Kinetico-Mechanistic Information about Alkene Hydroamination with Aniline in Bromide-Rich Ionic Media: Importance of Solvolysis
Résumé
The study of the [PtBr4]2– reactivity with hexene and aniline in highly ionic (Bu4P)Br/CH2Br2 media has been studied from a Kinetico-Mechanistic perspective. The results indicate bromide ion association to the square-planar starting material to produce a stable diamagnetic compound that can be described as an ion pair of a [PtBr5]3– square-pyramidal complex stabilized by several phosphonium countercations. While this species reacts rapidly with aniline, producing the known square-planar complex [PtBr3(PhNH2)]− with release of the apical bromide of the square-pyramidal intermediate, the reaction with hexene, producing the square-planar [PtBr3(hexene)]− complex, is much slower. The thermal and pressure activation parameters determined for these processes fully agree with the proposed reactivity. The gross features of the platinum-catalyzed hydroamination mechanism, occurring via much higher energy transition states, are not necessarily altered by these new findings, given the fact that all ligand exchange reactions occur with relatively low activation barriers. Nevertheless, the nature of the catalyst resting state needs revision as demonstrated. The importance of explicitly considering the solvent for reactions conducted in noninnocent highly organized media is also highlighted.
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