Metal-Catalyzed Annulations through Activation and Cleavage of C−H Bonds

Theoretical study of rhodium(III)-catalyzed synthesis of benzoxepine and coumarin

The mechanisms of the rhodium-catalyzed cycloaddition of 2-vinylphenol with diphenylacetylene and carbon monoxide have been studied using density functional theory calculations at the B3LYP/6-31G (d, p) (Lanl2dz for Rh) level of theory. The SMD solvation model was used in MeCN solvents at M06-2X/6-311 ++ G (d, p) (Lanl2dz (f) for Rh) levels using a single-point calculation to consider the solvent effect. The calculation results show that there are two competitive reaction pathways for the cycloaddition reaction of rhodium-catalyzed synthesis of benzohexine and coumarin. Starting from the precursor reaction complex, the reaction channel is more favorable for the carbon atoms of diphenylacetylene and carbon monoxide to attack the Rh-C bond (the barriers of 9.88 and 10.01 kcal/mol) rather than attack the Rh-O bond (the barriers of 15.37 and 30.17 kcal/mol), and carbon monoxide in two different reaction channels has a greater energy difference than diphenylacetylene. The results show that the computational study of the rhodium-catalyzed cycloaddition reaction has a high catalytic activity consistent with the high yield of the experiment of Gulías et al.

Palladium(II)-Catalyzed Oxidative Annulation of 2-Hydroxynaphthalene-1,4-diones and Internal Alkynes via C-H Functionalization

An efficient Pd(II)-catalyzed oxidative annulation of 2-hydroxynaphthalene-1,4-diones and internal alkynes has been developed with high step efficiency. A broad range of functional groups are compatible with this reaction, thus providing a new entry to diverse naphtho[2,3-b]furan-4,9-dione derivatives in good to high yields.

Azaruthena(II)-bicyclo[3.2.0]heptadiene: Key Intermediate for Ruthenaelectro(II/III/I)-catalyzed Alkyne Annulations

A ruthenium-catalyzed electrochemical dehydrogenative annulation reaction of imidazoles with alkynes has been established, enabling the preparation of various bridgehead N-fused [5,6]-bicyclic heteroarenes through regioselective electrochemical C-H/N-H annulation without chemical metal oxidants. Novel azaruthenabicyclo[3.2.0]heptadienes were fully characterized and identified as key intermediates. Mechanistic studies are suggestive of an oxidatively induced reductive elimination pathway within a ruthenium(II/III) regime.

Oxidative C-H/N-H Annulation of Aromatic Amides with Dialkyl Malonates: Access to Isoindolinones and Dihydrobenzoindoles

A copper-mediated oxidative C-H/N-H annulation of aromatic amides with dialkyl malonates has been presented to afford synthetically valuable dihydrobenzoindoles and isoindolinones. The reaction proceeds through direct oxidative C(sp²)-H/C(sp³)-H coupling followed by an intramolecular N-H/C(sp³)-H dehydrogenative coupling to deliver the target motifs with broad scope and functional group tolerance.

Rh-Catalyzed C-H activation/intramolecular condensation for the construction of benzo[f]pyrazolo[1,5-a][1,3]diazepines

A novel and mild Rh(iii)-catalyzed C-H activation/intramolecular condensation of 1-aryl-1H-pyrazol-5-amines with cyclic 2-diazo-1,3-diketones was developed, giving access to various important benzo[f]pyrazolo[1,5-a][1,3]diazepine scaffolds through sequential C-C/C-N bond formation in a one-pot procedure under additive- and oxidant-free conditions. Furthermore, 3-([1,1'-biphenyl]-2-ylamino)-2-ethoxycyclohex-2-enones can be obtained in good yields by constructing C-O and C-N bonds through 1,1'-insertion, dehydration, and isomerization processes.

Enantioselective Twofold C-H Annulation of Formamides and Alkynes without Built-in Chelating Groups

Twofold C-H annulation of readily available formamides and alkynes without built-in chelating groups was achieved. Ni-Al bimetallic catalysis enabled by a bulky BINOL-derived chiral secondary phosphine oxide (SPO) ligand proved to be critical for high reactivity and high selectivity. This reaction uses readily available formamides as starting materials and provides a concise synthetic pathway to a broad range of chiral ferrocenes in 40-98 % yield and 93-99 % ee.

