Aromatic metamorphosis: conversion of an aromatic skeleton into a different ring system

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Sodium-Mediated Reductive C-C Bond Cleavage Assisted by Boryl Groups

In contrast to the well-established oxidative C=C double bond cleavage to give the corresponding carbonyl compounds, little is known about reductive C=C double bond cleavage. Here we report that C-C single bond cleavage in 1,2-diaryl-1,2-diborylethanes proceeds by reduction with sodium metal to yield α-boryl benzylsodium species. In combination with our previous reductive diboration of stilbenes, the overall transformation represents reductive cleavage of the C=C double bonds of stilbene to yield α-boryl-α-sodiated toluenes. This reductive two-step C=C double bond cleavage is applicable to ring-opening or ring-expansion reactions of polycyclic aromatic hydrocarbons.

Intramolecular Cobalt/Visible Light Cocatalyzed Reductive Coupling of Unactivated Arenes with Unactivated Alkenes

A protocol for the intramolecular reductive coupling of unactivated arenes with unactivated alkenes has been developed with the aid of a cooperative visible light/cobalt catalytic system. This coupling is achieved via radical cascade cyclization using amines as terminal reducing reagents and water as the main hydrogen source. In their form, readily available N-allyl benzamides are converted to the corresponding spiro cyclohexadiene-lactam or β-phenethylamine analogues in moderate to excellent yields.

One-pot cascade synthesis of dibenzothiophene-based heterobiaryls from dibenzothiophene-5-oxide

A sulfoxide directed C-H metalation/boration/B2Pin2 mediated reduction/Suzuki coupling process to synthesize 4-substituted dibenzothiophene (DBT) in one-pot from dibenzothiophene-5-oxide (DBTO) was developed. A variety of DBT-based heterobiaryls were prepared in satisfactory to good yields. A mechanism was proposed. The application of this methodology was demonstrated by synthesizing a luminescent material.

Enantioselective Au-Catalyzed Synthesis of Thia[5]- and Thia[6]helicenes and Their Transformation into Bowl-shaped Pleiadenes

A series of helically shaped benzo[b]chryseno[4,3-d]thiophenes, naphtho[1,2-b]phenanthro[4,3-d]thiophenes, and chryseno[3,4-b]naphtho[1,2-d]thiophenes is synthesized via a highly enantioselective Au-catalyzed intramolecular alkyne hydroarylation reaction. The inversion barriers of the structures obtained are determined both theoretically and experimentally, and their chiroptical properties are reported. Preliminary studies on the post-synthetic functionalization of these thiahelicenes and their transformation into azahelicenes are also presented. In addition, a straightforward one-step protocol is developed, which wraps the initially obtained chryseno[3,4-b]naphtho[1,2-d]thiophenes into bowl-shaped pleiadene derivatives without erosion of the enantiopurity. The number of structurally related products that are obtained with high enantioselectivity enables the establishment of comprehensive correlations between the structure and conformational stability or (chir)optical properties.

Streptocyanine as an activation mode of amine catalysis for the conversion of pyridine rings to benzene rings

Amine catalysts have emerged as an invaluable tool in organic synthesis. Iminium, enamine, and enamine radical cation species are representative activation modes of amine catalysis. However, the development of new amine catalysis activation modes that enable novel synthetic strategies remains highly desirable. Herein, we report streptocyanine as a new amine catalysis activation mode, which enables the skeletal editing of pyridine rings to benzene rings. N-Arylation of pyridines bearing an alkenyl substituent at the 3-position generates the corresponding N-arylpyridiniums. The resulting pyridinum reacts with a catalytic amount of piperidine to afford a streptocyanine intermediate. Catalytically generated streptocyanine forms a benzene ring via a ring-closing reaction, thereby releasing the amine catalyst. Consequently, the alkene moiety in the starting pyridines is incorporated into the benzene ring of the products. Pyridiniums bearing various alkene moieties were efficiently converted to formyl-substituted benzene derivatives. Mechanistic studies support the postulation that the present catalytic process was intermediated by streptocyanine. In this reaction system, streptocyanine could be regarded as a new activation mode of amine catalysis.

