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(CH2NH2 tBu)2][HB(C6F5)3]2 (5). Reactions with CO2 gave PhCH2NMe2CO2B(C6F5)3 (6), [PhNiPrH2][PhNiPrCO2B(C6F5)3] (8), [1,4-C6H4(CH2NH2 tBu)(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.

Structurally simple complexes of CO2

“…Furthermore, the O-C-O angle of the fragment was found to be 133.1(2)°, which is wider than that of any carbamate yet reported. The angle is also wider than that in any known NHC-CO2 adduct, with the sole exception of the super-bulky imidazol-2-carboxylate shown in Figure 6B 113 and (B) the carboxylate adduct of the super-bulky carbene, 1,3-bis(2,6-dibenzhydryl-4-methylphenyl)imidazol-2-ylidene. 114 Both of these complexes have O-C-O bond angles greater than 133°.…”

“…The combination of N,N-dimethylaniline (a tertiary amine) and tris(pentafluorophenyl)borane reacts with carbon dioxide at low temperatures (−32°C) to give the adduct shown in Figure 6A. 113 As with some other FLP combinations, CO2 is released at low temperatures (> −20°C), but it is not this fact alone that makes this particular example so remarkable. The crystal structure of the adduct reveals an O=C bond length in the CO2 fragment of only 1.193(3) Å, much closer to that of gaseous CO2 (1.1602(8) Å)6 than many of the other structures discussed thus far.…”

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…[92,93] Similarly, the base can be replaced by primary and secondary amines, [94] or N-heterocyclic carbenes (NHCs).…”

“…[161] In an intermolecular addition reaction, the amine adduct PhCH 2 NMe 2 B(C 6 F 5 ) 3 was shown to react with ethylene to give the zwitterion, PhCH 2 NMe 2 CH 2 CH 2 B(C 6 F 5 ) 3 (286; Scheme 92). [94] In a similar fashion, this strategy can be These are not the final page numbers! Ü Ü…”

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

“…It is known from previous publications that FLPs react with terminal alkynes via deprotonative borylation pathway, producing the respective onium alkynylborates 41,42,43,44,45,46 . Indeed, 6 reacts with hex-1-yne giving the respective adduct 31 (Scheme 6).…”