Solvent-Mediated C- or N-Center Selective Syn-Hydroheteroarylation of Ynamides with N-Heteroaromatics: Access to a Solvatofluorochromic, Mechanofluorochromic, and Thermochromic Material

Abstract

We report a solvent (HFIP)-mediated, C- or N-center selective, and 100% atom-economic syn-hydroheteroarylation of ynamides with N-heteroaromatics to access a wide variety of stereodefined α-heteroarylenamides at room temperature in good to excellent yield of up to 98%. While indole, pyrrole, and furan underwent the reaction with ynamides via a C-center, other N-heterocycles, such as pyrazole, triazole, and phenothiazine, underwent the same via the N-center, selectively. This chemo-, regio-, and stereoselective transformation required only HFIP, which played a couple of crucial roles, such as Brønsted acid to protonate ynamide regioselectively to generate the reactive keteniminium intermediate and then stabilize the intermediate as an indispensable solvent through H-bonding. Notably, the reactions were very clean, and the products were obtained pure without requiring column chromatography in many cases. HFIP is recovered through distillation (∼90% recovery) and recycled for subsequent reactions without any compromise in the reaction outcome. The green chemistry metrics of this protocol were excellent. Significantly, this green protocol enabled the direct synthesis of highly substituted N-H carbazoles from N-H indoles via a one-pot, two-step strategy by using only a couple of solvents, i.e., HFIP and toluene. Interestingly, a synthesized product, i.e., (Z)-N-(1-(1H-indol-3-yl)-2-(4-nitrophenyl)vinyl)-N-benzylbenzenesulfonamide, exhibited interesting properties, such as solvatofluorochromism, mechanofluorochromism, and reversible thermochromism.