Frustrated Lewis Pair Behavior of Intermolecular Amine/B(C₆F₅)₃ Pairs

Reactions of <i>N</i>,<i>N</i>-dimethylaniline, <i>N-</i>isopropyl­aniline, 1,4-C₆H₄(CH₂NH<i>t</i>Bu)₂, and benzyldimethylamine with the Lewis acid B­(C₆F₅)₃ have been studied. In the case of <i>N</i>,<i>N</i>-dimethylaniline the combination of the Lewis acid and base forms an almost completely noninteracting frustrated Lewis pair, while the corresponding reactions of <i>N-</i>isopropylaniline and benzyldimethylamine with B­(C₆F₅)₃ afford the adducts (PhNH<i>i</i>Pr)­B­(C₆F₅)₃ (<b>1</b>) and PhCH₂NMe₂B­(C₆F₅)₃ (<b>2</b>), respectively. 1,4-C₆H₄­(CH₂NH<i>t</i>Bu)₂ reacts with 2 equiv of B­(C₆F₅)₃ to give rise to an inseparable mixture of the mono- and bis-amine–borane adducts as well as an iminium salt derived from hydride abstraction from a benzylic carbon of the diamine. Subsequent selected reactions of the Lewis acid/base combinations with H₂ afforded [PhNMe₂H]­[HB­(C₆F₅)₃] (<b>3</b>), [PhCH₂NMe₂H]­[HB­(C₆F₅)₃] (<b>4</b>), and [1,4-C₆H₄(CH₂NH₂<i>t</i>Bu)₂]­[HB­(C₆F₅)₃]₂ (<b>5</b>). Reactions with CO₂ gave PhCH₂NMe₂CO₂B­(C₆F₅)₃ (<b>6</b>), [PhN<i>i</i>PrH₂]­[PhN<i>i</i>PrCO₂B­(C₆F₅)₃] (<b>8</b>), [1,4-C₆H₄(CH₂NH₂<i>t</i>Bu)­(CH₂N<i>t</i>BuCO₂B­(C₆F₅)₃] (<b>9</b>), and [C₅H₆Me₄NMeH]₂[1,4-C₆H₄(CH₂N<i>t</i>BuCO₂B­(C₆F₅)₃)₂] (<b>10</b>). Interestingly, the species <b>4</b> also reacts with CO₂ to give [PhCH₂NMe₂H]­[HCO₂B­(C₆F₅)₃] (<b>7</b>). Species <b>8</b> reacts with HSiEt₃ to yield [PhN<i>i</i>PrC­(OSiEt₃)­OB­(C₆F₅)₃] (<b>11</b>). The adduct <b>2</b> also reacts with ethene, 2,4-hexadiyne, and the alkynes HCCR (R = <i>n</i>-Bu, Ph, <i>t</i>Bu, SiMe₃) to give the 1,2- addition products PhCH₂NMe₂CH₂CH₂B­(C₆F₅)₃ (<b>12</b>), PhCH₂NMe₂C­(CCMe)=CMeB­(C₆F₅)₃ (<b>13</b>), and the ammonium alkynylborate salts [PhCH₂NMe₂H]­[RCCB­(C₆F₅)₃] (R = <i>n</i>-Bu (<b>14a</b>), Ph (<b>14b</b>), <i>t</i>Bu (<b>14c</b>), SiMe₃ (<b>14d</b>)). In contrast, the reaction of 1-hexyne with the Lewis pair PhNMe₂ and B­(C₆F₅)₃ affords the trans-1,2-addition product PhNMe₂C­(C₄H₉)=CH­(B­(C₆F₅)₃) (<b>15</b>). This chemistry demonstrates the generality of such FLP chemistry, extending it to N-based Lewis bases and thereby expanding the scope for applications of FLP chemistry