Synthesis of Highly Substituted Acenes through Rhodium-Catalyzed Oxidative Coupling of Arylboron Reagents with Alkynes

Rhodium-Catalyzed Chemodivergent Pyridylation of Alkynes with Pyridylboronic Acids

The pyridylation of alkynes with pyridylboronic acids is realized under rhodium catalysis. Chemodivergent pyridylation products, including alkenylpyridines produced via the hydropyridylation pathway and cyclopenta[c]pyridines produced via the pyridylation/cyclization pathway, were selectively produced by fine-tuning the reaction conditions. A mechanistic study revealed that 1,4-rhodium migration to the pyridine ring was involved as the key step in the chemodivergent synthesis.

Nickel-catalyzed [2 + 2 + 2] benzannulation of alkynes: a new route to the synthesis of highly substituted naphthalenes

The Ni(II)-catalyzed aromatic homologation of various directing groups with alkynes to afford highly substituted naphthalene products via C-X bond cleavage followed by alkyne insertion and C-H activation is described. This reaction proceeded with a wide range of directing groups such as pyrazoles, imidazopyridines, benzoimidazothiazoles, thiazoles and triazoles with alkynes to afford highly substituted naphthalenes in moderate to good yields. This transformation is promoted by a simple, straightforward combination of a Ni(II)-complex, Zn dust and a base.

Synthesis of spiropyrans via the Rh(III)-catalyzed annulation of 3-aryl-2H-benzo[b][1,4]oxazines with diazo ketoesters

Rh(III)-Catalyzed 1 : 2 coupling of 3-aryl-2H-benzo[b][1,4]oxazines with α-diazo-β-ketoesters has been realized for the mild synthesis of spiropyrans. The reaction proceeded via twofold C-H activation followed by unusual [3+3] and [4+2] annulation with decent functional group tolerance. Moreover, a pyranoid-skeleton intermediate was isolated as a key intermediate as a result of monoalkylation and enol oxygen annulation, which offers direct mechanistic insight.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

DFT study on the ruthenium-catalyzed decarbonylative annulation of an alkyne with a six-membered hydroxychromone via C–H/C–C activation

Both steady-state approximation and the modified energy span model have been employed to determine the TOF-determining transition state and TOF-determining intermediate for the title catalytic reaction.

DFT study on ruthenium-catalyzed N-methylbenzamide-directed 1,4-addition of the ortho C–H bond to maleimide via C–H/C–C activation

B3LYP-D3a+IDSCRF/tzp-dkh(-dfg) calculations indicate that CO as a directing group is much more favourable than the N–H group, and the real active catalyst is an ionic type with one [SbF6]− group.

Rh(III)-Catalyzed Annulation of 2-Biphenylboronic Acid with Diverse Activated Alkenes

Rhodium(III)-catalyzed annulation of 2-biphenylboronic acids with three classes of activated alkenes has been realized, leading to the synthesis of fused or bridged cyclic skeletons via transmetalation-initiated C-H activation. In the annulative coupling of 2-biphenylboronic acid with a CF₃-substituted enone, the bulky cyclopentadienyl ligand (Cp^tBu) in the catalyst proved effective to promote the reductive elimination process prior to protonolysis, affording the [4 + 2] annulated products instead of the simple 1,4-addition product. Seven-membered rings were obtained when disubstituted cyclopropenones were employed. Bridged cycles were isolated from the coupling of 2-biphenylboronic acid with benzoquinones as a result of 2-fold Michael additions. The substrate scopes were found to be broad with up to 99% yield under air-tolerant conditions.

One-Pot Arylative Benzannulation of 2-Carbonyl-3-propargyl Indoles with Boronic Acids Leading to Arylated Carbazoles

Arylative annulation of 2-carbonyl-3-propargyl indoles with boronic acids under sequential palladium/triflic acid catalysis is described. The present strategy to provide di- and triaryl carbazoles in one pot involves benzannulation through difunctionalization of alkynes. The strategy showed a good substrate scope with respect to boronic acids as well as 2-carbonyl-3-propargyl indoles to afford the corresponding carbazoles in decent yields.

