Polarity of the Transition State Controls the Reactivity of Related Charged Phenyl Radicals Toward Atom and Group Donors

• Published in: Journal of organic chemistry 2001-04, Vol.66 (8), p.2726-2733
• Main Authors: Tichy, Shane E, Thoen, Kami K, Price, Jason M, Ferra, Joseph J, Petucci, Chris J, Kenttämaa, Hilkka I
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo001634r

Polar effects are demonstrated to be a key factor in controlling the reactivities of related charged phenyl radicals in different exothermic atom and group abstraction reactions in the gas phase. The effects of various meta substituents on the phenyl radicals' reactivity were probed via the measurement of bimolecular reaction rate constants by using Fourier transform ion cyclotron resonance mass spectrometry. This approach requires an additional, charged substituent to be present in the phenyl radical to allow mass spectrometric manipulation. The m-pyridinium group was chosen for this purpose. The substrates studied were allyl iodide, dimethyl disulfide, and tert-butyl isocyanide. Two of the reactions of interest, •I and •SCH3 transfer, are thought to occur by concerted bimolecular homolytic substitution (SH2), and the third one, •CN transfer, by an addition/elimination mechanism. For all three substrates, the reaction rate was found to increase in the following order for the differently substituted phenyl radicals:  CH3 ≈ H < Br ≈ Cl ≈ COOH < NO2 ≈ CN. This trend does not arise from differences in reaction exothermicities or bond dissociation energies but via lowering the reaction barrier by electronic effects. The stabilization of the transition state is attributed to its increased polar character. A semiquantitative measure of the barrier lowering effect for each substituent is obtained from its influence on the electron affinity of the charged radical, as the calculated (B3LYP/6-31+G(d)) adiabatic electron affinities of the radical model systems (ammonium instead of pyridinium charge site) follow the same trend as the reactivities.