gram scale synthesis of a deuterated analogue of biphenyl acetic acid (3), known to have activity against epileptic seizures, has also been successfully accomplished in high yields and with excellent isotope enrichment via implementing this protocol.

Recent advances in the development of palladium nanocatalysts for sustainable organic transformations

In this review, we highlighted Pd nanocatalysts which have been used in the development of sustainable organic transformations including transfer hydrogenation, C–H bond activation, and some carbon–carbon couplings in the last five years.

Palladium catalyzed 8-aminoimidazo[1,2-a]pyridine (AIP) directed selective β-C(sp2)–H arylation

An unprecedented palladium(ii) catalyzed selective β-C(sp²)–H arylation reaction using an 8-AIP (aminoimidazo[1,2-a]pyridine) auxiliary as a new N,N-bidentate directing group has been demonstrated.

Multiple Catalytic C-H Bond Functionalization for Natural Product Synthesis

In the past decade, multiple catalytic C-H bond functionalization has been successfully applied in natural product synthesis as a strategy to reduce the number of steps, increase overall yield and employ more easily available starting materials. This minireview presents selected examples making use of multiple C-H bond functionalization in conceptually different ways. First, linear syntheses are discussed, wherein multiple C-H functionalization is employed either from simple (hetero)cyclic cores, at a late stage, or to build polycyclic systems. Second, the use of multiple C-H functionalization as a strategic tool in convergent synthesis to access and couple complex fragments is discussed. Information on the scalability of the employed methods is provided when available. The presented cases indicate that multiple C-H functionalization strategies should play a great role to shape the future synthesis of functional complex molecules with improved sustainability.

The Mechanism of Rhodium-Catalyzed Allylic C-H Amination

Herein, the mechanism of catalytic allylic C-H amination reactions promoted by Cp*Rh complexes is reported. Reaction kinetics experiments, stoichiometric studies, and DFT calculations demonstrate that the allylic C-H activation to generate a Cp*Rh(π-allyl) complex is viable under mild reaction conditions. The role of external oxidants in the catalytic cycle is elucidated. Quantum mechanical calculations, stoichiometric reactions, and cyclic voltammetry experiments concomitantly support an oxidatively induced reductive elimination process of the allyl fragment with an acetate ligand proceeding through a Rh(IV) intermediate. Stoichiometric oxidation and bulk electrolysis of the proposed π-allyl intermediate are also reported to support these analyses. Lastly, evidence supporting the amination of an allylic acetate intermediate is presented. We show that Cp*Rh(III)²⁺ behaves as a Lewis acid catalyst to complete the allylic amination reaction.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Nickel(II/IV) Manifold Enables Room-Temperature C(sp3)-H Functionalization

This Article demonstrates a mild oxidatively induced C(sp³)-H activation at a high-valent Ni center. In contrast with most C(sp³)-H activation reactions at Ni^II, the transformation proceeds at room temperature and generates an isolable Ni^IV σ-alkyl complex. Density functional theory studies show two plausible mechanisms for this C-H activation process involving triflate-assisted C-H cleavage at either a Ni^IV or a Ni^III intermediate. The former pathway is modestly favored over the latter (by ∼3 kcal/mol). The Ni^IV σ-alkyl product of C-H cleavage reacts with a variety of nucleophiles to form C(sp³)-X bonds (X = halide, oxygen, nitrogen, sulfur, or carbon). These stoichiometric transformations can be coupled using N-fluoro-2,4,6-trimethylpyridinium triflate as a terminal oxidant in conjunction with chloride as a nucleophile to achieve a proof-of-principle Ni^II/IV-catalyzed C(sp³)-H functionalization reaction.

Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp2)-H and C(sp3)-H Functionalization of Aminoquinoline Substrates

This Article describes the synthesis and characterization of cyclometalated aminoquinoline Ni^II σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These Ni^II complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I₂. They also react stoichiometrically with I₂ to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with Ag^I salts at -30 °C to afford isolable five-coordinate Ni^III species. The Ni^III σ-aryl complexes proved inert toward C(sp²)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a Ni^III σ-alkyl analogue underwent C(sp³)-N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter Ni^III species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of Ni^III species in these C-H functionalization reactions.

σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of -2 at a transition-metal center. For a series of formal high-valent NiIV complexes, aryl-CF3 bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034-8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal NiIV centers are better described as approaching a +II oxidation state, originating from highly covalent metal-ligand bonds, a phenomenon attributable to σ-noninnocence. A direct consequence is that the elimination of aryl-CF3 products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of σ-noninnocence in metal-ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

Computational Study of the Ni-Catalyzed C-H Oxidative Cycloaddition of Aromatic Amides with Alkynes

The mechanism of Ni-catalyzed ortho C(sp²)-H oxidative cycloaddition of aromatic amides with internal alkynes containing 2-pyridinylmethylamine directing group was investigated using density functional theory (DFT) calculations. The C-H cleavage step proceeds via σ-complex-assisted metathesis (σ-CAM) with an alkenyl-Ni(II) complex. This is in contrast to the more common carboxylate/carbonate-assisted concerted metalation-deprotonation mechanism in related Ni-catalyzed C-H bond functionalization reactions with N,N-bidentate directing groups. In this reaction, the alkyne not only serves as the coupling partner, but also facilitates the σ-CAM C-H metalation both kinetically and thermodynamically. The subsequent functionalization of the five-membered nickelacycle proceeds via alkyne insertion into the Ni-C bond to form a seven-membered nickelacycle. This process proceeds with high levels of regioselectivity to form a C-C bond with sterically more encumbered alkyne terminus. This unusual regioselectivity is due to steric repulsions with the directing group that is coplanar with the alkyne in the migratory insertion transition state. The C-N bond reductive elimination to form the isoquinolone cycloadduct is promoted by PPh₃ complexation to the Ni center and the use of flexible 2-pyridinylmethylamine directing group. The origin of the cis-trans isomerism of alkene byproduct was also explained by computations.

Direct Sulfoxidation of Aromatic Methyl Thioethers with Aryl Halides by Copper-Catalyzed C(sp3)–H Bond Activation

A copper-catalyzed direct sulfoxidation reaction by C(sp3)–H bond activation has been developed. Starting from sample aromatic methyl thioethers with aryl halides, versatile biologically-active arylbenzylsulfoxide derivatives were efficiently synthesized in good to high yields under mild conditions. This new methodology provides an economical approach toward C(sp3)–C(sp2) bond formation.

A computational mechanistic study of Ni(0)-catalyzed annulation of aromatic amides with alkynes: the effects of directing groups

The mechanism of Ni(0)-catalyzed annulation of aromatic amides with alkynes was studied theoretically to investigate the effects of directing groups.