Hydroformylation, ruthenium catalyzed selectivity

The hydroformylation reaction or 0x0 process is an important industrial synthetic tool. Starting from an alkene and using syngas, aldehydes with one or more carbon atoms are obtained. In almost all industrial processes for the hydroformylation of alkenes, rhodium or cobalt complexes are used as catalysts [33]. A number of studies on ruthenium complex-catalyzed hydroformylation have been reported [34]. One of the reasons for the extensive studies on ruthenium complex catalysts is that, although the rhodium catalysts used in industry are highly active, they are very expensive, and hence the development of a less-expensive catalytic system is required. Since inexpensive ruthenium catalysts can achieve high selectivity for desired u-alde-hydes or n-alcohols, if the catalytic activity can be improved to be comparable with that of rhodium catalysts, it is possible that a ruthenium-catalyzed 0x0 process would be realized. 

Laine (43-47) used potassium hydroxide to promote the catalytic activity of [Ru3(CO),2] and [H4Ru4(CO),2] for the hydroformylation of pent-1-ene. Under 64 bar of CO pressure, at 135 or 150°C, high selectivities for straight-chain aldehydes were obtained, for example, 97%. As the subsequent reduction of aldehydes to alcohols is lower, important aldol condensations occurred owing to the presence of a base in solution. Analysis of the reaction mixtures has shown that the anionic [H3Ru4(CO),2] cluster is likely to be the active species. Since this complex was recognized as the major component of the low pressure ruthenium-catalyzed water gas shift... 

The ruthenium complex-catalyzed hydroformylation of 1-alkene was first examined by Wilkinson s group. Ru(CO)3(PPh3)2/phosphine catalysts were found to have moderate catalytic activity [35-37]. Ru3(CO)i2 [38] and anionic hydridocluster complexes such as [NEt4][Ru3H(CO)ii] [39] have also been shown to have catalytic activity. In molten phosphonium salt, Ru3(CO)i2/2,2 -bipyridine has high catalytic activity [40]. The Ru3(CO)i2/l,10-phenanthroline catalyst in N,N-dimethylacetamide (DMAC) shows excellent activity and selectivity for u-aldehydes (Eq. 11.10) [41]. 

Knifton has also shown (36 - 38,40) that nitrogen- or phosphorus-ligand modified ruthenium complexes, in a phosphonium salt matrix, can conveniently catalyze the hydroformylation of terminal alkenes with high selec-tivities in linear oxo products. Usually selectivities better than 80% were achieved. In the best case (160°C, 95 bar. CO/H2= 1/2) a linearity in nonanol of 94% was obtained starting from [Ru3(CO),2], 2,2 -bipyridine. and [PBu4]Br. The main products were alcohols and not aldehydes. However, it is often difficult to reduce the isomerization of oct-l-ene as well as its hydrogenation. The [Ru3(CO),2l/2,2 -bipyridine (bipy) system has been extensively explored. Two equilibria have been proposed to account for the infrared data and the effects of the bipy ligand [eqs. (8) and (9)]. 

A comparison of Rh and Ru catalysts in the hydroformylation of linear butenes [4] or the strong electron-deficient substrate 3,3,3-trifluoropropene led to the conclusion that the latter are less active [5]. Moreover, in the hydroformylation of propene in comparison with Co and Rh catalysts, an inferior selectivity was noted [6]. In a competition experiment with the iridium-catalyzed hydroformylation of several a-olefins at 13 bar syngas pressure and 100 C, a related PPhj-modified Ru complex revealed no activity [7]. On the other hand, unmodified ruthenium based catalysts were shown to be more active than osmium complexes [8], thus the following rough order of reactivity results ... 

The addition of other metals to the heterogeneously cobalt-catalyzed reaction can have a beneficial effect on hydroformylation. For example, small amounts of ruthenium added to a carbon-supported cobalt catalyst (Co/AC) increased activity as well as Hb selectivity [64]. The effect was rationalized by the high dispersion and reducibUity of supported cobalt. When ruthenium was added, small particles of an unbalanced alloy were formed. These particles keep more CO in a nondisso-ciative state and lower the surface hydrogen pressure. This was in contrast to the related but uniformly distributed Pt-Co or Pd-Co alloys. Activity and regioselectivity increased with increased Ru loading. 

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