In their Communication the authors stated that "non-transition-metal catalysts for hydrogenation reactions are all but unknown." However, it should be noted that non-transition-metal systems have been shown to effect hydrogenation under more forcing conditions. For example, DeWitt, Ramp, and Trapasso demonstrated hydrogenation with iPr 3 B under 67 atm (1000 psi) H 2 at 220 o C. [1] Similarly, Haenel and co-workers [2] among others [3] showed hydrogenation of coal under almost 148 atm (15 MPa) H 2 and at 280-350 8C using BI 3 or alkyl boranes. As well, superacid systems have also been shown to effect hydrogenation of alkenes at H 2 pressures of at least 35 atm. [4] [1] a) E.

The Lewis acid B(C(6)F(5))(3) has been found to be an efficient catalyst for the direct hydrogenation of imines and the reductive ring-opening of aziridines with H(2) under mild conditions; addition of a bulky phosphine allows for the reduction of protected nitriles.

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

In 2006, our group reported the first metal-free systems that reversibly activate hydrogen. This finding was extended to the discovery of "frustrated Lewis pair" (FLP) catalysts for hydrogenation. It is this catalysis that is the focal point of this article. The development and applications of such FLP hydrogenation catalysts are reviewed, and some previously unpublished data are reported. The scope of the substrates is expanded. Optimal conditions and functional group tolerance are considered and applied to targets of potential commercial significance. Recent developments in asymmetric FLP hydrogenations are also reviewed. The future of FLP hydrogenation catalysts is considered.

Size matters: The frustrated Lewis pair derived from B(C6F5)3 and the sterically encumbered N‐heterocyclic carbene N,N′‐tBu2C3H2N2 (1) cleaves dihydrogen heterolytically to give a imidazolium borate (see scheme, left), and cleaves amine NH bonds to form aminoborate salts (right) or aminoboranes.