| Abstract
| - The 1,3-dioxolane-2-phosphenium ion, 1,3-benzodioxole-2-phosphenium ion, and o-biphenylenephosphenium ion are reported to react in a stereoselective manner with cis- and trans-1,2-diaminocyclohexanes in the gas phase in a Fourier transform ion cyclotron resonance massspectrometer. Elimination of NH3 from an addition product was observed only for the trans isomer.Several reaction mechanisms were experimentally and computationally examined (B3LYP/6-31G(d)//HF/6-31G(d) + ZPVE level of theory). The most plausible mechanism is initiated by additionof one of the amino groups to the electrophilic phosphorus atom followed by proton transfer betweenthe amino groups. A change to a diaxial conformation for the trans isomer facilitates anchimericassistance by the now nucleophilic phosphorus atom as the C−N bond breaks to release NH3.Intramolecular proton transfer competes with the conformational change and ultimately leads toethylene glycol elimination. The transition states for the critical steps of these two reactions arecalculated to be nearly equal in magnitude, which rationalizes the observation of both reactionsfor the trans-diamine. In contrast, the adduct of the cis isomer can eliminate NH3 via a concerted1,2-hydride shift without a need for a conformational change. However, the barrier associated withthis reaction was found to be substantially greater than for proton transfer between the N- andO-atoms. The latter reaction dominates and ultimately leads to ethylene glycol elimination.
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