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 n-dehydropyridinium cation (n = 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 2 > 3 > 4. 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 2 and 3 also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of 4 with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of 4 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 2 is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals 2 and 3 abstract a hydrogen atom from the substituent in both phenol and toluene, but 4 abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical 4 reacts with tert-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for 2 and 3. The different reactivity observed for 4 (as compared to 2 and 3) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.
Adeuya
Anthony
A
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
Price
Jason M
JM
Jankiewicz
Bartłomiej J
BJ
Nash
John J
JJ
Kenttämaa
Hilkka I
HI
D000438:Alcohols; D002412:Cations / Q000737:chemistry; D003510:Cyclohexanes; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry*; D005663:Furans; D005740:Gases / Q000737:chemistry*; D006860:Hydrogen Bonding; D013058:Mass Spectrometry; D009930:Organic Chemicals / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*
D000438:Alcohols; D002412:Cations; D003510:Cyclohexanes; D005609:Free Radicals; D005663:Furans; D005740:Gases; D009930:Organic Chemicals; D011725:Pyridines; C506365:Cyclohexane