Gas-phase CαCβ double bond cleavage in the dissociation of protonated 2-benzylidenecyclopentanones: Dissociative proton transfer and intramolecular proton-transport catalysis

Among gas-phase dissociation reactions, double bond cleavage reaction appears to happen extremely rare, especially in the case of CC double bond. In the dissociation reaction of protonated 2-benzylidenecyclopentanones in tandem mass spectrometry, the formation of benzyl cations was observed, resulting from the cleavage of Cα=Cβ double bonds, which is different from the general cleavage route seen in most α, β-unsaturated ketone cases. A combined experimental and theoretical investigation on intramolecular hydrogen transfers was carried out to illustrate the mechanisms. The external proton is initially localized on the carbonyl oxygen (the thermodynamically-preferred protonation site). Upon collisional activation, the mobile proton stepwise migrates to the Cα position to achieve the reduction and subsequent cleavage of the Cα=Cβ double bond. The stepwise proton transfer is achieved via intramolecular proton-transport catalysis with the assistance of the phenyl ring. The ortho position of the phenyl accepts the proton from the carbonyl oxygen via a 1,6-H shift, and then donates it to the Cα stepwise. The conventional 1,3-H shift from the carbonyl oxygen to the Cα position can be excluded in this case due to its significant energy barrier. Further isotope-labeling experiments are applied to confirming the reaction mechanism. Last but not least, the scope-expansion experiments indicates that the aromatic and cycloalkanonyl moieties play a crucial roles in the cleavage reaction of Cα=Cβ double bond.

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