Changing the Reaction Pathway by NHC/Brønsted Base Cooperative Catalysis: Highly Stereoselective Synthesis of Multifunctional Benzo[a]fluoren-11-ones from the Dimerization of 2‑(Aroylvinyl)arylaldehydes

The unprecedented NHC/Brønsted base-cocatalyzed dimerization reaction of 2-(aroylvinyl)­arylaldehydes was reported. In the presence of a triazole carbene catalyst alone, no reaction of 2-(aroylvinyl)­arylaldehydes was observed. However, the combination of triazole carbene and 4-methoxyphenolate efficiently catalyzed the dimerization of 2-(aroylvinyl)­arylaldehydes to proceed through a benzoin–Michael–Michael reaction cascade, producing 6-aroyl-5-(aroylmethyl)-11a-hydroxybenzo­[a]­fluoren-11-ones as the sole diastereomers in good yields.