Catalytic Ring Expansion of Furyl Hydrosilanes: Synthesis of 1,2-Oxasilines as Diels-Alder Diene Components

The development of new methods for aromatic ring-expansion offers a strategic means of transforming abundant aromatic feedstocks into intricate structures via skeletal editing, thereby unlocking valuable avenues for organic synthesis. We report herein a mild and efficient ruthenium-catalyzed ring-expansion of 2-silylfurans to form 1,2-oxasilines with wide substrate scope and excellent functional group tolerance. Density functional theory (DFT) calculations suggest the reaction proceeds through regioselective insertion of furan double bond into a Ru─H bond, followed by skeletal rearrangement, 1,2-hydride transfer, and elimination steps. The resulting 1,2-oxasilines are electron-rich cyclic dienes with an extrudable and transformable Si─O linker, which exhibit unique and versatile reactivity in Diels-Alder cyclization with electron-deficient dienophiles, enabling modular assembly of molecular skeletons with higher complexity.

Aromatic Metamorphosis: Skeletal Editing of Aromatic Rings

ConspectusAromatic rings are fundamental structural motifs found in natural products, synthetic intermediates, pharmaceuticals, agrochemicals, and functional materials. While transformations at the periphery of these rings are well-established, modifying their core frameworks has remained an underexplored frontier. Our group has pioneered the concept, termed "aromatic metamorphosis", enabling skeletal transformations of aromatic rings by replacing an endocyclic atom with a different atom or inserting an atom into aromatic rings, which leads to novel synthetic strategies and diverse molecular architectures.The concept of aromatic metamorphosis was first demonstrated in the stepwise conversion of dibenzothiophenes and dibenzofurans into triphenylenes. These transformations, facilitated by palladium and nickel catalysts, involve the strategic activation of robust C-S and C-O bonds as the key steps. Next, the approach was extended to the two-step conversions of dibenzothiophenes into carbazoles, dibenzophospholes, fluorenes, etc., which involve oxidation into the corresponding sulfones and subsequent sequential inter- and intramolecular nucleophilic aromatic substitution reactions. These new synthetic routes have provided efficient access to optoelectronic materials. Especially, the SNAr-based aromatic metamorphosis facilitated the construction of a heterohelicene library with systematic variation in endocyclic atoms. This strategy has revolutionized the way molecular libraries are constructed and enables the rapid discovery of functional molecules.In addition to the endocyclic substitutions, ring-expanding aromatic metamorphosis through atom insertion has also been explored. We developed nickel-catalyzed boron insertion into benzofurans, generating benzoxaborins, which are important scaffolds for medicinal chemistry. This novel catalytic transformation has been successfully scaled to industrial synthesis by companies, which demonstrates the practical utility of aromatic metamorphosis. Furthermore, manganese-catalyzed and lithium-metal-promoted methodologies have expanded the ranges of heteroatoms inserted and aromatic frameworks cleaved, providing methods to access heterocycles with a diversity in element compositions.Reductive dilithiation of thiophenes efficiently yields 1,4-dilithiobutadienes, which react with a variety of electrophiles to produce a series of nonbiogenic heteroles, such as boroles, phospholes, and siloles. In principle, this method should allow the sulfur atom in readily available thiophenes to be replaced with any atom and is therefore considered an ideal example of aromatic metamorphosis in terms of rapid construction of diverse chemical spaces with a variety of elements.Aromatic metamorphosis proposes many new synthons and retrosynthetic disconnections that defy the conventional wisdom of organic synthesis. By making full use of metamorphosing the aromatic skeleton, a library with skeletal diversity can be constructed directly with minimal effort and time investment. Its applications span from pharmaceuticals to materials science, paving the way for a new paradigm in molecular design as well as synthetic strategy.

