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

A direct organocatalytic carbonyl/olefin oxo‐metathesis has been developed. The reaction is catalyzed by trityl tetrafluoroborate (TrBF 4 ) 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.