An Enamine/HB(C₆F₅)₂ Adduct as a Dormant State in Frustrated Lewis Pair Chemistry

The enamine piperidinocyclopentene reacts with HB­(C6F5)2 by formation of the C-Lewis base/B-Lewis acid adduct 10. It shows a zwitterionic iminium ion/hydridoborate structure. However, this adduct formation is apparently reversible and may generate the “invisible” frustrated Lewis pair 11 as a reactive intermediate by hydroboration of the enamine CC bond in an equilibrium situation at room temperature. Consequently, the FLP 11 was trapped by typical FLP reactions, namely by the reaction with dihydrogen to give the ammonium/hydridoborate 12, the acetylene deprotonation products 13 and 14, and simple borane adducts with pyridine (15) and with an isonitrile (17). The products 10 and 12–15 and the isonitrile adduct 17 were characterized by X-ray diffraction. A DFT study determined the thermodynamic features of the 10 ⇄ 11 equilibrium and of a previously discussed reference system (18 ⇄ 19) derived by reacting piperidinocyclohexene with HB­(C6F5)2.