Ultrasound-Assisted Metal-Free Synthesis of Functionalized 1,2-Teraryls by Ring Transformation Methodology

The function of styryl acetate as a carbanion nucleophile source has been explored in the ring transformation reaction of 2H-pyranones. The ultrasound assisted carbanion induced ring transformation of 4-amino-6-aryl-3-cyano-2H-pyran-2-ones using (E)-styryl acetate in the presence of KOH and DMF at room temperature is presented as an alternative strategy for the synthesis of functionalized 1,2-teraryls in high yields. This metal-free methodology provides easy access to a series of 1,2-teraryls and features broad functional group tolerance and mild reaction condition. Single crystal X-ray diffraction analysis of 4-(piperidin-1-yl)-6-(thiophen-2-yl)-[1,1'-biphenyl]-3-carbonitrile indicated that the crystal belongs to monoclinic system with a P21 space group having twisted aryl rings.

Sulfonium-aided coupling of aromatic rings via sigmatropic rearrangement

Biaryl synthesis continues to occupy a central role in chemical synthesis. From blockbuster drug molecules to organic electronics, biaryls present numerous possibilities and new applications continue to emerge. Transition-metal-catalyzed coupling reactions represent the gold standard for biaryl synthesis and the mechanistic steps, such as reductive elimination, are well established. Developing routes that exploit alternative mechanistic scenarios could give unprecedented biaryl structures and expand the portfolio of biaryl applications. We have developed metal-free C-H/C-H couplings of aryl sulfoxides with phenols to afford 2-hydroxy-2'-sulfanylbiaryls. This cascade strategy consists of an interrupted Pummerer reaction and [3,3] sigmatropic rearrangement. Our method enables the synthesis of intriguing aromatic molecules, including oligoarenes, enantioenriched dihetero[8]helicenes, and polyfluorobiaryls. From our successes in aryl sulfoxide/phenol couplings and a deeper understanding of sigmatropic rearrangements for biaryl synthesis, we have established related methods, such as aryl sulfoxide/aniline and aryl iodane/phenol couplings. Overall, our fundamental interests in underexplored reaction mechanisms have led to various methods for accessing important biaryl architectures.

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Enantioselective Synthesis of Dithia[5]helicenes and their Postsynthetic Functionalization to Access Dithia[9]helicenes

A highly enantioselective synthesis of 5,13-disubstituted dibenzo[d,d']benzo[1,2-b:4,3-b']dithiophenes is reported. Key for the successful assembly of these helical architectures is the last two successive Au-catalyzed intramolecular alkyne hydroarylation events. Specifically, the second cyclization is the enantiodetermining step of the whole process and provides the desired helicenes with excellent ee values when a TADDOL-derived 1,2,3-(triazolium)phosphonite moiety (TADDOL: α,α,α',α'-tetraaryl-1,3-dioxolane-4,5-dimethanol) is employed as an ancillary ligand. The absolute stereochemistry of the newly prepared structures has been determined by X-ray crystallography to be P; the optical properties of these heterohelicenes are also reported. A three-step procedure was subsequently developed that allows the transformation of the initially obtained dithia[5]helicenes into dithia[9]helicenes without erosion of the enantiopurity.

Achieving Complexity at the Bottom: Molecular Metamorphosis Generated by Anthocyanins and Related Compounds

The concept of molecular metamorphosis is described. A molecule (flavylium cation) generates a sequence of other different molecules by means of external stimuli. The reversibility of the system allows for the flavylium cation to be recovered by other external stimuli, completing one cycle. Differently from supramolecular chemistry, molecular metamorphosis is not a bottom-up approach. All events occur at the bottom. The procedures to characterize the kinetics and thermodynamics of the cycles are summarized. They are based on direct pH jumps (addition of a base to the flavylium cation) and reverse pH jumps (addition of an acid to equilibrated solutions at higher pH values). Stopped flow is an indispensable tool to characterize these systems. The following metamorphic cycles will be described to illustrate the concept: (i) introducing the flavanone in the metamorphic system and illustrating the concept of a timer at the molecular level; (ii) response of the flavylium-based metamorphosis to light inputs and the write-lock-read-unlock-erase molecular system; (iii) a one-way cycle of direct-reverse pH jumps; (iv) interconversion of the flavylium cation with 2,2'-spirobis[chromene] derivatives; (v) 6,8 A-ring substituent rearrangements.

