Borane Catalysed Annulative Sulfenylation of Internal Alkynes: Towards the Synthesis and Study of Fused Heterocycles

Abstract

Herein, we disclose a B(C₆F₅)₃-catalysed intramolecular cyclisation reaction of N-protected alkynyl amines and phenols to generate 5-membered heterocycles, including 3-sulfenyl indoles (16 examples, up to 90% yield) and benzo[b]furans (10 examples, up to 90% yield), in good yields with broad functional group tolerance. This protocol was adapted into an annulative π-extension (APEX) reaction when using diyne derivatives of aniline and phenol, which effectively led to sulfenylated polyaromatic heterocycles, such as benzo[a]carbazole or naphtho[1,2-b]benzofurans. These products exhibit fluorescence from locally excited states, consistent with their large singlet-triplet energy gaps. Additionally, the cyclisation of propargyl ethers affords sulfenylated 6-membered 2H-chromenes (7 examples, up to 55% yield). Extensive density functional theory (DFT) calculations were conducted and support a mechanism in which thiirenium ions were identified as key intermediates for the formation of the observed products.