Metal-free dehydrogenation of amine–boranes by an N-heterocyclic carbene

Abstract

The dehydrogenation of primary and secondary amine-boranes (RNH(2)·BH(3) and R(2)NH·BH(3); R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-(i)Pr(2)C(6)H(3)) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr·BH(2)-NH(R)-BH(3) were obtained in place of the desired polymers [RNH-BH(2)](n). The secondary amine-borane (i)Pr(2)NH·BH(3) participated in dehydrogenation chemistry with IPr to afford the aminoborane [(i)Pr(2)N=BH(2)] and the dihydroaminal IPrH(2) as products. Attempts to induce H(2) elimination from the arylamine-borane DippNH(2)·BH(3) yielded a reaction mixture containing the known species IPr·BH(2)NHDipp, IPr·BH(2)NH(Dipp)-BH(3), free DippNH(2) and IPrH(2). The new hindered aryl-amine borane adduct Ar*NH(2)·BH(3) [Ar* = 2,6-(Ph(2)CH)(2)-4-MeC(6)H(2)] underwent a reaction with IPr to give IPr·BH(3) and free Ar*NH(2), consistent with the presence of a weaker N-B dative bond in Ar*NH(2)·BH(3) relative to its less hindered amine-borane analogues.

Dehydrogenation of Amine–Boranes Using p‐Block Compounds

“…The group of Rivard extended the scope of this NHC chemistry and found that N‐heterocyclic carbene 27 (Scheme ) can dehydrogenate MeNH 2 ⋅BH 3 and i PrNH 2 ⋅BH 3 forming the expected NHC–H 2 adduct 28 together with the carbene‐bound B–N–B adduct NHC⋅BH 2 NH(R)−BH 3 ( 29 ) in a 1:1 ratio . The formation of 29 was proposed to proceed through a sequence of events.…”

Metal‐Free Dehydrogenation of Amine‐Boranes by Tunable N‐Heterocyclic Iminoboranes

“…Poly( N ‐methylaminoborane) [MeNH‐BH 2 ] n has been reported to give a broad 11 B NMR resonance at δ =−6.5 ppm in CDCl 3 with no discernable B,H coupling in the proton‐coupled 11 B{ 1 H} NMR spectrum . This chemical shift is similar to that of the six‐membered [MeNH‐BH 2 ] 3 ring (triplet at δ =−5.4 ppm in [D 6 ]acetone, 1 J (B,H)=107 Hz; δ =−5.9 ppm in [D 8 ]THF) . As a broad triplet resonance was located at δ =−4.7 ppm that featured a spectral tail that extends upfield to approximately δ =−6.2 ppm, we decided to investigate the possible formation of [MeNH‐BH 2 ] x oligomers.…”

“…The metal‐free dehydrogenation of amine‐boranes has also been reported by Manners and coworkers, where they observed that the monomeric aminoborane i Pr 2 N=BH 2 can behave as a hydrogen acceptor; thus, allowing stoichiometric hydrogen transfer to occur from amine‐borane adducts under ambient conditions . Moreover, our group has shown that N‐heterocyclic carbenes (NHCs) are able to effectively remove H + /H − equivalents from amine‐boranes . In addition, very promising examples of both Lewis acid (B(C 6 F 5 ) 3 ) and Lewis base (proton sponge; 1,8‐(Me 2 N) 2 C 10 H 6 ) promoted dehydrocoupling of amine‐boranes have been developed by the groups of Baker and Sneddon, respectively.…”

Systematic Investigation of the Ring‐Expansion Reaction of N‐Heterocyclic Carbenes with an Iminoborane Dihydride

“…Upon reacting the less bulky L Mes (NH)BH 3 (1 A) with the sterically unhindered L Me4 , a triplet in the 11 B NMR spectrum (d = À19.7 ppm, J = 89 Hz) indicates BDF and in the 1 H NMR spectrum L Me4 (H 2 ) is clearly identified. [12] As already demonstrated by the successful isolation of 2 Ab as well as of 2 Ba, the boost of steric hindrance for either ligand (the imino group or the NHC) results in clean BDF (entries Ab and Ba). Apparently, a certain degree of steric congestion at the boron center is required for the BDF to proceed in a controlled manner.…”

“…Inspired by related reports of Rivard, [4,12] Manners, [13] and Harder, [14] we targeted the synthesis of electron-donating imino ligand-stabilized borane dihydrides of type IV (Scheme 1) as a precursor for the RER through a dehydro-genative route. We introduced the imidazolin-2-imino group {L R1 (N), with L R1 = 1,3-R 1 2 -imidazolin-2-ylidene, R 1 = 2,6-diisopropylphenyl (Dipp) or 2,4,6-trimethylphenyl (Mes)} to borohydride by conversion of L R1 (NH) with borane dimethylsulfide adduct (see the Supporting Information for full experimental details).…”