Ligand-Promoted ortho -CH Amination with Pd Catalysts

Palladium-Catalyzed para-Selective C-H Olefination and Acetoxylation of Benzylamines Enabled by an Unmasked Benzoyl Template

Benzylamines belong to an important class of building blocks in the synthesis of biologically active natural products and drugs. Encumbered by amide-directed ortho-C-H activation, remote para-selective C-H functionalization of benzylamines has hitherto not been realized to date. Here, we report a palladium-catalyzed para-selective C-H olefination and acetoxylation of benzylamines using a functionalized benzoyl template. Enhancing the coordination strength of the directing group in the template significantly improved the site selectivity of C-H functionalization.

Mechanistic studies of the palladium-catalyzed S,O-ligand promoted C-H olefination of aromatic compounds

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has been shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions.

Ligand-Enabled Sequential C(sp3)-H and C(sp2)-H Diolefination Reaction via Palladium Catalyst

Palladium-catalyzed sequential C(sp³)-H and C(sp²)-H bond diolefination reaction of o-toluidine has been realized for the first time using acetyl-protected aminoethyl phenyl thioether ligands. This novel reaction allows for preparation of the conjugated diene structure via an immediate second olefination on the basis of the first C(sp³)-H olefination in one pot. Various triflyl-protected anilines and acrylates were used as coupling partners elegantly. Furthermore, the unpurified diolefination products can be easily converted to tetrahydroquinoline derivatives.

Palladium-Catalyzed Regioselective B(9)-Amination of o-Carboranes and m-Carboranes in HFIP with Broad Nitrogen Sources

Amination of carboranes has a good application prospect in organic and pharmaceutical synthesis. However, the current methods used for this transformation suffer from limitations. Herein, we report a practical method for a highly regioselective formation of a B-N bond by Pd(II)-catalyzed B(9)-H amination of o- and m-carboranes in hexafluoroisopropanol (HFIP) with different nitrogen sources under air atmosphere. The silver salt and HFIP solvent play critical roles in the present protocol. The mechanistic study reveals that the silver salt acts as a Lewis acid to promote the electrophilic palladation step by forming a heterobimetallic active catalyst PdAg(OAc)₃; the strong hydrogen-bond-donating ability and low nucleophilicity of HFIP enhance the electrophilic ability of Pd(II). It is believed that these N-containing carboranes are potentially of great importance in the synthesis of new pharmaceuticals.

Polysubstituted Indole Synthesis via Palladium/Norbornene Cooperative Catalysis of Oxime Esters

Polysubstituted indoles are prevalent in pharmaceuticals, agrochemicals, and organic materials. Presented herein is the fact that polyfunctionalized indoles can be efficiently constructed from easily accessible oxime esters and aryl iodides, involving a palladium/norbornene synergistic synthesis. The reaction is enabled by a unique class of electrophiles in palladium/norbornene cooperative catalysis, which are oxime esters derived from simple ketone. The broad substrate scope and high functional group tolerance could make this method attractive for the synthesis of polysubstituted indoles.

C–N coupling reactions with arenes through C–H activation: the state-of-the-art versus the principles of green chemistry

Herein, we discuss the state-of-the-art in arene C–N coupling through C–H activation and to what extent it complies with the principles of green chemistry, with a focus on heterogeneously catalysed systems.

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔG_het (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔG_het (Pd-X) scales span a range of 2.6-9.0 kcal mol^-1 for ΔG_het (Pd-O) values and of 14.5-19.5 kcal mol^-1 for ΔG_het (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔG_het (Pd-X) and pK_a (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Rh(iii)-catalyzed tandem annulative redox-neutral arylation/amidation of aromatic tethered alkenes

Transition-metal-catalyzed directed C–H functionalization has emerged as a powerful and straightforward tool to construct C–C bonds and C–N bonds. Among these processes, the intramolecular annulative alkene hydroarylation reaction has received much attention because this intramolecular annulation can produce more complex and high value-added structural motifs found in numerous natural products and bioactive molecules. Despite remarkable progress, these annulative protocols developed to date remain limited to hydroarylation and functionalization of one side of alkenes, thus largely limiting the structural diversity and complexity. Herein, we developed a rhodium(iii)-catalyzed tandem annulative arylation/amidation reaction of aromatic tethered alkenes to deliver a variety of 2,3-dihydro-3-benzofuranmethanamine derivatives bearing an all-carbon quaternary stereo center by employing 3-substituted 1,4,2-dioxazol-5-ones as an amidating reagent to capture the transient C(sp³)–Rh intermediate. Notably, by simply changing the directing group, a second, unsymmetrical ortho C–H amidation/annulation can be achieved to provide tricyclic dihydrofuro[3,2-f]quinazolinones in good yields.

