A four-membered cyclic intramolecular phosphane-borane adduct activates dihydrogen to yield the respective ethylene-bridged zwitterionic phosphonium-hydridoborate system, which reduces benzaldehyde.

The metal‐free hydrogen activator 1 catalyzes the unique P/B hydrogenation of the frustrated Lewis pair 3, which itself is inactive toward H2 under the applied conditions, to yield the hydrogenation product 4. System 1/2 (5 mol %) also catalyzes the hydrogenation of a bulky ketimine and of enamines under mild conditions (2.5 bar H2, RT) to yield the respective amines.

Hydroboration of allyl(dimesityl)phosphane with HB(C(6)F(5))(2) gives the intramolecular five-membered P-B Lewis pair 7, that was characterized by X-ray diffraction. Similarly, HB(C(6)F(5))(2) addition to the substrates (mesityl)(2)P-CR[double bond, length as m-dash]CH(2) (R = CH(3), Ph) yield the corresponding (mesityl)(2)P(micro-CHRCH(2))B(C(6)F(5))(2) products 9a (R = CH(3)) and 9b (R = Ph) that show a weak intramolecular P...B interaction. The activation energy of the (reversible) P...B cleavage of these substrates was determined by dynamic (19)F NMR spectroscopy (9a: Delta G(not equal)(inv) (280 K) = 11.7 +/- 0.4 kcal mol(-1)). Compounds 9b and 9c show similar values. Compound 9c was prepared by HB(C(6)F(5))(2) addition to (mesityl)(2)P-CH=CHSiMe(3). Compound 9a reacts rapidly with dihydrogen (2.5 bar) at room temperature in pentane to give the zwitterionic H(2)-activation product (mesityl)(2)PH(+)(micro-CHMeCH(2))BH(-)(C(6)F(5))(2) (11).

Frustbewältigung: Der metallfreie Wasserstoffaktivator 1 katalysiert die ungewöhnliche P/B‐Hydrierung des „frustrierten“ Lewis‐Paars 3, das unter den Reaktionsbedingungen selbst nicht mit H2 reagiert, und liefert das zwitterionische Produkt 4. Das System 1/2 (5 Mol‐%) katalysiert auch die Hydrierung eines sperrigen Ketimins und von Enaminen unter milden Bedingungen (2.5 bar H2) zu den entsprechenden Aminen.

The series of phosphine-borane adducts, Ph2(H3C--C[triple chemical bond]C)P--B(C6F5)3 (8 c), Ph(H3C--C[triple chemical bond]C)2P--B(C6F5)3 (8 b) and (H3C--C[triple chemical bond]C)3P--B(C6F5)3 (8 a), was prepared. The X-ray crystal structure analyses revealed close to eclipsed conformations for all members of this series with average dihedral angles theta(C-P-B-C) of 8.1 degrees (8 c), 12.3 degrees (8 b) and 20.3 degrees (8 a). Quantum chemical analysis of these compounds revealed the importance of a subtle interplay between competing attractive and repulsive secondary interactions, causing the surprising eclipsed conformational preference for systems of this degree of complexity. Some cyclic phosphine-borane adducts were studied for comparison.