Frustrated Lewis pairs derived from N-heterocyclic carbenes and Lewis acids.

The chemistry of frustrated Lewis pairs derived from N-heterocyclic carbenes and a number of Lewis acids has been probed. The combination of 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene (IDipp) (1) with B(C6F5)3 was shown to give the classical Lewis acid-base adduct (IDipp)B(C6F5)3 (2) which was unreactive. In contrast, the combination 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (3) with B(C6F5)3 proved to form a frustrated Lewis pair, and reacts with H2 to give the salt [ItBuH][HB(C6F5)3] (4) in high yield. In a similar fashion, addition of (3) to a series of amine-borane adducts including H3NB(C6F5)3 (5), PhH2NB(C6F5)3 (6) and PhH2NB(C6F5)3 (7) led to deprotonation and formation of imidazolium amido-borate salts [ItBuH][H2NB(C6F5)3] (8), [ItBuH][PhHNB(C6F5)3] (9) and [ItBuH][Ph2NB(C6F5)3] (10). Similar reactions of the amine-borane adducts EtNH2B(C6F5)3 (11) tBuNH2B(C6F5)3 (12) and Et2NHB(C6F5)3 (13) gave the amidoboranes EtHNB(C6F5)2 (14) tBuHNB(C6F5)2 (15) and Et2NB(C6F5)2 (16) respectively, with liberation of C6F5H and an equivalent of unreacted carbene. Mechanistically these reactions are thought to proceed through transient imidazolium salts similar to (8)-(10). In addition, the combination of carbene (3) and the cation [CPh3]+ in frustrated Lewis pair chemistry has been probed. Reaction of this combination at room temperature results in the immediate formation of [C3H2N2tBu2(C6H5)CPh2][B(C6F5)4] (17) stemming from carbene attack at a para-carbon of one of the phenyl rings of trityl. Addition of carbene to the benzyl amine adduct of [CPh3][B(C6F5)4] results in the formation of [ItBuH][B(C6F5)4] (18) and the secondary amine Ph3CNHCH2Ph (19), thus providing the first example of an all carbon-based frustrated Lewis pair. Crystallographic data for (2), (4), (9) and (13) are reported.