Rhodium(III)-Catalyzed Redox-Neutral Coupling of α-Trifluoromethylacrylic Acid with Benzamides through Directed C-H Bond Cleavage

A rhodium(III)-catalyzed redox-neutral coupling of α-trifluoromethylacrylic acid with bezamides proceeds smoothly accompanied by amide-directed C-H bond cleavage to produce β-[2-(aminocarbonyl)phenyl]-α-trifluoromethylpropanoic acid derivatives. One of the products can be transformed to a trifluoromethyl substituted heterocyclic compound. In addition, the redox-neutral coupling of α-trifluoromethylacrylic acid with related aromatic substrates possessing a nitrogen-containing directing group can also be conducted under similar conditions.

Ligand-Enabled Ni-Al Bimetallic Catalysis for Nonchelated Dual C-H Annulation of Arylformamides and Alkynes

A bifunctional secondary phosphine oxide (SPO) ligand-controlled method was developed for Ni-Al-catalyzed nonchelated dual C-H annulation of arylformamides with alkynes, providing a series of substituted amide-containing heterocycles in ≤97% yield. The SPO-bound bimetallic catalysis proved to be critical to the reaction efficiency.

Rhodium(III)-Catalyzed Decarboxylative Aminomethylation of Glycine Derivatives with Indoles via C-H Activation

A rhodium(III)-catalyzed decarboxylative aminomethylation of glycine derivatives with indoles bearing a pyrimidine directing group was developed via C-H activation, exclusively affording the corresponding aminomethylated products with C3 selectivity. In this synthetic protocol, easily available and resourceful arylglycines were utilized as aminomethyl functional moieties.

Divergent C-H activation synthesis of chalcones, quinolones and indoles

We here report a condition-controlled divergent synthesis strategy of chalcones, quinolones and indoles, which was achieved via a C-H activation reaction of N-nitrosoanilines and cyclopropenones. Variations of Ag salts are observed to be crucial for divergently constructing the three distinct chemical scaffolds. A Rh(i)- and Rh(iii)-cocatalyzed decarbonylation/C-H activation/[3+2] annulation cascade reaction was developed for the synthesis of indoles. These methodologies are characterized by mild reaction conditions, high functional group tolerance, and amenability to gram-scale synthesis, providing a reference for future derivation of new chemical scaffolds by C-H activation.

Palladium-catalyzed enol/enolate directed oxidative annulation: functionalized naphthofuroquinone synthesis and bioactivity evaluation

A palladium-promoted oxidative annulation reaction for the synthesis of structurally diverse naphthoquinone-containing heterocycles has been developed, providing switchable access to 1,2-naphthofuroquinones and densely functionalized cyclobutene-fused 1,4-naphthofuroquinones by selective enol/enolate-directed processes. The synthetic application was extended by late-stage functionalization of an anti-HIV drug. The practical value of 1,2-naphthofuroquinone synthesis was highlighted in endothelial protective lead compound development.

Cobaltaelectro-catalyzed oxidative allene annulation by electro-removable hydrazides

An efficient C-H/N-H functionalization with allenes was enabled via versatile electro-oxidative cobalt catalysis. Thus, electrochemical C-H activations were accomplished with high levels of chemoselectivity and regioselectivity in an operationally simple undivided cell setup. The user-friendly nature of this protocol was highlighted by excellent functional group tolerance, an electro-reductive removable hydrazide directing group and easy scalability. Experimental mechanistic studies were indicative of a facile BIES C-H cobaltation event.

Rhodium-catalyzed multiple C–H activation/highlymeta-selective C–H amination between amidines and alkynes

A tandem process of multiple C–H activation, intermolecular highlymeta-selective C–H amination, and intramolecular C–H amination between amidines and alkynes has been developed.

Synthesis of 4-ethenyl quinazolines via rhodium(iii)-catalyzed [5 + 1] annulation reaction of N-arylamidines with cyclopropenones

An unprecedented synthesis of 4-ethenyl quinazolines/2-benzoyl quinazolines via Rh(iii)-catalyzed C–H annulation reaction was reported.

