Metal‐Free Catalytic Hydrogenation of Enamines, Imines, and Conjugated Phosphinoalkenylboranes

Abstract

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.

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation

“…There are examples in the literature showing that discrete molecules comprising Lewis acid-base pairs separated at a distance that they do not undergo neutralization ('frustrated Lewis acid-base pairs') can activate hydrogen and act as hydrogenation catalysts 21,[45][46][47][48][49][50] . On the basis of this known activity of frustrated Lewis acid-base pairs in organic molecules acting as metal-free hydrogenation catalysts, a reasonable proposal to rationalize the catalytic activity of Gr materials as hydrogenation catalysts would be the existence on the Gr layer of similar type of frustrated Lewis acid-base pairs consisting of the case of Gr by independent acid and basic pairs located on the Gr sheet at an adequate distance to promote the splitting of H 2 molecules.…”

Synthesis, Reactivity, and Computational Analysis of Halophosphines Supported by Dianionic Guanidinate Ligands

“…9 Since then, many different FLP systems have been developed 10 and reactivity with a variety of substrates has been observed, including B-H bonds, 11 CO2, 12,13 unsaturated bonds 10 and even catalytic, metal-free hydrogenation. [14][15][16][17][18] The silylene LSi (L = CH{(C=CH2)(CMe)(2,6-i Pr2C6H3N)2}) will oxidatively add ammonia to give four-coordinate LSi(H)NH2 19 and is the only reported main group molecule that has been successfully used in the activation of PH3, yielding LSi(H)PH2. 20 Phosphenium cations [R2P] + are a class of phosphorus-containing molecules that have a rich history of reactivity, including reactions with Lewis bases, 21-23 inorganic and organic unsaturated bonds, 24 and as ligands on transition metals.…”

Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

“…8 This zwitterionic salt was then shown to release dihydrogen when heated to elevated temperatures, representing the first example of reversible, metalfree dihydrogen activation. Since this landmark discovery, H 2 activation has been extended to a number of other FLP systems and has been used for the catalytic hydrogenation of unsaturated substrates such as imines, 9 N-protected nitriles, 9 aziridines, 9 enamines, 10 silyl enol ethers, 11 oxime ethers, 12 olefins, 1317 allenes, 16 acetylenes, 18 polycyclic aromatic compounds, 19 ketones, 20,21 and aldehydes 21 (Scheme 2b) as well as the stoichiometric hydrogenation of anilines to cyclohexylammonium derivatives. 22 Computational studies by the groups of Papai 6,2325 and Grimme 7 proposed that the heterolytic cleavage of H 2 proceeds via the initial polarization of the dihydrogen molecule in an "encounter complex": cleavage of the HH bond is driven by the interaction of the · H 2 electron pair with the vacant orbital of the Lewis acid and the donation of the HOMO of the Lewis base in to the · H2 * orbital (Figure 2).…”