I2-Induced Metal-Free C(sp2)-H Functionalization of Indoles via One-Pot and Two-Step Reaction with 1-(2-Hydroxyphenyl)-propargyl Alcohols: Access to 3-(Benzofuran-3-yl)-1H-indoles

The I2-catalyzed reaction of 1-(2-hydroxyphenyl)-propargyl alcohols with indoles and subsequent treatment with K2CO3 provided (benzofuran-3-yl)-1H-indoles in good to excellent yields with high regioselectivity. This one-pot and two-step strategy proved to be suitable for a wide range of substrates except for aliphatic alkynyl alcohols as well as the indoles bearing N-protected groups such as the Ts group possessing strong electron-withdrawing feature. The reaction procedure involved a cross-coupling and the construction of a benzofuran framework via 5-exo-dig cyclization.

Skeletal Editing of Aromatic N-Heterocycles via Hydroborative Cleavage of C-N Bonds-Scope, Mechanism, and Property

Skeletal editing of the core structure of heterocycles offers new opportunities for chemical construction and is a promising yet challenging research topic that has recently gained increasing interest. However, several limitations of the reported systems remain to be addressed. For example, the reagents employed are generally in high-energy, such as chlorocarbene precursors, nitrene species, and metal carbenes, which are also associated with low atomic efficiencies. Thus, the development of simple systems for the skeletal editing of heterocycles is still desired. Herein, a straightforward and facile BH3-mediated skeletal editing of readily available indoles, benzimidazoles, and several other aromatic heterocycles is reported. Structurally diverse products were readily obtained, including tetrahydrobenzo azaborinines, diazaboroles, O-anilinophenylethyl alcohols, benzene-1,2-diamines, and more. Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis revealed a BH3-induced C-N bond cleavage reaction pathway. An exciting and counterintuitive indole hydroboration phenomenon of -BH2 shift from C3-position to C2-position was disclosed. Moreover, the photophysical properties of the synthesized diazaboroles were studied, and an interestingly and pronounced aggregation-induced emission (AIE) behavior was disclosed.

One-Step Palladium-Catalyzed Heterocyclic Ring Closure of Benzofurans with Aryl Iodides through a Heck-Type Pathway

An operationally simple and robust method for the direct arylation and ring closure of benzofurans is reported. Besides the mild conditions and good reaction yields, the methodology is applicable for a wide range of derivatives using commercially available aryl iodides with complete C2 regioselectivity. The reaction is proposed to follow a Heck-type oxyarylation mechanism. The facile synthesis method will enable the development of new materials for diverse applications in biology and catalysis and as precursors for organic semiconductors.

Ring-opening silylation of N-arylindoles via endocyclic C-N bond cleavage triggered by silylboranes

The cleavage of heteroaromatic endocyclic carbon-heteroatom bonds to assemble C-Si bonds is scarce. Here, we demonstrate an unprecedented dearomatization silylation of N-arylindoles arising from reductive activation initiated by electron-rich silylboronic complexes to deliver silyl styrenes with perfect stereoselectivity.

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.

Room Temperature C-H Arylation of Benzofurans by Aryl Iodides

A robust method of room temperature direct arylation for benzofuran is reported. This discovery allows for mild arylation by commercially available aryl iodides with complete C-2 regioselectivity and tolerates a range of functional groups, including heat sensitive groups. Mechanistically, a Heck-type oxyarylation product from a direct arylation process is reported as a key piece of evidence for a carbopalladation intermediate.

Synthesis of [6,6,m]-Tricyclic Compounds via [4+2] Cycloaddition with Au or Cu Catalyst

We synthesized [6,6,6]- and [6,6,7]-tricyclic compounds via intramolecular [4+2] cycloaddition by gold or copper catalysts. Substrates for cyclization were prepared by coupling reactions between eight types of diyne and four types of aromatic moieties. We have successfully synthesized eleven tricyclic compounds.

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.

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.

New reductive rearrangement of N-arylindoles triggered by the Grubbs-Stoltz reagent Et3SiH/KO t Bu

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide. Studies indicate that the pathway involves (i) the formation of indole radical anions followed by fragmentation of the indole C2–N bond, and (ii) a ring-closing reaction that follows a potassium-ion dependent hydrogen atom transfer step. Unexpected behaviors of ‘radical-trap’ substrates prove very helpful in framing the proposed mechanism.

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide.

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.