General and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones via a formal [3 + 2] cycloaddition initiated by C–H activation

A [Cp*RhCl₂]₂-catalyzed formal [3 + 2] cycloaddition involving a sequential coupling reaction initiated by C–H activation and aza-Michael addition has been developed for the general and efficient synthesis of 1,2-dihydropyrrolo[3,4-b]indol-3-ones.

Rh(iii)-catalyzed spiroannulation of 3-arylquinoxalin-2(1H)-ones with alkynes: practical access to spiroquinoxalinones

The use of imines as a H acceptor for Rh(iii)-catalyzed spirocyclization of 3-arylquinoxalinones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been achieved.

Ruthenium(ii)-catalyzed [5 + 1] annulation reaction: a facile and efficient approach to construct 6-ethenyl phenanthridines utilizing a primary amine as a directing group

A ruthenium(ii)-catalyzed [5 + 1] annulation reaction between 2-arylanilines and cyclopropenones employing a free amine as a directing group has been developed.

Rhodium-catalyzed oxidative annulation of 1H-indazoles with alkynes for the synthesis of indazolo[3,2-a]isoquinolines via C–H bond functionalization

A Rh(iii)-catalyzed oxidative annulation of 1H-indazoles with internal alkynes via C–C and C–N coupling for the preparation of highly functionalized indazolo[3,2-a]isoquinolines is disclosed.

Double annulation of ortho- and peri-C–H bonds of fused (hetero)arenes to unusual oxepino-pyridines

Direct difunctionalization of chemically distinct ortho- and peri-C–H bonds of fused hetero(arenes) is illustrated through an unusual one-pot domino {[4 + 2] & [5 + 2]} double annulation with alkynes for the first time.

The ruthenium(ii)-catalyzed C–H olefination of indoles with alkynes: the facile construction of tetrasubstituted alkenes under aqueous conditions

An environmentally-friendly and facile protocol for the construction of tetrasubstituted alkenes has been established with Ru(ii)-catalyzed C–H bond functionalizations under mild conditions.

Merging Cu-catalysed C–H functionalisation and intramolecular annulations: computational and experimental studies on an expedient construction of complex fused heterocycles

A copper-catalysed protocol for the synthesis of fused dihydrobenzofuran-isoquinolone compounds through an intramolecular annulation of readily accessible benzamide substrates is reported, along with a full DFT study into the mechanism.

Rhodium(iii)-catalyzed synthesis of 3-trifluoromethylindanones from N-methoxybenzamides via C–H activation and Claisen/retro-Claisen reaction

An efficient synthesis of 3-trifluoromethylindanones via C–H annulation of N-methoxybenzamides with β-trifluoromethyl-α,β-unsaturated ketones under Rh(iii)-catalysis is described.

π-Extended Polyaromatic Hydrocarbons by Sustainable Alkyne Annulations through Double C-H/N-H Activation

The widespread applications of substituted diketopyrrolopyrroles (DPPs) call for the development of efficient methods for their modular assembly. Herein, we present a π-expansion strategy for polyaromatic hydrocarbons (PAHs) by C-H activation in a sustainable fashion. Thus, twofold C-H/N-H activations were accomplished by versatile ruthenium(II)carboxylate catalysis, providing step-economical access to diversely decorated fluorogenic DPPs that was merged with late-stage palladium-catalyzed C-H arylation on the thus-assembled DPP motif.

Rhodium-Catalyzed Successive C-H Bond Functionalizations To Synthesize Complex Indenols Bearing a Benzofuran Unit

An efficient rhodium-catalyzed redox-neutral annulations of N-phenoxyacetamides and ynones via successive double C-H bond activations has been developed. A series of novel and complex indenols bearing a benzofuran unit were generated with moderate to excellent regioselecetivities under mild conditions. Adding N-ethylcyclohexanamine (CyNHEt) could restrict the formation of the mono C-H bond activation byproduct, which is not the intermediate of the reaction demonstrated via the mechanistic investigations.

Rhodium(III)-Catalyzed C(sp2)-H Functionalization of Cyclobutenes. Access to Cyclobuta[c]pyridones and -pyridines

O-(ω-Alkynyl) hydroxamates derived from cyclobutenyl carboxylic acids were identified as viable substrates in intramolecular rhodium(III)-catalyzed heteroannulations, which led to diversely substituted cyclobuta[c]pyridones. Further functionalization of the resulting cyclobutapyridones enabled the synthesis of cyclobuta[c]pyridines and other nitrogen heterocycles after electrocyclic ring opening of the four-membered ring.