A rhodium(iii)-catalyzed tandem annulative arylation/amidation reaction of aromatic tethered alkenes was developed to deliver a variety of 2,3-dihydro-3-benzofuranmethanamine derivatives.

Dirhodium(II)-Catalyzed C(sp2 )-H Azidation of Benzaldehydes

Multiple steps are needed to achieve the C-H functional of aromatic aldehyde, since the C-H functional reaction usually occurs preferentially at the aldehydic C-H bond over the aryl C-H bond. We report an efficient azidation method mediated by dirhodium(II) catalysts to achieve the direct aryl azidation of aromatic aldehydes avoiding the simultaneous use of protected aldehydes and prefunctionalized arenes. The regioselectivity of this method is similar to those of typical aromatic electrophilic substitution reactions. The resulting azidobenzaldehyde products are versatile building blocks or precursors for the synthesis of many biologically active compounds. The mechanism studies indicate that the one-electron oxidative intermediate Rh₂ ^(II,III) N₃ is responsible for the azide transfer.

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.

Nickel-Catalyzed Asymmetric Reductive 1,2-Carboamination of Unactivated Alkenes

Starting from diverse alkene-tethered aryl iodides and O-benzoyl-hydroxylamines, the enantioselective reductive cross-electrophilic 1,2-carboamination of unactivated alkenes was achieved using a chiral pyrox/nickel complex as the catalyst. This mild, modular, and practical protocol provides rapid access to a variety of β-chiral amines with an enantioenriched aryl-substituted quaternary carbon center in good yields and with excellent enantioselectivities. This process reveals a complementary regioselectivity when compared to Pd and Cu catalysis.

tert-Butyl peroxide (TBHP)/KI-mediated dual C(sp2)-H bond amination of arylamines with α-diazo carbonyls toward 1,2,4-benzotriazines

A new radical-induced dehydrogenative heterocyclization of arylamines with α-diazo carbonyls has been established under metal-free oxidative conditions, enabling two-fold C(sp2)-H bond amination to access a wide range of functionalized 1,2,4-triazine derivatives with generally good yields by combining KI/tert-butyl peroxide (TBHP). The present protocol features wide substrate scope, commercial accessibility, and mild reaction conditions. Mechanistic details of this radical process are rendered by conducting systematic theoretical calculations.

Transition Metal-Free Intermolecular C(sp2)-H Direct Amination of Furanones via a Redox Pathway

A direct C(sp²)-H amination of 2-furanones under metal-free conditions was realized. This unprecedented intermolecular C-H to C-N conversion provides rapid access to 4-amino-furanone derivatives and novel aza-heterocycle fused furanone skeletons. A redox mechanism based on a double-Michael-addition intermediate INT2 is proposed and detected by spectrometry.

Cobalt-catalysed unactivated C(sp3)–H amination: two-state reactivity and multi-reference electronic character

A remarkable two-state reactivity scenario and an unusual multi-reference character have been computationally found in Co-catalysed C(sp³)–H amination. In addition, the investigation on the additive, aminating reagent, metal center, and auxiliary ligand provides implications for development of new catalytic C–H functionalization systems.

