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

Reactions of N, N -dimethylaniline, N- isopropylaniline, 1,4-C 6 H 4 (CH 2 NH t Bu) 2, and benzyldimethylamine with the Lewis acid B(C 6 F 5 ) 3 have been studied. In the case of N, N -dimethylaniline the combination of the Lewis acid and base forms an almost completely noninteracting frustrated Lewis pair, while the corresponding reactions of N- isopropylaniline and benzyldimethylamine with B(C 6 F 5 ) 3 afford the adducts (PhNH i Pr)B(C 6 F 5 ) 3 ( 1 ) and PhCH 2 NMe 2 B(C 6 F 5 ) 3 ( 2 ), respectively. 1,4-C 6 H 4 (CH 2 NH t Bu) 2 reacts with 2 equiv of B(C 6 F 5 ) 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 H 2 afforded [PhNMe 2 H][HB(C 6 F 5 ) 3 ] ( 3 ), [PhCH 2 NMe 2 H][HB(C 6 F 5 ) 3 ] ( 4 ), and [1,4-C 6 H 4 (CH 2 NH 2 t Bu) 2 ][HB(C 6 F 5 ) 3 ] 2 ( 5 ). Reactions with CO 2 gave PhCH 2 NMe 2 CO 2 B(C 6 F 5 ) 3 ( 6 ), [PhN i PrH 2 ][PhN i PrCO 2 B(C 6 F 5 ) 3 ] ( 8 ), [1,4-C 6 H 4 (CH 2 NH 2 t Bu)(CH 2 N t BuCO 2 B(C 6 F 5 ) 3 ] ( 9 ), and [C 5 H 6 Me 4 NMeH] 2 [1,4-C 6 H 4 (CH 2 N t BuCO 2 B(C 6 F 5 ) 3 ) 2 ] ( 10 ). Interestingly, the species 4 also reacts with CO 2 to give [PhCH 2 NMe 2 H][HCO 2 B(C 6 F 5 ) 3 ] ( 7 ). Species 8 reacts with HSiEt 3 to yield [PhN i PrC(OSiEt 3 )OB(C 6 F 5 ) 3 ] ( 11 ). The adduct 2 also reacts with ethene, 2,4-hexadiyne, and the alkynes HC≡CR (R = n -Bu, Ph, t Bu, SiMe 3 ) to give the 1,2- addition products PhCH 2 NMe 2 CH 2 CH 2 B(C 6 F 5 ) 3 ( 12 ), PhCH 2 NMe 2 C(CCMe)=CMeB(C 6 F 5 ) 3 ( 13 ), and the ammonium alkynylborate salts [PhCH 2 NMe 2 H][RCCB(C 6 F 5 ) 3 ] (R = n -Bu ( 14a ), Ph ( 14b ), t Bu ( 14c ), SiMe 3 ( 14d )). In contrast, the reaction of 1-hexyne with the Lewis pair PhNMe 2 and B(C 6 F 5 ) 3 affords the trans-1,2-addition product PhNMe 2 C(C 4 H 9 )=CH(B(C 6 F 5 ) 3 ) ( 15 ). 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.