Stereoelectronic Influence of a “Spectator” Propargylic Substituent Can Override Aromaticity Effects in Radical Peri-Cyclizations en Route to Expanded Polyaromatics

Tethering Three Radical Cascades for Controlled Termination of Radical Alkyne peri-Annulations: Making Phenalenyl Ketones without Oxidants

Although Bu3Sn-mediated radical alkyne peri-annulations allow access to phenalenyl ring systems, the oxidative termination of these cascades provides only a limited selection of the possible isomeric phenalenone products with product selectivity controlled by the intrinsic properties of the new cyclic systems. In this work, we report an oxidant-free termination strategy that can overcome this limitation and enable selective access to the full set of isomerically functionalized phenalenones. The key to preferential termination is the preinstallation of a "weak link" that undergoes C-O fragmentation in the final cascade step. Breaking a C-O bond is assisted by entropy, gain of conjugation in the product, and release of stabilized radical fragments. This strategy is expanded to radical exo-dig cyclization cascades of oligoalkynes, which provide access to isomeric π-extended phenalenones. Conveniently, these cascades introduce functionalities (i.e., Bu3Sn and iodide moieties) amenable to further cross-coupling reactions. Consequently, a variety of polyaromatic diones, which could serve as phenalenyl-based open-shell precursors, can be synthesized.

Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar's Sextets

This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar's π-sextets. To illustrate the utility of this concept to the design of Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar's sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y₀ value of 0.98 (y₀ = 0 for a closed-shell molecule, y₀ = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet-triplet gap, i.e., the ΔE_ST values, in both of these systems approaches zero: -0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C-H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔE_ST value of -0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.

Cascade Transformations of 1-R-Ethynyl-9,10-anthraquinones with Amidines: Expanding Access to Isoaporphinoid Alkaloids

The interaction of acetamidine and phenylamidine with peri-R-ethynyl-9,10-anthraquinones in refluxing n-butanol leads to the formation of cascade transformations products: addition/elimination/cyclization-2-R-7H-dibenzo[de,h]quinolin-7-ones and(or) 2-R-3-aroyl-7H-dibenzo[de,h]quinolin-7-ones. The anti-inflammatory and antitumor properties of the new 2-R-7H-dibenzo[de,h]quinolin-7-ones were investigated in vivo, in vitro, and in silico. The synthesized compounds exhibit high anti-inflammatory activity at dose 20 mg/kg (intraperitoneal injection) in the models of exudative (histamine-induced) and immunogenic (concanavalin A-induced) inflammation. Molecular docking data demonstrate that quinolinones can potentially intercalate into DNA similarly to the antitumor drug doxorubicin.

Twofold π-Extension of Polyarenes via Double and Triple Radical Alkyne peri-Annulations: Radical Cascades Converging on the Same Aromatic Core

A versatile synthetic route to distannyl-substituted polyarenes was developed via double radical peri-annulations. The cyclization precursors were equipped with propargylic OMe traceless directing groups (TDGs) for regioselective Sn-radical attack at the triple bonds. The two peri-annulations converge at a variety of polycyclic cores to yield expanded difunctionalized polycyclic aromatic hydrocarbons (PAHs). This approach can be extended to triple peri-annulations, where annulations are coupled with a radical cascade that connects two preexisting aromatic cores via a formal C-H activation step. The installed Bu₃Sn groups serve as chemical handles for further functionalization via direct cross-coupling, iodination, or protodestannylation and increase solubility of the products in organic solvents. Photophysical studies reveal that the Bu₃Sn-substituted PAHs are moderately fluorescent, and their protodestannylation results in an up to 10-fold fluorescence quantum yield enhancement. DFT calculations identified the most likely possible mechanism of this complex chemical transformation involving two independent peri-cyclizations at the central core.

Coligand driven diverse organometallation in benzothiazolyl-hydrazone derivatized pyrene: ortho vs. peri C–H activation

Divergent aromatic C–H activation at both ortho and peri positions in polyaromatic hydrocarbons has been successfully accomplished by suitable variation of the coligand.