Copper-Mediated Amination of Aryl C-H Bonds with the Direct Use of Aqueous Ammonia via a Disproportionation Pathway

The direct amination of C-H bonds with ammonia is a challenge in synthetic chemistry. Herein, we present a copper-mediated approach that enables a chelation-assisted aromatic C-H bond amination using aqueous ammonia. A key strategy was to use soft low-valent Cu(I) species to avoid the strong coordination of ammonia. Mechanistic investigations suggest that the catalysis is initiated by a facile deprotonation of bound ammonia, and the C-N coupling is achieved by subsequent reductive elimination of the resultant copper-amido intermediate from a Cu(III) intermediate that is readily generated by disproportionation of low-valent copper analogues. This mechanistic postulate was supported by a preliminary kinetic isotope effect study and computations. This new chelation-assisted, copper-mediated C-H bond amination with aqueous ammonia was successfully applied to a broad range of substrates to deliver primary anilines. Moreover, the mild conditions required for this transformation allowed the reaction to operate even under substoichiometric conditions to enable a late-stage application for the preparation of pharmaceutical agents.

Radical-mediated direct C-H amination of arenes with secondary amines

Direct radical-based substitution of aryl C–H bonds allows one-pot access to valuable polycyclic aryl amines from simple secondary amines.

Aryl dialkyl amines, valuable subunits of a wide range of effect chemicals, are accessed by intramolecular amination of aromatic C–H bonds employing UV photolysis of N-chloroamines. The reactions show good functional group tolerance and allow access to a range of fused and bridged polycyclic structures. The homogeneous reaction conditions allow for the one-pot conversion of secondary amines to their arylated derivatives. Experimental and theoretical evidence supports the involvement of electrophilic aminium radicals which react via direct ortho-attack on the arene.

Modular ipso/ ortho Difunctionalization of Aryl Bromides via Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene (Pd/NBE) cooperative catalysis has emerged as a useful tool for preparing poly substituted arenes; however, its substrate scope has been largely restricted to aryl iodides. While aryl bromides are considered as standard substrates for Pd-catalyzed cross coupling reactions, their use in Pd/NBE catalysis remains elusive. Here we describe the development of general approaches for aryl bromide-mediated Pd/NBE cooperative catalysis. Through careful tuning the phosphine ligands and quenching nucleophiles, ortho amination, acylation and alkylation of aryl bromides have been realized in good efficiency. Importantly, various heteroarene substrates also work well and a wide range of functional groups are tolerated. In addition, the utility of these methods has been demonstrated in sequential cross coupling/ ortho functionalization reactions, consecutive Pd/NBE-catalyzed difunctionalization to construct penta-substituted aromatics and two-step meta functionalization reactions. Moreover, the origin of the ligand effect in ortho amination reactions has been explored through DFT studies. It is expected that this effort would significantly expand the reaction scope and enhance the synthetic potential for Pd/NBE catalysis in preparing complex aromatic compounds.

Copper mediated C-H amination with oximes: en route to primary anilines

Here we report an efficient Cu(i)-mediated C-H amination reaction with oximes as amino donors to introduce NH2 groups directly. Various strongly coordinating heterocycles including quinoline, pyrimidine, pyrazine, pyrazole and triazole were tolerated well. The potential utility was further demonstrated in a late-stage modification of telmisartan (an antagonist for the angiotensin II receptor).

Emerging Developments Using Nitrogen-Heteroatom Bonds as Amination Reagents in the Synthesis of Aminoarenes

Aminoarenes constitute valuable building blocks in organic synthesis and an essential skeleton ubiquitously found in ligands, agrochemicals, and pharmaceuticals. This Synopsis presents recent amination methods using nitrogen-heteroatom bonds as a powerful and versatile platform to rapidly synthesize diverse aminoarenes, with a focus on aryne amino functionalization and transition-metal-catalyzed arene C-H amination.

Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications

Catalytic transformation of ubiquitous C-H bonds into valuable C-N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C-H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Pd-Catalyzed regioselective C–H halogenation of quinazolinones and benzoxazinones

A Pd-catalyzed ortho-selective halogenation of benzoxazinone and quinazolinone scaffolds has been described employing N-halosuccinimide via C–H bond activation.

Palladium-catalyzed interannular meta-C–H arylation

The interannularmeta-selective C–H arylation of biaryl-2-trifluoroacetamides using Pd(ii)/norbornene catalysis is described.

Palladium-catalyzed C(sp2)–H aminoimidoylation of isocyano-containing arenes: synthesis of amino substituted N-heterocycles

An efficient access to amino substituted phenanthridine and isoquinoline derivatives, through palladium-catalyzed C(sp²)–H aminoimidoylation, has been developed.