Electrochemical cascade access to hetero[8]circulenes as potent organophotocatalysts for diverse C-X bond formations

Abstract

The chemistry of hetero[8]circulenes has been limited to three main types, constrained by synthetic challenges in creating unsymmetrical variants. Herein, we introduce an electrochemical approach to a type of hetero[8]circulene, featuring five hexagons and three pentagons. Our method capitalizes on the sustainability and selectivity of electrochemistry, utilizing differential oxidation potentials to generate dioxaza[8]circulenes through selective intermolecular and intramolecular couplings under mild conditions, achieving yields of up to 83% with good functional group tolerance. We further refine this process into a one-pot protocol using commercially available substrates, forming six new bonds. Comprehensive structural, optical, and electrochemical characterizations, including X-ray crystallography, spectrophotometric analysis, and DFT calculations, are conducted. Inspired by their distinct structural and redox properties, we explore the application of dioxaza[8]circulenes as organophotocatalysts for diverse C-X (X = C, B, S, P) bond formation achieving up to 97% yields under LED light irradiation (365 nm) without transition metals.

Fig. 1. Synthesis of different types of hetero[8]circulenes.

a Synthetic approaches to dihetero[8]circulenes (Type I, six hexagons and two pentagons); (b) Tetrahetero[8]circulenes (Type II, four hexagons and four pentagons); (c) Octahetero[8]circulenes (Type III, eight pentagons); (d) Electrochemical approach to trihetero[8]circulenes (Type IV, five hexagons and three pentagons) and their application as organophotocatalysts for diverse C–X bond formations; Pink ring fill highlights the hexagons, while gray ring fill highlights the pentagons; Newly formed bonds are indicated by bold lines.

Fig. 2. Substrate-scope studies for the electrochemical sequential synthesis of hetero[8]circulenes.

a Electrosynthesis conditions using carbazoles [1 (X = OTf) or 1’ (X = H)] and 2,7-dihydroxynaphthalene 2 as substrates: Pt anode, Pt cathode, constant current = 1.0 mA (J  =  0.51 mA/cm²), Bu₄NClO₄ (0.2 M), CH₂Cl₂ (10 mL) at 25 °C; ^aUsing (2.0 equiv.) of 2; b One-pot synthesis of dioxaza[8]circulenes 3 from commercially available substrates 4 and 5; FE: Faradic efficiency; Newly formed bonds are indicated by bold lines.

Fig. 3. Structural, optical, and electrochemical features of dioxaza[8]circulenes 3a.

a Top-view of the crystal structure of 3a along with selected bond lengths illustrating the irregular shape of the aromatic rings; (b) Side-view of the crystal structure of 3a with ellipsoids at 30% probability (H atoms were omitted for clarity); (c) Packing structure of 3a is viewed along the b-axis to show the cofacial lamellar pattern; (d) Aromaticity of dioxaza[8]circulene 3a: NICS(1)_zz calculated at the MN15/cc-PVTZ level of theory and AICD plots calculated at the B3LYP/6-311 G(d,p) level of theory (isosurface value: 0.05); (e) UV/Vis absorption and PL spectra of 3a in chloroform (20 µM); (f) Frontier Kohn-Sham molecular orbitals (HOMO & LUMO) of 3a optimized in the lowest energy excited state (S₁) and TD-DFT calculated transitions at MN15/ cc-PVTZ level of theory (for further information); (g) The cyclic voltammetry profile of 1a, 2, and 3a in MeCN with n-Bu₄NClO₄ (0.1 M) using ferrocene as external reference.

Fig. 4. Substrate-scope of the photocatalytic arylation reactions towards diverse C–X bond formations.

^a Without Cs₂CO₃ (base); ^b Using Ar–Cl instead of Ar–Br; ^c Using Ar-I instead of Ar–Br; ^d Using 3.0 equiv. of 7 instead of 20.0 equiv.; ^e NMR yield; Newly formed bonds are indicated by bold lines.

Fig. 5. Control experiments and plausible mechanism for the circulene-photo-catalyzed arylation reactions of 6 and 7.

a Radical inhibition and no light experiments; b Plausible photo-induced radical-mediated mechanistic pathway.