Reactions of 2-bromopropene radical cation with amines-a study by FT-ICR spectrometry and DFT calculation

摘要

The reactions of the 2-bromopropene radical cation 1~(·+) with four amines (CH_3NH_2, CH_3CH_2NH_2, (CH_3)_2NH, and (CH_3)_3N) and CH_3SH were studied by Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry and molecular orbital calculations. The ionization energy (IE) of (CH)_3)_2NH, and (CH_3(_3N), respectively, are considerably below the IE(1). Consequently, fast electron transfer (ET) is the only reaction detected with 1~(·+). The amine radical cation formed undergoes a (known) fast secondary reaction with the neutral amine and generates the corresponding ammonium ion. ET is also exothermic (0.14-0.78 eV) for the other reactants, but in these reaction systems substitution of the Br atom of 1~(·+) by the nucleophile and oxidation of the nucleophile by hydride transfer to 1~(·+) competes efficiently with ET. The minimum energy reaction path (MERP) for the reaction system 1~(·+)/CH_3NH_2 was analyzed by theoretical calculation of stationary points using DFT at the level B3LYP/3-31+G(3df,2p)//B3LYP/6-31G(d). The calculated MERP agrees to the general reaction scheme developed for the reactions of ionized alkenes with nucleophiles. In particular, the initial reaction step for substitution and oxidation, which competes with ET, is addition of the nucleophile to the ionized double bound. No energy minimum along the MERP was found with B3LYP/6-31G(d) corresponding to the encounter complex of 1~(·+) and CH_3NH_2, while both UHF/6-31(Gd) and MP2/6-31G(d) show this complex as an energy minimum. The charge and spin density of the complex is almost completely localized at the 2-bromopropene component, confuting and "hidden charge transfer" in the complex prior to further reactions. The initial addition step, which is strongly exothermic, can be viewed as an "inner sphere" ET which combines the single ET from the nucleophile to 1~(·+) with simultaneous bond formation between reactants. This "inner sphere" ET competes with "outer sphere" ET resulting only in charge exchange between the reactants. In the case of simple amines as nucleophiles, the "outer sphere" ET requires considerable reorganization of the amine structure. This is indicated by significantly higher vertical IV_V of the amines. Therefore, a fast ET prevails over fast addition of the nucleophile to the ionized double bond of 1~(·+) only if the IE_v (amine) is distinctly below IE(1).