The Role of the Carbonyl Group in C–C Triple Bond Cleavage of Ynones via Grob-Type Fragmentation under Acidic Conditions

Cleavage of the C-C triple bond is a unique pathway for the generation of new synthons. Nevertheless, the high bond dissociation energy, coupled with the acidic nature of the alkyne proton, renders the highly selective cleavage of the C-C triple bond of ynone via direct activation of the σ bond a challenging task. Herein, we report cleavage of the C-C triple bond via direct activation of the C-C triple bond of ynone by trifluoroacetic acid (TFA)-mediated lowest unoccupied molecular orbital (LUMO) lowering of the carbonyl group. Dehydrogenation of dihydroquinolinone was observed under the same reaction conditions, and this is the first report of this kind. Broad functional group tolerance was observed for trifluoroacetic acid-mediated C-C triple bond cleavage and the dehydrogenation reaction. A detailed mechanistic study using combined experimental and computational studies confirmed Grob-type fragmentation for the C-C triple bond cleavage of ynones.