Recent Advances in Sulfur-Containing Heterocycle Formation via Direct C–H Sulfuration with Elemental Sulfur

The synthesis of sulfur heterocycles via the construction of C–S bonds has received considerable attention due to their biological value and extensive pharmaceutical application. While diverse sulfurating agents have been developed over the past few decades, in this regard, elemental sulfur, with advantages of low toxicity, odorless nature and chemical stability, has great potential for the construction of diverse sulfur heterocycles through its direct incorporation into the target molecules in a concise way. Direct functionalization of inert C–H bonds can shorten the number of reaction steps and minimize the amount of waste formed. Hence, heteroannulations via direct C–H sulfuration is considered to be an attractive strategy for the synthesis of sulfur heterocycles. In the last few years, a vast array of concise systems have been reported for the synthesis of some valuable sulfur heterocycles such as thiophenes, thienoindoles, thienothiazoles, thiazoles, benzothiazoles, and thiadiazoles through direct C–H sulfuration/annulations with elemental sulfur. These are discussed in detail in this review.

1 Introduction

2 Thiophenes

3 Thienoindoles

4 Thienothiazoles

5 Other Fused Thiophenes

6 Thiazoles

7 Benzothiazoles

8 Thiadiazoles

9 Others

10 Summary and Outlook

Effect of Cp-Ligand Methylation on Rhodium(III)-Catalyzed Annulations of Aromatic Carboxylic Acids with Alkynes: Synthesis of Isocoumarins and PAHs for Organic Light-Emitting Devices

An efficient protocol was developed for the synthesis of π-extended isocoumarins and polycyclic aromatic hydrocarbons based on the oxidative coupling of aromatic carboxylic acids with internal alkynes catalyzed by (cyclopentadienyl)rhodium complexes. The coupling chemoselectivity strongly depends on whether Cp or the methylated Cp* ligands are used. The pentamethyl derivative [Cp*RhCl₂ ]₂ predominantly gives isocoumarins, while the non-methylated complex [CpRhI₂ ]_n produces naphthalene derivatives. The polyaromatic carboxylic acids (such as 1-naphthoic acid, 1-pyrenecarboxylic acid, fluorene-1-carboxylic acid, and dibenzofuran-4-carboxylic acid) are suitable for this approach. A mixture of Cp*H/RhCl₃ can be used as a catalyst instead of [Cp*RhCl₂ ]₂ . The structures of 3,4-diphenylindeno[1,2-h]isochromen-1(11H)-one and 7,10-dimethyl-8,9-diphenylbenzo[pqr]tetraphene were determined by X-ray diffraction. In addition, the optical properties of the prepared compounds were studied. 7,8-Diphenyl-10H-phenaleno[1,9-gh]isochromen-10-one was employed as an emissive layer for OLED manufacturing. The OLED emits yellow-green light with a maximum intensity 1740 cd ⋅ m^-2 at 15 V.

Group 4 Diarylmetallocenes as Bespoke Aryne Precursors for Titanium-Catalyzed [2 + 2 + 2] Cycloaddition of Arynes and Alkynes

Despite the ubiquity of reports describing titanium (Ti)-catalyzed [2 + 2 + 2] cyclotrimerization of alkynes, the incorporation of arynes into this potent manifold has never been reported. The in situ generation of arynes often requires fluoride, which instead will react with the highly fluorophilic Ti center, suppressing productive catalysis. Herein, we describe the use of group 4 diarylmetallocenes, Cp^R₂MAr₂ (Cp^R = C₅H₅, C₅Me₅; M = Ti, Zr), as aryne precursors for the Ti-catalyzed synthesis of substituted naphthalenes via coupling with 2 equiv of an alkyne. Fair-to-good yields of the desired naphthalene products could be obtained with 1% catalyst loadings, which is roughly an order of magnitude lower than similar reactions catalyzed by palladium or nickel. Additionally, naphthalenes find broad applications in the electronics, photovoltaics, and pharmaceutical industries, urging the discovery of more economic syntheses. These results indicate that aryne transfer from a Cp^R₂M(η²-aryne) complex to another metal is a viable route for the introduction of aryne fragments into organometallic catalytic processes.

Pd-Catalyzed Annulation of 1-Halo-8-arylnaphthalenes and Alkynes Leading to Heptagon-Embedded Aromatic Systems

A palladium-catalyzed heptagon-forming annulation reaction between 1-halo-8-arylnaphthalene and diarylacetylene is reported. The reaction is promoted using a catalytic system comprised of Pd(OAc)₂ , moderately electron-deficient triarylphosphine P(4-ClC₆ H₄ )₃ , and Ag₂ CO₃ to afford benzo[4,5]cyclohepta[1,2,3-de]naphthalene derivatives in moderate to good yields, in preference to fluoranthene as a competing byproduct. Twofold annulation can also be achieved to access a novel heptagon-embedded polycyclic aromatic hydrocarbon compound.

