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

Reactions of N,N-dimethylaniline, N-isopropyl-aniline, 1,4-C6H4(CH(2)NHtBu)(2), and benzyldimethylamine with the Lewis acid B(C6F5)(3) have been studied. In the case of N,N-dirnethylaniline 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-(CH(2)NHtBu)(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 H-2 afforded [PhNMe2H][HB(C6F5)(3)] (3), [PhCH2NMe2H][HB(C6F5)(3)] (4), and [1,4-C6H4(CH(2)NH(2)tBu)(2)][HE(C6F5)(3)](2) (5). Reactions with CO2 gave PhCH2NMe2CO2B(C6F5)(3) (6), [PhNiPrH2]-[PhNiPrCO2B(C6F5)(3)] (8), [1,4-C6H4(CH(2)NH(2)tBu)(CH(2)NtBuCO(2)B(C6F5)(3)] (9), and [C5H6Me4NMeH](2)[1,4-C6H4(CH(2)NtBuCO(2)B(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.