Manganese-Catalyzed Ring Opening of Benzofurans and Its Application to Insertion of Heteroatoms into the C2-O Bond

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.

C-to-N atom swapping and skeletal editing in indoles and benzofurans

Skeletal editing comprises the structural reorganization of compounds. Such editing can be achieved through atom swapping, atom insertion, atom deletion or reorganization of the compound's backbone structure1,2. Conducted at a late stage in drug development campaigns, skeletal editing enables diversification of an existing pharmacophore, enhancing the efficiency of drug development. Instead of constructing a heteroarene classically from basic building blocks, structural variants are readily accessible directly starting from a lead compound or approved pharmacophore. Here we present C to N atom swapping in indoles at the C2 position to give indazoles through oxidative cleavage of the indole heteroarene core and subsequent ring closure. Reactions proceed through ring-opened oximes as intermediates. These ring deconstructed intermediates can also be diverted into benzimidazoles resulting in an overall C to N atom swapping with concomitant skeletal reorganization. The same structural diverting strategies are equally well applicable to benzofurans leading to either benzisoxazoles or benzoxazoles. The compound classes obtained through these methods-indazoles3,4, benzisoxazoles5, benzimidazoles6,7 and benzoxazoles8-are biologically relevant moieties found as substructures in natural products and pharmaceuticals. The procedures introduced substantially enlarge the methods portfolio in the emerging field of skeletal editing.

Photocatalytic Boron Insertion into Thiaarenes via Boryl Radicals

Skeletal editing of aromatic heterocycles represents a straightforward approach to rapidly expand the accessible chemical space. While notable progress has been made on the direct modification of various nitrogen- and oxygen-heterocycles, editing of prevalent sulfur-containing heteroarenes, especially for single-atom insertion, remains exceedingly rare. This disparity is primarily attributed to the sulfur atom's inherent nucleophilicity and high susceptibility to oxidation. Here we present a conceptually distinct photocatalytic strategy that enables the insertion of a boron atom into a diverse range of thiaarenes, furnishing previously inaccessible cyclic thioborane scaffolds and synthetically valuable alkyl boronates in an efficient manner. Furthermore, mechanistic studies have revealed that the boron insertion step proceeds through an unprecedented mechanism.

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.

Reimagining Dearomatization: Arenophile-Mediated Single-Atom Insertions and π-Extensions

ConspectusDearomatization of simple aromatics serves as one of the most direct strategies for converting abundant chemical feedstocks into three-dimensional value-added products. Among such transformations, cycloadditions between arenes and alkenes have historically offered effective means to construct complex polycyclic architectures. However, traditionally harsh conditions, such as high-energy UV light irradiation, have greatly limited the scope of this transformation. Nevertheless, recent progress has led to the development of visible-light-promoted dearomative photocycloadditions with expanded scope capable of preparing complex bicyclic structures.A fundamentally distinct approach to dearomative photocycloadditions involves the visible-light activation of arenophiles, which undergo para-photocycloaddition with various aromatic compounds to produce arene-arenophile cycloadducts. While only transiently stable and subject to retro-cycloaddition, further functionalization of the photocycloadducts has allowed for the development of a wide collection of dearomatization methodologies that access products orthogonal to existing chemical and biological processes. Central to this strategy was the observation that arene-arenophile photocycloaddition reveals a π-system that can be functionalized through traditional olefin chemistry. Coupled with subsequent [4 + 2]-cycloreversion of the arenophile, this process acts to effectively isolate a single π-system from an aromatic ring. We have developed several transformations that bias this methodology to perform dearomative single-atom insertion and π-extension reactions to prepare unique products that cannot be prepared easily through traditional means.Through the application of a dearomative epoxidation, we were able to develop a method for the epoxidation of arenes and pyridines to arene-oxides and pyridine-oxides, respectively. Notably, when this arenophile chemistry is applied to polycyclic arenes, photocycloaddition reveals a π-system transposed from the site of native olefinic reactivity, enabling unique site-selectivity for dearomative functionalization. As a result, we were able to perform a single-atom insertion of oxygen into polycyclic (aza)arenes to prepare 3-benzoxepines. When applying this strategy in the context of cyclopropanations, we were able to accomplish a dearomative cyclopropanation of polycyclic (aza)arenes which yield benzocycloheptatrienes upon cycloreversion. Notably, while the Buchner ring expansion is a powerful method for the direct single-atom insertion of carbon into arenes, the corresponding cyclopropanation of polycyclic arenes does not yield ring-expanded products. Furthermore, this strategy could be utilized for the synthesis of novel nanographenes through the development of an M-region annulative π-extension (M-APEX) reaction. Traditionally, methods for π-extension rely on the native reactivity of polycyclic aromatics at the K- and bay-region. However, photocycloaddition of polycyclic aromatics with arenophiles acts as a strategy to activate the M-region for further reactivity. As a result, arenophile-mediated dearomative diarylation, followed by cycloreversion, could deliver π-extended nanographenes with exclusive M-region site selectivity.

