The Multieffects of DMF and DBU on the [5+1] Benzannulation of Nitroethane and α-Alkenoyl Ketene-(S,S)-Acetals: Hydrogen Bonding and Electrostatic Interactions

A density functional theory (DFT) study was performed to elucidate the mechanism for the [5+1] benzannulation of nitroethane and -alkenoyl ketene-(S,S)-acetals. The calculation results are consistent with experimental findings, showing that the reaction proceeds via deprotonation of nitroethane, nucleophilic addition, intramolecular cyclization, elimination of HNO2, and the keto-enol tautomerization sequence. It was disclosed that N,N-dimethylformamide (DMF) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) act as not only solvent and nonnucleophilic base, respectively, but also catalysts in the reaction by stabilizing the transition states (TSs) and intermediates via intermolecular hydrogen bonds and electrostatic interactions. Besides, the effective orbital interaction of the reaction site in TS also contributes to the intramolecular cyclization step. The new mechanistic insights obtained by DFT calculations highlight that the hydrogen bonds and electrostatic interactions are key factors for the [5+1] benzannulation of nitroethane and -alkenoyl ketene-(S,S)-acetals. (c) 2015 Wiley Periodicals, Inc.