Rhodium-Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids with Diynes through Decarboxylation and C≡C Triple Bond Cleavage

It has been established that an electron-deficient cyclopentadienyl rhodium(III) (Cp^E Rh^III ) complex catalyzes the oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic acids with diynes through C≡C triple bond cleavage, leading to fused naphthalenes. This cyclotrimerization is initiated by directed ortho C-H bond cleavage of a benzoic acid, and the subsequent regioselective alkyne insertion and decarboxylation produce a five-membered rhodacycle. The electron-deficient nature of the Cp^E Rh^III complex promotes reductive elimination giving a cyclobutadiene-rhodium(I) complex rather than the second intermolecular alkyne insertion. The oxidative addition of the thus generated cyclobutadiene to rhodium(I) (formal C≡C triple bond cleavage) followed by the second intramolecular alkyne insertion and reductive elimination give the corresponding [2+1+2+1] cycloaddition product. The synthetic utility of the present [2+1+2+1] cycloaddition was demonstrated in the facile synthesis of a donor-acceptor [5]helicene and a hemi-hexabenzocoronene by a combination with the chemoselective Scholl reaction.

Rhodaelectrocatalysis for Annulative C-H Activation: Polycyclic Aromatic Hydrocarbons through Versatile Double Electrocatalysis

Rapid access to structurally diversified polycyclic aromatic hydrocarbons (PAHs) in a controlled manner is of key significance in materials sciences. Herein, we describe a strategy featuring two distinct electrocatalytic C-H transformations for the synthesis of novel nonplanar PAHs. The combination of rhodaelectrooxidative C-H activation/[2+2+2] alkyne annulation of easily accessible boronic acids with electrocatalytic cyclodehydrogenation provided modular access to diversely substituted PAHs with electricity as a sustainable oxidant. The unique molecular topology as well as the photophysical and electronic properties of the thus obtained PAHs were fully analyzed. The unique power of this metallaelectrocatalysis method was demonstrated by the chemoselective assembly of synthetically useful iodo-substituted PAHs.

A computational study on ruthenium-catalyzed [4 + 1] annulation via C–H activation: the origin of selectivity and the role of the internal oxidizing group

Internal alkynes with β-H and hydroxyl groups lead to the [4 + 1] annulation.

Rh(III)-Catalyzed C-H Activation of Boronic Acid with Aryl Azide

A Rh(III)-catalyzed C-H activation of boronic acid with aryl azide to obtain unsymmetric carbazoles, 1 H-indoles, or indolines has been developed. The reaction constructs dual distinct C-N bonds via sp²/sp³ C-H activation and rhodium nitrene insertion. Synthetically, this approach represents an access to widely used carbazole derivatives. The practical application to CBP and unsymmetric TCTA derivatives has also been performed. Mechanistic experiments and DFT calculations demonstrate that a five-membered rhodacycle species is the key intermediate.

Carbo-biphenyls and Carbo-terphenyls: Oligo(phenylene ethynylene) Ring Carbo-mers

Ring carbo-mers of oligo(phenylene ethynylene)s (OPEn, n=0-2), made of C₂ -catenated C₁₈ carbo-benzene rings, have been synthesized and characterized by NMR and UV-vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT-optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo-terphenyls (n=2) are reversibly reduced at ca. -0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo-benzene (-0.83 V/SCE), thus revealing efficient inter-ring π-conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (-0.30 V/SCE) for a carbo-quaterphenyl congener (n=3). Increase with n of the effective π-conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT-calculated excited states, one of them restricting to a HOMO→LUMO main one-electron transition.

Weak Directing Group Steered Formal Oxidative [2+2+2]-Cyclization for Selective Benzannulation of Indoles

A double C-H activation and double insertion process to achieve the synthesis benzo[e]indole frameworks has been disclosed. This type of benzannulation is directed by a trifluoromethylketone moiety, which is easy to install on the indole C3-position. Overall the reaction takes places as an oxidative cyclization of two alkynes with the C4-C5 position of indole.

Monomer stoichiometry imbalance-promoted formation of multisubstituted polynaphthalenes by palladium-catalyzed polycouplings of aryl iodides and internal diynes

A novel palladium-catalyzed polymerization route toward multisubstituted polynaphthalenes with high thermal stability and advanced functionalities in a monomer stoichiometry imbalance-promoted manner.

