Electrochemical Access to Aza-Polycyclic Aromatic Hydrocarbons: Rhoda-Electrocatalyzed Domino Alkyne Annulations

Abstract

Nitrogen-doped polycyclic aromatic hydrocarbons (aza-PAHs) have found broad applications in material sciences. Herein, a modular electrochemical synthesis of aza-PAHs was developed via a rhodium-catalyzed cascade C-H activation and alkyne annulation. A multifunctional O-methylamidoxime enabled the high chemo- and regioselectivity. The isolation of two key rhodacyclic intermediates made it possible to delineate the exact order of three C-H activation steps. In addition, the metalla-electrocatalyzed multiple C-H transformation is characterized by unique functional group tolerance, including highly reactive iodo and azido groups.

Keywords: C−H activation; aza-PAHs; domino reactions; metalla-electrocatalysis; rhodium catalysis.

Figure 1

Electrochemical synthesis of aza‐nanographene via rhoda‐electrocatalyzed domino C−H annulations. GF: graphite felt.

Scheme 1

Rhoda‐electrocatalyzed C−H activation with amidoxime 1. Rh cat.=[RhCp*(CH₃CN)₃](SbF₆)₂.

Scheme 2

Rhoda‐electrocatalyzed C−H activation with alkyne 2.

Figure 2

Cyclic voltammetry of complex 5 (2 mm, black), a mixture of 5 (2 mm) and 2 b (10 mm, red), a mixture of 5 (2 mm) and 2 b (10 mm) after heating to 40 °C for 20 minutes (green), and this mixture after addition of KOAc (10 mm, blue), all in methanol with n‐Bu₄NPF₆ (0.1 m) at 0.1 V s⁻¹.

Scheme 3

Mechanistic studies.

Figure 3

Cyclic voltammetry of compound 6 (2 mm) at varied scan rates, in dichloromethane with n‐Bu₄NPF₆ (0.1 m).

Scheme 4

Synthesis of cationic aza‐nanographene 6. DCE=1,2‐dichloroethane.

Scheme 5

Synthesis of dendrimer 8 via sixfold aza‐alkyne addition between 3 aj and 7. DIPEA=N,N‐diisopropylethylamine.

Scheme 6

Gram‐scale synthesis of 3 aa.