“Thermometer” Ions Can Fragment Through an Unexpected Intramolecular Elimination: These Are Not the Fragments You Are Looking For

Benzylpyridinium analogs are effective
thermometer ions since monitoring
the formation of the benzylium fragment produced from heterolytic
cleavage of the C–N bond can be linked to the ion’s
internal energy. In this study, three para-substituted
benzylpyridinium ions containing ethoxy (OEt), isopropoxy (OiPr) and tert-butoxy (OtBu) substitutents were synthesized and evaluated as chemical thermometers.
Intriguingly, the product ion spectra of the three benzylpyridinium
ions were dominated by m/z 107 instead
of the anticipated benzylium species. Deuterium labeling suggested
that the m/z 107 fragment resulted
from an intramolecular elimination (Ei), which formed via
a four-membered transition state (TS). The fragmentation pathway appears
to be an anomaly within the mass spectrometry literature, as four-membered
pericyclic TSs are usually accompanied by the formation of an exceptionally
stable neutral molecule (e.g., CO2). Quantum-chemical calculations
confirmed our hypothesis that stabilization of the strained TS is
afforded by hyperconjugation (ΔG‡ tert-butoxy < isopropyoxy < ethoxy).