Naphthazarin-Polycyclic Conjugated Hydrocarbons and Iptycenes

The synthesis of a set of naphthazarin-containing polycyclic conjugated hydrocarbons is described herein. Sequential Diels-Alder reactions on a tautomerized naphthazarin core were employed to access the final conjugated systems. Complete conjugation across the backbone can be achieved through complexation with BF₂, as observed by ¹H NMR analysis and UV/vis spectroscopy. Precise synthetic control over the degree of oxidation of naphthazarin quinone Diels-Alder adduct 10 is additionally demonstrated and enables us to direct its subsequent reactivity. Finally, this work serves to demonstrate the potential for naphthazarin as a building block in the synthesis of novel organic electronic materials.

Iridium-catalyzed polymerization of benzoic acids and internal diynes: a new route for constructing high molecular weight polynaphthalenes without the constraint of monomer stoichiometry

A green and powerful polymerization tool for constructing functional multisubstituted polynaphthalenes without strict control on monomer stoichiometry.

Twisted Polycyclic Aromatic Systems Prepared by Annulation of Bis(arylethynyl)arenes with Biphenylboronic Acids

Twisted polycyclic aromatic hydrocarbons (PAHs) were prepared by the successive rhodium-catalyzed annulation and dehydrogenative cyclization of bis(arylethynyl)arenes with di-tert-butylbiphenyl-2-ylboronic acid. The molecular structures of the PAHs were determined by single-crystal XRD analysis. The PAHs showed up to four fjord regions, and the twisting angle was 46.7°. The nonplanarity (NP) and harmonic oscillator model of aromaticity (HOMA) were calculated by using the structural data obtained from XRD analysis. The PAHs derived from dialkynyl naphthalenes showed low planarity and HOMA of the central ring. The optical properties of the PAHs were investigated by UV/Vis absorption and fluorescence spectroscopy analyses. The absorption and emission maxima of the PAHs with a larger planar region appeared at a longer wavelength. DFT calculations support that the absorption band at λ≈450 nm can be mainly attributed to the HOMO-LUMO transition.

Rhodium-catalyzed oxidative coupling of N-acyl anilines with alkynes using an acylamino moiety as the traceless directing group

A rhodium-catalyzed oxidative annulation of N-acyl anilines with alkynes was developed by using the acylamino group as a traceless directing group for the first time.

Hydrazone-palladium catalyzed annulation of 1-allyl-2-bromobenzene derivatives with internal alkynes

Annulation of 1-allyl-2-bromobenzene derivatives with internal alkynes using a hydrazone-palladium catalyst system proceeded smoothly and gave the corresponding polysubstituted naphthalene derivatives in good to high yields.

RhI/RhIII catalyst-controlled divergent aryl/heteroaryl C-H bond functionalization of picolinamides with alkynes

The ability to establish switchable site-selectivity through catalyst control in the direct functionalization of molecules that contain distinct C-H bonds remains a demanding challenge that would enable the construction of diverse scaffolds from the same starting materials. Herein we describe the realization of this goal, namely a divergent heteroaryl/aryl C-H functionalization of aromatic picolinamide derivatives, targeting two distinct C-H sites, either at the pyridine ring or at the arene unit, to afford isoquinoline or ortho-olefinated benzylamine (or phenethylamine) derivatives. This complementary reactivity has been achieved on the basis of a RhIII/RhI switch in the catalyst, resulting in different mechanistic outcomes. Notably, a series of experimental and DFT mechanistic studies revealed important insights about the mechanism of the reaction and reasons behind the divergent regiochemical outcome.

Phosphine-Mediated Iterative Arene Homologation Using Allenes

A PPh3-mediated multicomponent reaction between o-phthalaldehydes, nucleophiles, and monosubstituted allenes furnishes functionalized non-C2-symmetric naphthalenes in synthetically useful yields. When the o-phthalaldehydes were reacted with 1,3-disubstituted allenes in the presence of PPh2Et, naphthalene derivatives were also obtained in up to quantitative yields. The mechanism of the latter transformation is straightforward: aldol addition followed by Wittig olefination and dehydration. The mechanism of the former is a tandem γ-umpolung/aldol/Wittig/dehydration process, as established by preparation of putative reaction intermediates and mass spectrometric analysis. This transformation can be applied iteratively to prepare anthracenes and tetracenes using carboxylic acids as pronucleophiles.