Iridium(III)-Catalyzed Diarylation/Annulation of Benzoic Acids: Facile Access to Multi-Aryl Spirobifluorenes as Pure Hydrocarbon Hosts for High-Performance OLEDs

Herein disclosed is the first example of diarylation/annulation of benzoic acids via an iridium catalyst system. This protocol provides a step-economic and highly efficient pathway to 1-aryl, 1,3-diaryl, 1,7-diaryl and 1,3,7-triaryl spirobifluorenes from readily available starting materials. The applications of multi-aryl spirobifluorenes as pure hydrocarbon (PHC) hosts for red, green, and blue (RGB) phosphorescent organic light-emitting diodes (PhOLEDs) were explored. Due to high triplet energies, 1,3-diaryl spirobifluorenes exhibit the potential as the host material of blue PhOLEDs. 1,7-Diaryl spirobifluorene can serve as the host of green PhOLEDs. 1,3,7-Triaryl spirobifluorene is a high-performance host for red PhOLEDs, which exhibits a high external quantum efficiency (EQE) up to 27.3 %. This work not only exemplifies the great potential of multi-aryl spirobifluorenes as PHC hosts, but also offers a new approach for the synthesis of these PHC hosts.

Palladium-Catalyzed Fluoroalkylation via C(sp3)-S Bond Cleavage of Vinylsulfonium Salts

An interrupted Pummerer/palladium-catalyzed fluoro-alkylation strategy was developed for alkenyl C-H fluoroalkylthiolation. Palladium-catalyzed ring-opening fluoroalkylation via aliphatic C-S bond cleavage of the vinylsulfonium salts efficiently afforded fluoroalkylthiolated alkene derivatives from readily available alkene substrates and CsF. The protocol features broad substrate scopes and good functional group tolerance under an air atmosphere. The practicability of the synthetic method was demonstrated by transforming the multisubstituted alkene products to diverse fluoroalkylthiolated N-heterocycles.

Sequential Ring-Opening and Ring-Closing Reactions for Converting para-Substituted Pyridines into meta-Substituted Anilines

Herein we report a method for converting para-substituted pyridine rings into meta-dialkylamino-substituted benzene rings through sequential ring-opening and ring-closing reactions. The nitrogen atom in the pyridine rings was replaced with a methine group, and a dialkylamino substituent was introduced onto the original unsubstituted carbon atom in the pyridine rings. This process can be formally regarded as a hybrid of the skeletal editing and C-H amination of pyridine rings.

Synthesis, structural characterization, and optical properties of benzo[f]naphtho[2,3-b]phosphoindoles

Phosphole-fused π-conjugated acenes have been attracting interest because of the attractive features of the phosphole moiety, such as fluorescence and chemically modifiable properties. Herein, 6-phenyl-6H-benzo[f]naphtho[2,3-b]phosphoindole was prepared by reacting dichlorophenylphosphine with a dilithium intermediate derived from 3,3′-dibromo-2,2′-binaphthyl. Various derivatives, such as a phospholium salt and a borane–phosphole complex with functional groups on the phosphorus atom were synthesized using the obtained phosphole as a common starting material. Single-crystal X-ray analysis of the parent benzo[f]naphtho[2,3-b]phosphoindole revealed that the pentacyclic ring is almost planar. Fluorescence spectroscopy data showed that the phosphole derivatives, such as phosphine oxide and the phospholium salt and borane complex exhibited photoluminescence in chloroform.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Palladium-catalysed annulative allylic alkylation for the synthesis of benzannulated heteroarenes

A conceptually novel intramolecular allylic alkylation strategy is developed for the synthesis of carbazoles and dibenzothiophenes.

Cut and sew: benzofuran-ring-opening enabled cyclopentenone ring formation

A facile approach to the fully substituted cyclopentenones involving an unprecedented benzofuran-ring-opening is described. The cleavage of a benzofuran endocyclic C2-O bond proceeded smoothly in the absence of any transition metal catalyst or highly reactive organometallic reagent. Such benzofuran-ring-opening is delicately incorporated into an acid-catalyzed cascade process, orchestrating a novel synthetic strategy for complex cyclopentenones with excellent yields and diastereoselectivities.

Facile Construction of Furanoacenes by a Three-Step Sequence Going through Disilyl-exo-cyclic Dienes

Facile synthesis of various benzonaphthofurans was achieved by intramolecular hydroarylation of 1,4-disilyl-2-aryloxy-1,3-enynes followed by cycloaddition with arynes or alkenes and finally desilylaromatization. The three-step transformation can be operated sequentially in one-pot, providing with a range of furanoacenes easily and highly effectively.

