Gas-Phase Reactivity of Protonated 2-, 3-, and 4-Dehydropyridine Radicals Toward Organic Reagents

To explore the effects of the electronic nature of charged phenyl radicals on their reactivity, reactions of the three distonic isomers of <i>n</i>-dehydropyridinium cation (<i>n</i> = 2, 3, or 4) have been investigated in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. All three isomers react with cyclohexane, methanol, ethanol, and 1-pentanol exclusively via hydrogen atom abstraction and with allyl iodide mainly via iodine atom abstraction, with a reaction efficiency ordering of <b>2</b> > <b>3</b> > <b>4</b>. The observed reactivity ordering correlates well with the calculated vertical electron affinities of the charged radicals (i.e., the higher the vertical electron affinity, the faster the reaction). Charged radicals <b>2</b> and <b>3</b> also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of <b>4</b> with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of <b>4</b> is likely to result from the contribution of an ionized carbene-type resonance structure that facilitates nucleophilic addition to the most electrophilic carbon atom (C-4) in this charged radical. The influence of such a resonance structure on the reactivity of <b>2</b> is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals <b>2</b> and <b>3</b> abstract a hydrogen atom from the substituent in both phenol and toluene, but <b>4</b> abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical <b>4</b> reacts with <i>tert</i>-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for <b>2</b> and <b>3</b>. The different reactivity observed for <b>4</b> (as compared to <b>2</b> and <b>3</b>) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.