Pd(ii)-catalyzed intramolecular diarylation of alkynes via dual C–H activation: modular access to azafulvalene-based bis(polycyclic) aromatic enes

Abstract

Bis(polycyclic) aromatic enes (BPAEs), comprising two polyaromatic units bridged by an ethylenic linker, exhibit distinctive electronic properties, yet heteroatom-containing analogues like azafulvalenes remain scarcely explored despite their tunable π-conjugation and redox versatility. Herein, we report an efficient Pd(II)-catalyzed intramolecular diarylation of alkynes that enables the selective synthesis of previously inaccessible azafulvalene-based BPAEs through dual C–H bond activation. This strategy adopts N-aryl and 2-aryl-substituted biarylalkynyl indole scaffolds, enabling modular access to structurally diverse azafulvalene-based BPAEs with precisely incorporated nitrogen atoms. Mechanistic investigations and DFT calculations revealed a pivalate-assisted concerted metallation–deprotonation (CMD) pathway, with the syn-insertion step as the rate-determining process. The addition of trifluoroacetic acid significantly lowers the activation barrier of this key step, enhancing the overall reaction efficiency. The synthesized azafulvalene-based BPAEs exhibit broad and red-shifted absorption bands extending up to 650 nm, low-lying LUMO levels (−3.47 to −3.63 eV), and narrow HOMO–LUMO gaps, consistent with donor–acceptor electronic structures predicted by DFT and TD-DFT calculations. This work establishes a modular and atom-economical synthetic approach for constructing π-extended nitrogen-containing fulvalene frameworks and highlights the potential of azafulvalene-based BPAEs as promising building blocks for advanced organic optoelectronic materials.