Recently, a new approach to dihydrogen activation known as frustrated Lewis pairs (FLPs) concept has been introduced 1, 2, 3 . A combination of highly Lewis acidic boranes and sterically hindered bases can split hydrogen heterolytically generating onium (phosphonium, ammonium, etc.) borohydrides. These compounds show reduction activity resembling that of inorganic borohydrides like NaBH 4 , i. e. they are suitable mostly for reduction of polarized multiple bonds. Imines, enamines, silyl ethers 4, 5,6 , α,β-enones 7 , ynones 8 , Nalkylanilines 9 were hydrogenated using stoichiometric or catalytic amounts of FLPs. Due to heterolytic nature of FLP-H 2 adducts, hydrogenation of unactivated multiple C-C bonds using FLPs has some natural limitations, since during the respective step of the catalytic cycle a proton transfer from catalyst to substrate should take place (Fig. 1a). Although Greb et al. have implemented this approach to hydrogenation of alkenes under ambient conditions, this method is predictably restricted to the alkenes with high proton affinity 10 .

The first ansa-aminoborane N-TMPN-CH2C6H4B(C6F5)2 (where TMPNH is 2,2,6,6-tetramethylpiperidinyl) which is able to reversibly activate H2 through an intramolecular mechanism is synthesized. This new substance makes use of the concept of molecular tweezers where the active N and B centers are located close to each other so that one H2 molecule can fit in this void and be activated. Because of the fixed geometry of this ansa-ammonium-borate it forms a short N-H...H-B dihydrogen bond of 1.78 A as determined by X-ray analysis. Therefore, the bound hydrogen can be released above 100 degrees C. In addition, the short H...H contact and the N-H...H (154 degrees) and B-H...H (125 degrees) angles show that the dihydrogen interaction in N-TMPNH-CH2C6H4BH(C6F5)2 is partially covalent in nature. As a basis for discussing the mechanism, quantum chemical calculations are performed and it is found that the energy needed for splitting H2 can arise from the Coulomb attraction between the resulting ionic fragments, or "Coulomb pays for Heitler-London". The air- and moisture-stable N-TMPNH-CH2C6H4BH(C6F5)2 is employed in the catalytic reduction of nonsterically demanding imines and enamines under mild conditions (110 degrees C and 2 atm of H2) to give the corresponding amines in high yields.

No abstract

Lifting the curtain on metal‐free hydrogen activation and hydrogenation: The successive splitting of molecular hydrogen under mild conditions by “frustrated Lewis pairs” based on inexpensive, stable amines in combination with B(C6F5)3 is presented by B. Rieger, T. Repo et al. in their Communication on The successful reduction of benzaldehyde is encouraging for the development of amine‐B(C6F5)3 systems for the catalytic hydrogenation of olefins, carbonyl functions, and imines.