Ruthenium hydroformylation

Phosphonium ionic liquids have been used several times for metal-catalyzed hydroformylations. Ruthenium and cobalt metal complexes catalyze the hydroformylation of internal olefins in [ Bu4P][Br] the major products are, however, the corresponding alcohols. Rhodium-catalyzed hydroformylations were conducted in [Bu3PEt][TsO] and [Ph3PEt][TsO] melts (meltingpoints 8UC and94 C, respectively). The products were easily isolated by decantation of the solid medium at room temperature. ... 

Hydration and Hydroformylation.—Ruthenium. Fluoro-olefins are catalyti-cally hydrated in the presence of chlororuthenate(ii) species. Ignorance of the exact nature of the species present in such solutions precludes a precise discussion of the mechanism. ... 

Ruthenium. Ruthenium, as a hydroformylation catalyst (14), has an activity signiftcandy lower than that of rhodium and even cobalt (22). Monomeric mthenium carbonyl triphenylphosphine species (23) yield only modest normal to branched regioselectivities under relatively forcing conditions. For example, after 22 hours at 120°C, 10 MPa (1450 psi) of carbon monoxide and hydrogen, biscarbonyltristriphenylphosphine mthenium [61647-76-5] ... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [PBuJBr was reported by Knifton as early as in 1987 [2]. The author described a stabilization of the active ruthenium-carbonyl complex by the ionic medium. An increased catalyst lifetime at low synthesis gas pressures and higher temperatures was observed. 

A similar type of intermediate in the ruthenium-catalyzed hydroformylation was suggested by Wilkinson and co-workers (36). 

Fig. 10. Proposed mechanism for the ruthenium-triphenylphosphine-catalyzed hydroformylation of olefins (36).

Catalysts and Catalyst Concentration. The most active catalyst for benzaldehyde reduction appears to be rhodium [Rh6(C0)i6 precursor], but iron [as Fe3(C0)i2] and ruthenium [as Ru3(C0)12] were also examined. The results of these experiments are shown in Table 1. Consistent with earlier results (12), the rhodium catalyst is by far the most active of the metals investigated and the ruthenium catalyst has almost zero activity. The latter is consistent with the fact that ruthenium produces only aldehydes during hydroformylation. Note that a synergistic effect of mixed metals does not appear to be present in aldehyde reduction as contrasted with the noticeable effects observed for the water-gas shift reaction (WGSR) and related reactions (13). 

KOH Concentration Studies. The effect of KOH concentration on benzaldehyde reduction was examined, and the results are shown in Figure 2 along with our previous results for ruthenium catalyzed hydroformylation (12). 

Studies analyzing the effects of the remaining reactants, H20 and C6HsCH0 indicate that the reaction appears to be zero order with respect to both reactants. It is interesting that in previous work we also found similar behavior for H20 in ruthenium catalyzed hydroformylation (12), as did Ungermann et al. with the WGSR (14). 

The effects of changes in KOH concentration on catalyst activity for benzaldehyde reduction are complex. Figure 2 compares the present work with KOH concentration studies for ruthenium catalyzed hydroformylation ... 

Small amounts of hydrocarbons added to the normal tetrahydrofuran or diglyme solvent system result in improved WGSR activity, but larger quantities inhibit the reaction (Table II). When 1-butene or 1-hexene is used, hydroformylation competes with the WGSR (4 ), but the rate of this process is small compared with the rate of H2 production. With pentane, no olefin or aldehyde products could be detected. Calderazzo (29) has reported that Ru(C0) is the principal product when the acetylacetonate of ruthenium is treated with synthesis gas in heptane,... 

Given the previous discussion on reductive amination, it is surprising that the potentially more complicated domino hydroformylation-reductive amination reactions have been more thoroughly developed. The first example of hydroaminomethylation was reported as early as 1943 [83]. The most synthetically useful procedures utilize rhodium [84-87], ruthenium [88], or dual-metal (Rh/Ir) catalysts [87, 89, 90]. This area was reviewed extensively by one of the leading research groups in 1999 [91], and so is only briefly outlined here as the second step in the domino process is reductive amination of aldehydes. Eilbrachfs group have shown that linear selective hydroaminomethylation of 1,2-disubstituted alkenes... 

A number of ruthenium-based catalysts for syn-gas reactions have been probed by HP IR spectroscopy. For example, Braca and co-workers observed the presence of [Ru(CO)3l3]", [HRu3(CO)ii]" and [HRu(CO)4] in various relative amounts during the reactions of alkenes and alcohols with CO/H2 [90]. The hydrido ruthenium species were found to be active in alkene hydroformylation and hydrogenation of the resulting aldehydes, but were inactive for alcohol carbonylation. By contrast, [Ru(CO)3l3]" was active in the carbonylation of alcohols, glycols, ethers and esters and in the hydrogenation of alkenes and oxygenates. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

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