Ionic Hydrogenation refers to hydrogenation achieved by the addition of a proton and a hydride to substrate; in contrast to traditional hydrogenation which is achieved using H2. The proton and hydride transfers can be either sequential or concerted. Usually ionic hydrogenation is shown to occur in two steps, starting with protonation.
Contents
R2C=Y + H+ → R2C+-YHR2C+-YH + "H−" → R2CH-YHSubstrates
Ionic hydrogenation is employed when the substrate can produce a stable carbonium ion. Polar double bonds are favored substrates. In the case of metal-catalyzed ionic hydrogenation, the substrates and their products must not bind to metal sites, as this would interfere with H2 activation. Ketones are the most common substrates. Less common are imines and N-heterocycles. The reaction can also be performed in reverse to effect hydrogenolysis. Liquid substrates can sometimes be hydrogenated without solvent, a goal of green chemistry.
Proton and hydride pairs
The most common hydrogenating pair is an organosilane as the hydride source (e.g. triethylsilane), and a strong oxyacid as the proton source (e.g. trifluoroacetic acid or triflic acid). The hydride and proton source cannot combine to give H2, which limits the hydricity and acidity of the H− and H+ sources, respectively.
Transition metal hydride complexes can be used in place of organosilanes as the hydride source. In these cases, triflic acid is a typical proton donor. Ketones such as benzophenones, and 1,1-disubstituted olefins are typical substrates. Hydrides of tungsten, chromium, osmium, and molybdenum complexes have also been reported. Tungsten dihydride complexes can hydrogenate ketones stoichiometrically with no external acids. One hydride serves as the hydride source, and the other serves as a proton source.
In the case of ionic hydrogenation, a dihydride complex is regenerated by hydrogen gas following hydrogenation. Typical catalysts are tungsten or molybdenum complexes. An example of such a catalyst is CpMo(CO)2(PR3)(OCR'2)]+ where M = W or Mo.
Related reactions
Transfer hydrogenation (TH) catalysts, e.g. Shvo catalyst, are related to catalysts used for ionic hydrogenation. TH catalysts however do not employ strong acids and both the H− and H+ components are covalently bonded to the complex prior to transfer to the unsaturated substrates. Typically, TH catalysts are more widely employed in organic synthesis.