An atom economical and environmentally benign protocol has been developed for the regioselective C(sp2)-H bondfunctionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing awide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broadsubstrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played acrucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafastnature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated.Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzedpathway. Furthermore, regioselectivity has also been explained theoretically.

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