Two-phase hydrogenation

Col I(CN)-, 1 is readily formed under mild conditions from Co(CN)2, KCN and H2 [Eqs. (1) and (2)]. It is an active catalyst for the hydrogenation of a variety of unsaturated substrates, and in fact in the first documented examples of two-phase hydrogenations this catalyst was used [48, 49]. The catalysis suffers from several drawbacks such as rapid aging with a loss of activity, and the need to use highly basic aqueous solutions. 

Aqueous two-phase hydrogenations are dominated by platinum group metal catalysts containing water-soluble tertiary phosphine ligands. The extremely stable and versatile N-heterocyclic carbene complexes attracted only limited interest, despite the fact that such complexes were described in the literature [62-65]. Recently, it was reported that the water-soluble [RuXY(l-butyl-3-methylimi-dazol-2-ylidene) ( 76-p-cymene)]n+ (X=Ch, H20 Y = C1-, H20, pta) complexes preferentially hydrogenated cinnamaldehyde and benzylideneacetone at the C = C double bond (Scheme 38.5) with TOF values of 30 to 60 h 1 in water substrate biphasic mixtures (80 °C, lObar H2) [66]. 

Aqueous Two-Phase Hydrogenations of Nitro-Compounds, Imines, Nitriles,... 

Aqueous two-phase hydrogenation may be a method of choice for synthetic purposes when no incompatibility problems between water and the substrates, products, or catalyst arise. It has already been proven by the success of the Ruhrchemie-Rhone-Poulenc hydroformylation process, that the catalyst can be retained in the aqueous phase with very high efficiency, and that aqueous-organic biphasic processes using organometallic catalysts are suitable for indus-... 

There now exists evidence for the extension of two-phase catalysis into the new area of Ci-chemistry. Thus, Leitner an co-workers [206] described the biphase hydrogenation of CO2 to formic acid (cf. Section 3.3.4). Two-phase hydrogenations of aromatic nitro compounds with Pd or Rh catalysts are examined by Tafesh and co-workers [207] and others [212 f, 218 d, 226]. 

Ding, Hanson, and Bakos [39] used an amphiphilic chiral rhodium-phosphine complex in the catalytic asymmetric two-phase hydrogenation of methyl (Z)-a-acetamidocinnamate in ethyl acetate/water and obtained enantioselectivities of up to 69% ee. 

Co-solvents also play a rate-enhancing role in two-phase hydrogenation reactions. In the two-phase hydrogenation of cyclohexene with a catalyst prepared in situ by the hydrogen reduction of RhCl3 3 H20 in the presence of monosulfonated tri-phenylphosphine, TPPMS, the rate of hydrogenation was observed to be dependent on the co-solvent [19]. Lower alcohols, methanol plus ethanol, were superior... 

Hydrogenation of simple alkenes often serves as a test reaction to characterize the catalytic performance of new hydrogenation catalysts, and only in relatively few cases was it employed for straightforward synthetic purposes. In aqueous solutions the most widely employed substrates for catalyst characterization are unsaturated carboxylic acids (such as maleic, fumaric, crotonic, and itaconic acids), alcohols (i.e., allyl alcohol) and sodium 4-styrenesulfonate, while in two-phase hydrogenations a-alkenes (1-hexene, 1-octene), cydohexene, and styrene are the conventional targets of catalytic reduction. These reactions will not be discussed here in detail. 

The two-phase hydrogen clustering persists in films of a-Si H that are alloyed with significant amounts of C, N, F, or B (Reimer et al, 198 Id Shimizu et al., 1983). Even in pure a-C H without silicon the two distinct hydrogen NMR lines remain. 

R. W. Albach, M. jautelat, in Two-Phase Hydrogenation Method and Colloidal... 

Fig. 2. Sonic velocity and Mach number in two-phase hydrogen.

Fig. 4. Volume ratio vapor volume fraction in two-phase hydrogen.

Iridium complexes with triphenylphosphine modified with poly(ethylene oxide)s 14 at one of the phosphine groups were active in two-phase hydrogenation of allylbenzene, although the catalyst activity and selectivity with respect to the hydrogenation product, propylbenzene, was substantially lower than in the case of its low molecular analogue and one of the main products was propenylbenzene. Similar results were obtained for an iridium complex with polyethylene oxide modified with pyridyl groups [53],... 

However, it had to be established first that the Martinelli flow parameter was applicable to the two-phase hydrogen flow model. In the original development of the parameter [3], the fluid model consisted of a gaseous core and a liquid annulus adjacent to the wall. By a similar derivation it was apparent that the Martinelli parameter was applicable to the hydrogen flow comprising a liquid core and a gaseous annulus. 

---