Ruthenium-Catalyzed Pyrrole Synthesis from 1,4-Alkynediols

Significance

Reported is a Ru(PPh₃)₃(CO)H₂- and XantPhos-catalyzed conversion of various 1,4-alkynediols into pyrrole derivatives. The reaction starts with an isomerization of the diol into a 1,4-diketone effected by the XantPhos-Ru complex and continues with a Paal-Knorr cyclization to the pyrrole. The amine component, as well as the alkyne component were quite thoroughly investigated. The reaction fails for anilines containing strong EWGs and for strongly hindered substrates. The conversion is in almost all cases complete (one exception), but the reaction sometimes stops at the diketone stage or leads to furan byproducts, thus the lower yields. Variation in catalyst loading was not studied.