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

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 (17 examples, up to 91% yield) and benzo[b]furans (9 examples, up to 90% yield), in good yields with a broad functional group tolerance. This protocol was adapted into an annulative πextension (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 aryl propargyl ethers, 4-diphenylbut-1-yne and a tosyl-protected propargylaniline afforded sulfenylated 6-membered products (7 examples, up to 94% yield). Extensive density functional theory (DFT) calculations were conducted and support a mechanism in which thiirenium ions are involved as key intermediates in the formation of the observed products.