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

Synthesis of substituent-free dioxadiaza[8]circulene to investigate intermolecular interactions and photophysical properties

Peripherally unsubstituted dioxadiaza[8]circulene, as the first example of structurally identified pristine hetero[8]circulene, was synthesized by the substituent detachment reactions. The solid-state structures and photophysical properties were analysed to elucidate intermolecular interactions. Herzberg-Teller type emission was considered to explain the optical behavior.

Photocatalytic furan-to-pyrrole conversion

The identity of a heteroatom within an aromatic ring influences the chemical properties of that heterocyclic compound. Systematically evaluating the effect of a single atom, however, poses synthetic challenges, primarily as a result of thermodynamic mismatches in atomic exchange processes. We present a photocatalytic strategy that swaps an oxygen atom of furan with a nitrogen group, directly converting the furan into a pyrrole analog in a single intermolecular reaction. High compatibility was observed with various furan derivatives and nitrogen nucleophiles commonly used in drug discovery, and the late-stage functionalization furnished otherwise difficult-to-access pyrroles from naturally occurring furans of high molecular complexity. Mechanistic analysis suggested that polarity inversion through single electron transfer initiates the redox-neutral atom exchange processes at room temperature.

Photocatalytic Sulfonylation: Innovations and Applications

Photosynthesis, converting sustainable solar energy into chemical energy, has emerged as a promising craft to achieve diverse organic transformations due to its mild reaction conditions, sustainability, and high efficiency. The synthesis of sulfonated compounds has drawn significant attention in the pharmaceuticals, agrochemicals, and materials industries due to the unique structure and electronic properties of the sulfonyl groups. Over the past decades, many photocatalytic sulfonylation reactions have been developed. In this review, the recent advances in photocatalyzed sulfonylation have been reviewed since 2020, with a primary focus on discussing reaction design and mechanism.

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.

Synthesis of Dithia[5]helicenes and Enantioselective Synthesis of Helically Chiral Thia[6]helicenes via Rh-Catalyzed Intramolecular [2 + 2 + 2] Cycloaddition of Triynes

Dithia[5]helicenes and helically chiral thia[6]helicenes were synthesized in high yields via a cationic Rh-catalyzed intramolecular [2 + 2 + 2] cycloaddition of triynes bearing sulfur-containing 1,6-diynes. Thia[6]helicene could be obtained with a high enantiomeric excess of P-isomers by using (S)-SEGPHOS as a chiral ligand. This protocol is the first example of the synthesis of thiahelicenes via [2 + 2 + 2] cycloaddition and can also be used for the asymmetric construction of an aza[6]helical skeleton.

Transformation of Thia[7]helicene to Aza[7]helicenes and [7]Helicene-like Compounds via Aromatic Metamorphosis

[n]Helicenes with helically twisted structures have attracted increasing interest owing to their unique properties. Therefore, it has been an important issue to develop facile synthetic methodologies which allow access to a variety of [n]helicenes. Here we report the synthesis of [7]helicenes and [7]helicene-like compounds from the thia[7]helicene as a common starting material. Desulfurative dilithiation of the thia[7]helicene and the subsequent reaction with silicon and phosphorus electrophiles afforded the silole- and phosphole-fused [7]helicene-like compounds, respectively. The cyclopentadiene-fused [7]helicene-like compound and the pyrrole-fused aza[7]helicenes were also successfully synthesized via twofold S_NAr reactions of the thia[7]helicene S,S-dioxide with the carbon and nitrogen nucleophiles, respectively. The thia[7]helicene S,S-dioxide showed a slightly red-shifted absorption spectrum than the parent thia[7]helicene, which was well demonstrated by the theoretical calculations. The substituents on the silicon atom of silole-fused [7]helicene-like compounds have little impact on the longest absorption maximum. Such little effect of the substituents on absorption properties was also observed for cyclopentadiene-fused [7]helicene-like compounds and aza[7]helicenes and was well demonstrated by the theoretical calculations. The thia[7]helicene S,S-dioxide and the silole-fused [7]helicene-like compound exhibited bright blue emission, and the cyclopentadiene-fused [7]helicene-like compound and the aza[7]helicenes showed strong violet emission. Each single enantiomer of the aza[7]helicenes showed circularly-polarized luminescence with the dissymmetry factors of 4.2~4.4 × 10⁻³.

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.

Divergent reactivity of sulfinates with pyridinium salts based on one- versus two-electron pathways

One of the main goals of modern synthesis is to develop distinct reaction pathways from identical starting materials for the efficient synthesis of diverse compounds. Herein, we disclose the unique divergent reactivity of the combination sets of pyridinium salts and sulfinates to achieve sulfonative pyridylation of alkenes and direct C4-sulfonylation of pyridines by controlling the one- versus two-electron reaction manifolds for the selective formation of each product. Base-catalyzed cross-coupling between sulfinates and N-amidopyridinium salts led to the direct introduction of a sulfonyl group into the C4 position of pyridines. Remarkably, the reactivity of this set of compounds is completely altered upon exposure to visible light: electron donor-acceptor complexes of N-amidopyridinium salts and sulfinates are formed to enable access to sulfonyl radicals. In this catalyst-free radical pathway, both sulfonyl and pyridyl groups could be incorporated into alkenes via a three-component reaction, which provides facile access to a variety of β-pyridyl alkyl sulfones. These two reactions are orthogonal and complementary, achieving a broad substrate scope in a late-stage fashion under mild reaction conditions.

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.

Highly Stable Radical Cations of N,N'-Diarylated Tetrabenzotetraaza[8]circulene

N,N'-Diarylated tetrabenzotetraaza[8]circulenes 3 a and 3 b were synthesized in good yields by a reaction sequence involving oxidation of tetrabenzodiazadithia[8]circulene 5-Oct and S_N Ar reaction with aniline derivatives. The obtained aza[8]circulenes 3 a and 3 b were easily oxidized to give their radical cations 3 a⁺ and 3 b⁺ , which are highly stable under ambient conditions. X-ray diffraction analysis of radical cation 3 a⁺ showed a face-to-face dimer arrangement with an interplanar separation of 3.320 Å. The spin density of 3 a⁺ was calculated to be delocalized over the whole circulene π-systems with spin-spin exchange integral (J=-144 cm^-1 ) in the dimeric part. These radical cations displayed far red-shifted absorption bands reaching to 2000 nm. Thus this study has proved the hetero[8]circulene scaffold to be a new entry of promising electronics and spin materials.

Hetero[8]circulenes: synthetic progress and intrinsic properties

Synthetic route for hetero[8]circulenes are classified into the annulative construction of eight-membered ring and the peripheral ring fusion of eight-membered ring. Post-transformation enables the tuning of structural features and electronic nature.

Triarylborane catalysed N-alkylation of amines with aryl esters

B(C₆F₅)₃ is demonstrated to be an active catalyst for N-alkylation reactions of amine substrates with aryl esters.