Spontaneous Metal-Free Transfer Hydrogenation of Iminoboranes with Ammonia Borane and Amine Boranes.

The first spontaneous, metal-free, room-temperature hydrogen transfer from ammonia borane to an iminoborane is reported. Mechanistic studies of the reaction indicate a concerted transfer of H+ and H- from donor to acceptor with an activation energy far below those of comparable concerted transfer hydrogenations. This reaction was employed in the syntheses and isolation of new B,N-disubstituted aminoboranes, a comparatively rare subset within the aminoborane family. This successful transfer hydrogenation to a highly dehydrogenated BN system may serve as a starting point for the design of new systems capable of reversible dehydrogenation/rehydrogenation.

Intriguing migrations in transient iminoborane adducts: two new pathways to aminoboranes.

Two unusual reactions were demonstrated with iminoboranes, both leading to aminoboranes of the type R2B[double bond, length as m-dash]NR2. In one case, a carboboration of di(tert-butyl)iminoborane with B(C6F5)3 led to (tBu)(C6F5)B-N(tBu)(B(C6F5)2). In the other, a transient IDipp (1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) adduct of MesBNtBu undergoes a shift of one carbene-bound Dipp substituent to the iminoborane nitrogen atom, yielding (1-Dipp-imidazol-2-yl)(Mes)B-N(Dipp)(tBu). Mechanistic DFT studies indicate the intermediacy of a borane·iminoborane adduct in the carboboration reaction.

Cleavage of BN triple bonds by main group reagents.

We report two rare instances of an insertion into the strong (ca. 170 kcal mol-1) BN triple bond of iminoboranes. In the first, a silylene inserts into di-tert-butyliminoborane to form an iminosilane. In the second, the highly crowded iminoborane Ter-NB-TMP (TMP = 2,2,6,6-tetramethylpiperidyl, Ter = 2,6-(diphenylmethyl)-4-tert-butylphenyl) can be forced to react with Pip-CC-Pip (Pip = piperidyl) at 60 °C. The reaction product is the apparent result of Pip-CC insertion into the iminoborane BN bond.

CuOTf-mediated intramolecular diborene hydroarylation.

Upon complexation to CuOTf, a PMe3-stabilized bis(9-anthryl) diborene slowly undergoes an intramolecular hydroarylation reaction at room temperature. Subsequent triflation of the B-H bond with CuOTf, followed by a PMe3 transfer, finally yields a cyclic sp2-sp3 boryl-substituted boronium triflate salt.