Direct Organocatalytic Oxo‐Metathesis, a trans‐Selective Carbocation‐Catalyzed Olefination of Aldehydes

Abstract

AbstractA direct organocatalytic carbonyl/olefin oxo‐metathesis has been developed. The reaction is catalyzed by trityl tetrafluoroborate (TrBF4) and utilizes unactivated alkenes for the olefination of aromatic aldehydes to give trans β‐alkylstyrenes in yields of 44–85 % with only acetone as the byproduct. The pronounced Lewis acidity of the carbocation results in unusual reactivity that is proposed to catalyze a stepwise [2+2] cycloaddition to give an oxetane intermediate. Fragmentation of the latter in a formal retro [2+2] reaction gives the oxo‐metathesis product.