Mechanochemical Metal-Free Synthesis of 3-Arylindenones via Unprecedented Aryl Swapping

Comprehensive Summary A triflic anhydride-induced annulation of benzoic acids bearing electron-withdrawing groups with arylalkynes leading to diverse 3-arylindenones via an unprecedented aryl swapping pathway under an air atmosphere and ball-milling conditions has been disclosed. Most of the obtained 3-arylindenones are fluorine-containing compounds. The present mechanochemical protocol employs easily available and inexpensive benzoic acids, which unexpectedly undergo aryl swapping with arylalkynes and serve as the source of the 3-aryl group. The current mechanochemical methodology shows advantages including no requirement of transition-metal catalysts, ambient conditions and excellent regioselectivity for the rearrangement products, providing expedient access to 3-arylindenones containing functional groups such as F, Cl, Br, CF3, OCF3, SCF3, OAc, Ac, CO2Et or NO2. Moreover, the applications of this method to construct analogues of PPAR gamma agonist and antiosteoporotic agent have been demonstrated. A plausible reaction mechanism involving aryl swapping through vinyl cation formation from arylalkyne and in situ generated benzoyl triflate, intramolecular 4-endo-dig cyclization, subsequent ring opening and final annulation is proposed to explain the formation of 3-arylindenones.