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

N-Propargyltetramethylpiperidine reacts with a series of trans-alkenyl-B­(C6F5)2 compounds to give the substituted alkenyl-bridged frustrated N/B Lewis pairs 5. Their structures and spectroscopic features indicate a pronounced participation of the mesomeric s-trans-iminium/borata-alkene resonance form. The compounds are thought to be formed in a stepwise addition/rearrangement process which is initiated by a trans-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 5a–c. 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 5a–c are active N/B FLPs. They split dihydrogen heterolytically under mild conditions to give the respective NH+/BH– products 9a–c. These contain Z-configurated core CC double bonds, which indicates rotational equilibration around the central C–C bond of 5a–c during this reaction. Structural and chemical features of the 5c system were analyzed by DFT calculations.