Developing highly active Cu(ii)-triazolyl-pyridine complexes for C–H functionalization of 9H-fluorene and indole via a borrowing hydrogen strategy

Abstract

Herein, we present the distinct capability of air-stable Cu(II)-complexes with N-bound triazolyl-pyridine ligands for the C–H functionalization of 9H-fluorene and indole with alcohols, which can be used as more reliable and greener alkylating agents with the formation of environmentally benign water as the sole byproduct. Methodologies have been developed, utilizing a broad spectrum of alcohols, including primary benzyl alcohols, aliphatic alcohols, and, impressively, even secondary alcohols. Importantly, a few drug molecules, like benflumetol (an antimalarial drug), turbomycin B, and an orphan nuclear receptor, were also synthesized using the developed borrowing hydrogen strategy. The Cu(II)-complexes were characterized by EPR spectroscopy, IR spectroscopy, and UV-visible spectroscopy. Hirshfeld analysis and spin density calculations were also performed to shed light on the electrophilic propensity of the Cu-centre. The molecular structure of one of the Cu(II)-complexes was also determined via the single-crystal X-ray diffraction method. A number of post-functionalization reactions successfully demonstrated the practicality of the method. Additionally, this easily scalable procedure, which follows an ionic pathway by utilizing a hydrogen borrowing strategy, confirmed through various control experiments, operates smoothly under mild conditions with minimal catalyst loading.