Elucidating the Mechanism of Coumarin Homodimerization Using 3-Acetylcoumarin Derivatives

The current study is a continuation of our previous investigations into the radical homodimeric reaction mechanism of 3-acetylcoumarin. In the current study, the effects of different substituents on the coumarin ring of 3-acetylcoumarin are investigated both experimentally and theoretically. Several 3-acetylcoumarin derivatives (substituted at C-6, C-7, and C-8) were tested in the optimized reaction conditions under ultrasound irradiation, and biscoumarin species were isolated and characterized. The elucidation of the substituent's effect was further investigated by means of DFT calculations (free-energy calculations, NBO analysis), both in the initial substituted coumarins and in the formed radicals. It was observed that the presence of substituents at the C-6 and C-8 positions in the coumarin moiety would not affect significantly the formation of a radical, while a group at position C-7 could either stabilize or destabilize the formed radical depending on the electronic properties of the substituent.