Hydroformylation with Mononuclear Ruthenium Complexes

This complex was also identified starting from other mononuclear 

In the presence of a large excess of triphenylphosphine there is strong competition between all the ligands so that coordination of the alkene in Step 1 is retarded, and, possibly, CO insertion (step 3) as well as the reductive elimination are less favored than in the previous mechanism 

and n/iso ratios of approximately 2.8. Carbon monoxide has an inhibiting effect on the catalytic activity although the n/iso ratio was improved to 4 when the partial CO pressure was increased. Below 150°C the cyclopentadienyl ligand remained attached to ruthenium. The authors proposed that the first step was the formation of a hydrido mononuclear ruthenium species according to Eq. (3). 

Creation of a vacant site by CO dissociation is favored at low CO pressures, whereas the CO insertion step seems not to be very sensitive to pressure. Above 150°C the cyclopentadienyl ruthenium complex was destroyed, and many complexes were formed, with [Ru3(CO),2] as the major component. This mixture of compounds is of poor activity, and [Ru3(CO),2] tested under the same conditions presents the same low level of catalytic activity. 

In a subsequent study, Ugo and co-workers (7) started from the monomeric species [( / -C5H5)Ru(CO)2X] (X = Cl, Br, 1) in order to generate more easily the hydride intermediate [( / -C5H5)Ru(CO)2H], employing a tertiary amine to remove HX [Eq. (4)]. The activity of the ruthenium 

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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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