New method for preparation of coumarins and quinolinones via Pd-catalyzed intramolecular hydroarylation of C-C triple bonds

A new and general method has been developed for preparation of coumarins an d quinolinones by intramolecular hydroarylation of alkynes. Various aryl al kynoates and alkynanilides undergo fast intramolecular reaction at room tem perature in the presence of a catalytic amount of Pd(OAc)(2) in a mixed sol vent containing trifluoroacetic acid (TFA), affording coumarins and quinoli nones in moderate to excellent yields with more than 1000 turnover numbers (TON) to Pd. The methodology proved to tolerate a number of functional grou ps such as Br and CHO. On the basis of isotope experiments, a possible mech anism involving ethynyl chelation-assisted electrophilic metalation of arom atic C-H bonds by in-situ generated cationic Pd(II) species has been discus sed. Also the involvement of vinylcationic species has been suggested.