Computational study of reactivity and transition structures in nucleophilic substitutions on benzyl bromides

Abstract

DFT computations on the mechanisms of nucleophilic substitutions on benzyl bromides were performed and the calculated activation parameters were compared with experimentally acquired data. In vacuo, the presence of electron-withdrawing (e-w) groups on the benzyl bromides accelerated the reactions and the calculated ΔG‡/ΔH‡/ΔS‡ vs. σ plots were linear, while in solvents there were breaks in the calculated and measured ΔG‡/ΔH‡ vs. σ+ plots of the reactions between the benzyl bromides and Br-, both with electron-donating (e-d) and with e-w substituents accelerating the reactions. The calculated ΔS‡ values appeared to be independent of the substituents. In solvents, the calculated ΔG‡/ΔH‡/ΔS‡ vs. σ+ plots for the reactions between benzyl bromides and pyridine were linear, whereas breaks were observed in the plots of the measured data. These reactions were promoted by e-d substituents, but the measured reactivities of substrates bearing e-w groups were higher than expected. No breaks in the ΔG‡/ΔH‡/ΔS‡ vs. σ plots were observed when the substituents on the pyridine nucleophile were changed. The best agreement between the calculated and measured values was obtained for the least solvent-dependent ΔG‡ parameter. The experimentally measured ΔS‡ and ΔH‡ data were influenced by the rearrangement of the solvent mole-cules. The calculated structural parameters of the transition states (TSs) varied linearly with the substituent constants, with loose and tight distorted trigonal-bipyramidal TSs being formed by benzyl bromides bearing e-d and e-w groups, respectively. The reactions proceeded by SN2 mechanisms; only the transition structures were changed with the substituents and the media. © Wiley-VCH Verlag GmbH & Co. KGaA, 2006.