Formation of Reactive π‑Conjugated Frustrated N/B Pairs by Borane-Induced Propargyl Amine Rearrangement

<i>N</i>-Propargyltetramethylpiperidine reacts with a series of <i>trans</i>-alkenyl-B­(C₆F₅)₂ compounds to give the substituted alkenyl-bridged frustrated N/B Lewis pairs <b>5</b>. Their structures and spectroscopic features indicate a pronounced participation of the mesomeric <i>s-trans</i>-iminium/borata-alkene resonance form. The compounds are thought to be formed in a stepwise addition/rearrangement process which is initiated by a <i>trans</i>-1,2-amine/borane FLP addition to the carbon–carbon triple bond to generate a reactive zwitterionic aziridinium/alkenylborate intermediate. Subsequent alkenylborate attack leads to opening of the activated three-membered heterocycle with clean formation of the products <b>5a</b>–<b>c</b>. Treatment of the propargyl-TMP substrate with B­(C₆F₅)₃ gave a stable example of such an aziridinium/borate betaine, which was isolated and amply characterized. The products <b>5a</b>–<b>c</b> are active N/B FLPs. They split dihydrogen heterolytically under mild conditions to give the respective NH⁺/BH^– products <b>9a</b>–<b>c</b>. These contain <i>Z</i>-configurated core CC double bonds, which indicates rotational equilibration around the central C–C bond of <b>5a</b>–<b>c</b> during this reaction. Structural and chemical features of the <b>5c</b> system were analyzed by DFT calculations.