Iron-Catalyzed C-H Alkylation/Ring Opening with Vinylbenzofurans Enabled by Triazoles

We report an unprecedented iron-catalyzed C-H annulation using readily available 2-vinylbenzofurans as the reaction pattern. The redox-neutral strategy, based on cheap, non-toxic, and earth-abundant iron catalysts, exploits triazole assistance to promote a cascade C-H alkylation, benzofuran ring-opening and insertion into a Fe-N bond, to form highly functionalized isoquinolones. Detailed mechanistic studies supported by DFT calculations fully disclosed the manifold of the iron catalysis.

A practical preparation of bicyclic boronates via metal-free heteroatom-directed alkenyl sp2-C‒H borylation

Bicyclic boronates play critical roles in the discovery of functional materials and antibacterial agents, especially against deadly bacterial pathogens. Their practical and convenient preparation is in high demand but with great challenge. Herein, we report an efficient strategy for the preparation of bicyclic boronates through metal-free heteroatom-directed alkenyl sp2-C‒H borylation. This synthetic approach exhibits good functional group compatibility, and the corresponding boronates bearing halides, aryls, acyclic and cyclic frameworks are obtained with high yields (43 examples, up to 95% yield). Furthermore, a gram-scale experiment is conducted, and downstream transformations of the bicyclic boronates are pursued to afford natural products, drug scaffolds, and chiral hemiboronic acid catalysts.

Dearomative ring expansion of thiophenes by bicyclobutane insertion

Skeletal ring enlargement is gaining renewed interest in synthetic chemistry and has recently focused on insertion of one or two atoms. Strategies for heterocyclic expansion through small-ring insertion remain elusive, although they would lead to the efficient formation of bicyclic products. Here, we report a photoinduced dearomative ring enlargement of thiophenes by insertion of bicyclo[1.1.0]butanes to produce eight-membered bicyclic rings under mild conditions. The synthetic value, broad functional-group compatibility, and excellent chemo- and regioselectivity were demonstrated by scope evaluation and product derivatization. Experimental and computational studies point toward a photoredox-induced radical pathway.

Rh-catalyzed ring-opening coupling of cyclic vinyl ethers with organometallic reagents

The rhodium-catalyzed ring-opening coupling of cyclic vinyl ethers, including 2,3-dihydrofuran and benzofuran, with organometallic reagents to give homoallylic alcohols and stilbenoids was reported. The suitable organometallic reagent for 2,3-dihydrofuran and benzofuran was found to be substrate-dependent, and a plausible mechanism involving different active organorhodium intermediates was proposed for these coupling reactions.

Palladium-Catalyzed Borylative Cyclizations of α-(2-Bromoaryl) Ketones to Form 1,2-Benzoxaborinines

Herein, we report that palladium catalyzes the borylative cyclization of α-(2-bromoaryl) ketones to afford 1,2-benzoxaborinines. The developed system is compatible with a variety of functionalities (Me, t-Bu, OMe, NMe₂, F, Cl, CN, CF₃, CO₂Me, and heteroaryl groups) and is applicable to the synthesis of B-O-containing tri- and tetracyclic fused-ring compounds.

Late-stage derivatization of a (B,O)2-doped perylene

Regioselective di- and tetrabrominations of the (B,O)₂-perylene 1 afford derivatives 2-4. Despite their poor solubility, 2 and 4 could be used in Stille-type coupling reactions to introduce two CCMe (5) or four CC(p-C₆H₄tBu) substituents (6), respectively. The alkynylated derivatives show blue-green photoluminescence with appreciable quantum efficiencies.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

The isoelectronic replacement of C[double bond, length as m-dash]C bonds with -B[double bond, length as m-dash]N+ bonds in polycyclic aromatic hydrocarbons (PAHs) is a widely used tool to prepare novel optoelectronic materials. Far less well explored are corresponding B,O-doped PAHs, although they have a similarly high application potential. We herein report on the modular synthesis of B,N- and B,O-doped PAHs through the [Au(PPh3)NTf2]-catalyzed 6-endo-dig cyclization of BN-H and BO-H bonds across suitably positioned C[triple bond, length as m-dash]C bonds in the key step. Readily available, easy-to-handle o-alkynylaryl boronic and borinic acids serve as starting materials, which are either cyclized directly or first converted into the corresponding aminoboranes and then cyclized. The reaction even tolerates bulky mesityl substituents on boron, which later kinetically protect the formed B,N/O-PAHs from hydrolysis or oxidation. Our approach is also applicable for the synthesis of rare doubly B,N/O-doped PAHs. Specifically, we prepared 1,2-B,E-naphthalenes and -anthracenes, 1,5-B2-2,6-E2-anthracenes (E = N, O) as well as B,O2-containing and unprecedented B,N,O-containing phenalenyls. Selected examples of these compounds have been structurally characterized by X-ray crystallography; their optoelectronic properties have been studied by cyclic voltammetry, electron spectroscopy, and quantum-chemical calculations. Using a new unsubstituted (B,O)2-perylene as the substrate for late-stage functionalization, we finally show that the introduction of two pinacolatoboryl (Bpin) substituents is possible in high yield and with perfect regioselectivity via an Ir-catalyzed C-H borylation approach.

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.

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.

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.

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.