Hydrogenation rhodium-phosphine complexes

Dialdehydes 8 have been converted to y-lactones 9 in the presence of a rhodium phosphine complex as catalyst. The example shown below demonstrates that this reaction works also with aldehydes that contain a-hydrogen atoms. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

In the hydrogenation of alkenes, rhodium-, ruthenium- and iridium-phosphine catalysts are typically used [2-4]. Rhodium-phosphine complexes, such as Wilkinson s catalyst, are effective for obtaining alkanes under atmospheric pres-... 

Initially, research focused on the use of C2-symmetric rhodium and ruthenium-phosphine and phosphinite complexes a rhodium-phosphine complex 3 (Fig. 30.2) was used in the first reported enantioselective hydrogenation of substrate 1 (Table 30.1, entry 1) [1],... 

Similarly, highly selective hydrogen reductions of rhodium-acyl intermediates are known, such as in hydroformylation reactions. Rhodium phosphine complexes are highly selective with little further reduction of products to alcohols (19). 

Table 2 Some Other Hydrogenations Catalyzed by Cationic Rhodium-Phosphine Complexes...

It has been reported that the chiral NMR shift reagent Eu(DPPM), represented by structure 19, catalyzes the Mukaiyama-type aldol condensation of a ketene silyl acetal with enantiose-lectivity of up to 48% ee (Scheme 8B1.13) [29-32]. The chiral alkoxyaluminum complex 20 [33] and the rhodium-phosphine complex 21 [34] under hydrogen atmosphere are also used in the asymmetric aldol reaction of ketene silyl acetals (Scheme 8BI. 14), although the catalyst TON is quite low for the former complex. 

Homogeneous hydrogenation in the fluorous phase has been so far reported only for a limited set of simple olefins (Richter et al., 1999, Rutherford et al., 1998), as exemplified with the neutral rhodium phosphine complex 18 as catalyst precursor (eq. 5.7). Isomerization of the substrate 1-dodecene (17a) was observed as a competing side reaction under the reaction conditions. The catalyst formed from 18 could be recycled using a typical FBS protocol, but deactivation under formation of metal deposits limited the catalyst lifetime. 

Most of the asymmetric homogeneous hydrogenations reported to date have used rhodium-phosphine complexes as catalysts. In a majority of these cases, it has been assumed that the active catalysts are similar to the well-studied... 

We previously prepared surface-bonded rhodium phosphine complexes in Al-MCM-41. In a solution of dichloromethane, [Rh(acac)(chiraphos)] and Al-MCM-41 react to a surface bonded [(Os)x-Rh(chiraphos)] complex due to an exchange reaction of the acetylacetonato ligand and surface oxygens of the acidic support12. Here we present a heterogeneous Rhodium diphosphine catalyst and its application in the enantioselective hydrogenation of dimethylitaconate. The results indicate the localisation of the complex inside of the mesoporous channel system. 

There are some reports on the asymmetric catalytic hydrogenation of C=N double bonds. Rhodium phosphine complexes do not have high activity, and the stereoselectivity is low. - Homogeneous hydrogenation of oximes, Schiff s bases and cyclic imines " have been reported. A cyano cobalt complex has been used for the homogeneous catalytic hydrogenation of oximes and Schiff s bases but the degree of asymmetric induction is unknown. 

There are a number of reviews available on homogeneous hydrogenation. Specialized reviews are available on hydrogenation with phosphine complexes of rhodium, " on hydrogenation of arenas, on hydrogenation with water-soluble catalysts, " and on mechanistic aspects. There are general reviews covering the older and more recent literature. 

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. 

When our studies commenced it had been assumed that the mechanism of asymmetric hydrogenation by chelating rhodium phosphine complexes followed a similar pathway. It has been demonstrated, however, that the timing is quite different, and that oxidative addition of hydrogen to metal does not occur in the initial stages of reaction. This conclusion follows from studies on the phosphorus-31 NMR spectra of hydrogenated complex solutions made separately by Halpern,... 

The rhodium-phosphine catalyst [Rh(T)-L] was prepared from [ RhCl(NBD) 2] and the chelating bisphosphines 141 and 142 [59] following chloride removal with silver triflate. In fact, the two systems on Scheme 3.52 and 3.53 could be mn together, with the net result of hydrogenation of ketones by molecular hydrogen. The same rhodium-phosphine complex catalyzed the direct hydrogenation of flavin mononucleotide to dihydroflavin mononucleotide [59],... 

Rhodium-Phosphine Complexes as Homogeneous Catalysts. Hydrogenation of Aromatic Nitro Compounds ... 

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