Transition-Metal-Free One-Pot Tandem Synthesis of 3-Ketoisoquinolines from Aldehydes and Phenacyl Azides

An efficient and transition-metal-free strategy for the synthesis of 3-keto-isoquinolines in one pot has been developed from the easily accessible 2-(formylphenyl)acrylates and phenacyl azides. Various substituted aldehydes and phenacyl azides were successfully employed in this transformation to furnish a variety 3-keto-isoquinolines in very good yields. A number of controlled experiments were conducted to postulate the reaction mechanism. Secondary functionalizations of 2-keto-isoquinolins were also performed to showcase the synthetic utility.

Palladium(II)/Lewis Acid Cocatalyzed Oxidative Annulation of 2-Alkenylanilines and Propargylic Esters: An Access to Benzo[b]azepines

An attractive approach to valuable yet synthetically challenging benzo[b]azepines was established via palladium(II)/Lewis acid cocatalyzed oxidative [5 + 2] annulation of readily available 2-alkenylanilines and propargylic esters. The protocol features mild reaction conditions and good functional group tolerance, constituting an array of benzo[b]azepines in yields of 30-75%.

Construction of Pyranoisoquinolines via Ru(II)-Catalyzed Unsymmetrical Double Annulation of N-Methoxybenzamides with Unactivated Alkynes

A ruthenium (Ru)-catalyzed double annulation of easily accessible N-methoxybenzamide derivatives with unactivated alkynes for the synthesis of unusual 6,6-fused pyranoisoquinolines is described. Both ortho-C-H bonds of arenes and the N- and O-moieties of N-methoxybenzamides are involved in the construction of four [(C-C)-(C-N) and (C-C)-(C-O)] bonds in one step under single catalytic conditions. The unsymmetrical annulation of N-methoxybenzamides with two distinct alkynes is also demonstrated. The oxidizable directing group N-methoxyamine (NHOMe) assists the unsymmetrical double annulations of arenes [that use both N- and O-heteroatoms] in a single operation. This synthetic method features excellent substrate scope and tolerates a wide range of functional groups. Peripheral modification of pyranoisoquinolines for the construction of complex heterocyclic compounds is also demonstrated.

An unusual [4 + 2] fusion strategy to forge meso-N/O-heteroarene-fused (quinoidal) porphyrins with intense near-infrared Q-bands

Divergent synthesis of meso-N/O-heteroarene-fused (quinoidal) porphyrins was established via rhodium-catalyzed β-C–H activation/annulation of quinoidal porphyrins with alkynes.

Here we present a divergent synthesis of brand-new types of meso-N/O-heteroarene-fused (quinoidal) porphyrins through Rh-catalyzed β-C–H activation/annulation of 5,15-dioxoporphyrins and dioxime derivatives with alkynes, in which the synthetic disconnections are difficult to access through the commonly used intramolecular cyclization strategy. Using the O-methyl oxime as a traceless oxidizing directing group, the meso-N-embedded pyridine-fused anti-quinoidal porphyrin 3 and pyridinium-fused cation 4 are formed with controllable chemoselectivity and complete anti-selectivity. Replacing the exocyclic oxime with a carbonyl group delivers the pyran-fused porphyrin 5, achieving structural conversion from a quinoidal conformation to a stable porphyrin macrocycle. Further oxidation of the expanded dimer 5ea gives the oxonium 6, which exhibits intense near-infrared (NIR) Q-bands up to 1300 nm. Theoretical studies demonstrate that the incorporation of a heteroatom at the meso-position enables more effective π-extension, resulting in a 22π aromatic (vs. 18π aromatic) character of pyran-fused porphyrins (syn/anti-5aa). Compared with the commercially available methylene blue (MB), syn-5al exhibits a better ability (Φ_Δ = 0.61) to sensitize singlet oxygen (¹O₂) when irradiated with a 680 nm laser beam, and has potential as a photodynamic therapy (PDT) photosensitizer in the body's therapeutic window (650–900 nm).

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.