Quantum-Chemical Study of the Assembly Mechanism of Furan from Chalcone and Acetylene in MOH/DMSO and MOtBu/DMSO (M = Na, K) Superbasic Media

The mechanism of furan assembly from chalcone and acetylene in MOH/DMSO and MOtBu/DMSO (M = Na, K) superbasic media has been studied theoretically employing a DFT approach [B2PLYP-D2/6-311+G**//B3LYP/6-31+G* + IEFPCM], with explicit modeling of the superbases by MOR·<i>n</i>DMSO (M = Na, K; R = H, tBu; <i>n</i> = 4, 5) complexes. The mechanisms of two possible routes have been investigated: (i) via the intramolecular O-vinylation occurring in the adducts of the addition of the ethynide ion to the C═C double bond of chalcone, and (ii) via the intramolecular <i>O</i>-vinylation occurring in the allene isomer of the adducts. The mechanisms of possible side reactions, namely, Favorskii reaction and retro-aldol reaction of chalcone, which are considered the main factors that decrease the yield of the target furans, have also been investigated. The reasons of the different activities of MOH/DMSO and MOtBu/DMSO (M = Na, K) in the assembly reaction observed in experiment are discussed and interpreted.