Oxidatively induced reactivity in Rh(iii)-catalyzed 7-azaindole synthesis: insights into the role of the silver additive.

Abstract

A typical synthetic protocol for preparing 7-azaindoles involves the coupling of 2-aminopyridine and alkyne substrates using a Rh(iii)-catalyst. The catalysis requires the assistance of an external Ag+ oxidant that is thought to regenerate the catalyst and increase the turnover efficiency. Density functional theory (DFT) simulations confirm that Ag+ can oxidize various neutral Rh(iii) intermediates encountered at different stages of the catalysis. Among them, the catalytically relevant species is a cationic Rh(iii)-pyridyl+ complex (2A), which undergoes C-H activation of pyridine and couples an internal alkyne substrate into the pyridyl ligand to form the desired 7-azaindole product. Computations reveal that the oxidation also accelerates the reaction steps, including C-H activation via concerted metalation deprotonation (CMD), 1,2-alkyne insertion, and reductive elimination, thus highlighting the role of Ag+ as a catalytic promoter for the oxidatively induced reactivity of the Rh-catalyst in 7-azaindole synthesis. DFT calculations show that the catalysis is inefficient without invoking an oxidatively induced reaction pathway.