Synthetic Applications of Sulfonium Salts

This minireview aims to cover the developments over the past two decades in the chemistry of sulfonium salts. Specifically, insight is provided into the synthetic strategies available for the preparation of these compounds, the different reactivity patterns that are expected depending on their structural features or the reaction conditions applied, and the diversity of organic scaffolds that can thereby be synthesized. Additionally, the pros and cons derived from the use of sulfonium salts are presented and critically compared, when possible, in relation to reagents not based on sulfur but depicting similar reactivity.

Catalyst control of selectivity in the C-O bond alumination of biomass derived furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp² C–O bond into a sp² C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH^‡ = +19.7 (±2.7) kcal mol⁻¹, ΔS^‡ = −18.8 (±7.8) cal K⁻¹ mol⁻¹ and ΔG^‡_{298 K} = +25.3 (±0.5) kcal mol⁻¹ and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp² C–O bond through population of the σ*-orbital. Inclusion of [Pd(PCy₃)₂] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to the sp²C–H bond. The aluminium(iii) dihydride reagent [{(MesNCMe)₂CH}AlH₂] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to the palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy₃)₂]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass.

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle.

Conjugated small organic molecules: synthesis and characterization of 4-arylpyrazole-decorated dibenzothiophenes

4-Arylpyrazole-decorated dibenzothiophenes have been synthesized.

Diels–Alder reaction of tetraarylcyclopentadienones with benzo[b]thiophene S,S-dioxides: an unprecedented de-oxygenation vs. sulfur dioxide extrusion

The Diels–Alder adduct, dihydrodibenzothiophene S,S-dioxides underwent aromatization either through de-oxygenation or extrusion of sulfur dioxide to furnish substituted dibenzothiophenes or benzenes.

Synthesis of N-Alkyl and N-H-Carbazoles through SN Ar-Based Aminations of Dibenzothiophene Dioxides

Alkyl amines have become available for the synthesis of diverse N-alkyl carbazoles through twofold S_N Ar aminations of dibenzothiophene dioxides by using alkali metal bases. Of particular importance is the choice of counter cations on alkali metal bases, that is, i) the use of Li base for the efficient intermolecular reaction and ii) the sequential addition of heavier alkali metal bases (Na, K, or Cs) to promote intramolecular cyclization in a one-pot manner. This protocol also enables the cascade synthesis of N-H-carbazoles by using 2-phenylethylamine by removal of the 2-phenethyl group from N-(2-phenethyl) carbazoles in a single operation.

Cu-Catalyzed Aromatic Metamorphosis of 3-Aminoindazoles

We present a novel Cu-catalyzed aromatic metamorphosis of 3-aminoindazoles via oxidative cleavage of two C-N bonds of 3-aminoindazoles. This unprecedented reactivity of 3-aminoindazoles allows one to forge diverse nitrile-containing triphenylenes in decent yields via generation of the cyano group in situ. The current study reveals that 3-aminoindazoles could be harnessed as radical precursors via oxidative denitrogenation, the reaction mechanism of which was supported by density functional theory calculations.

A remarkably strained cyclopyrenylene trimer that undergoes metal-free direct oxygen insertion into the biaryl C-C σ-bond

A remarkably strained cyclopyrenylene trimer CP3 was synthesized and it underwent the first biaryl C-C σ-bond cleavage by direct oxygen insertion without the aid of any metal agents. A priori highly strained CP3 exhibits the longest wavelength emission among all pyrene-based fluorophores due to the intensive electronic interactions between pyrenes. The color of the emission drastically changes from orange to light blue upon oxidation. Theoretical studies revealed that the release of ring strain reasonably drives the reaction between two CP3 molecules and O2. This strain-induced transformation could be also applied for sulfur atom insertion into a biaryl σ-bond.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

KHMDS mediated synthesis of 9-arylfluorenes from dibenzothiophene dioxides and arylacetonitriles by tandem SNAr-decyanation-based arylation

A straightforward KHMDS mediated synthetic route to 9-arylfluorenes from readily available starting materials has been developed. This reaction involves SNAr reactions of dioxide with arylacetonitriles, followed by decyanation reaction. The proposed transformation can also be used to furnish a densely arylated indene.