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

Abstract

Reactions of N,N-dimethylaniline, N-isopropylaniline, 1,4-C6H4(CH2NHtBu)2, and benzyldimethylamine with the Lewis acid B(C6F5)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(C6F5)3 afford the adducts (PhNHiPr)B(C6F5)3 (1) and PhCH2NMe2B(C6F5)3 (2), respectively. 1,4-C6H4(CH2NHtBu)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] (3), [PhCH2NMe2H][HB(C6F5)3] (4), and [1,4-C6H4(CH2NH2tBu)2][HB(C6F5)3]2 (5). Reactions with CO2 gave PhCH2NMe2CO2B(C6F5)3 (6), [PhNiPrH2][PhNiPrCO2B(C6F5)3] (8), [1,4-C6H4(CH2NH2tBu)(CH2NtBuCO2B(C6F5)3] (9), and [C5H6Me4NMeH]2[1,4-C6H4(CH2NtBuCO2B(C6F5)3)2] (10). Interestingly, the species 4 also reacts with CO2 to give [PhCH2NMe2H][HCO2B(C6F5)3] (7). Species 8 reacts with HSiEt3 to yield [PhNiPrC(OSiEt3)OB(C6F5)3] (11). The adduct 2 also reacts with ethene, 2,4-hexadiyne, and the alkynes HC≡CR (R = n-Bu, Ph, tBu, SiMe3) to give the 1,2- addition products PhCH2NMe2CH2CH2B(C6F5)3 (12), PhCH2NMe2C(CCMe)=CMeB(C6F5)3 (13), and the ammonium alkynylborate salts [PhCH2NMe2H][RCCB(C6F5)3] (R = n-Bu (14a), Ph (14b), tBu (14c), SiMe3 (14d)). 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) (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.