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There are very many investigations in the literature concerning the evaluation of different metals and associated organic ligands in hydroformylation. In 2013, Franke and Beller [1] provided a concise summary about the applicability of alternative metals in hydroformylation. In the same year, another survey was assembled by a joint French/Italian cooperation [2]. In order to avoid a full repetition, only some basic conclusions will be mentioned here, which are not in the focus of the reviews cited above.
Several hydrido metal carbonyl complexes are able to catalyze the hydroformylation reaction (Scheme 1.1). Preconditions are the ability for the formation of the relevant intermediates and the passage of crucial steps, such as a metal-alkyl complex by addition of the M-H bond to an olefin (a), subsequent insertion of CO into the M-alkyl bond by migration of a ligated CO ligand (b), and the final hydrogenolysis of the M-acyl bond to liberate the desired aldehyde and to reconstruct the catalyst (c). The type of the transient M-alkyl complex is responsible for the formation of isomeric aldehydes, here distinguished as Cycle I and II. For the successful passage of these catalytic events, besides the reaction conditions the choice of the appropriate metal and its coordinated ligands are pivotal.
Scheme 1.1 Simplified catalytic cycle for hydroformylation.
In the early (mainly patent) literature, besides Co and Rh, Ni, Ir, and other metals of the VIII group, also Cr, Mo, W, Cu, Mn, and even Ca, Mg, and Zn were suggested or claimed for hydroformylation [3]. However, several of them do not exhibit any activity.
Adequate hydroformylation activity of the hydrido carbonyl complexes is attributed to the polarity of the M-H bond [4]. It is assumed that high acidity facilitates the addition to an olefin and the hydrogenolysis of the transient metal-acyl complex in a later stage of the catalytic cycle. In this respect, HCo(CO)4 is a much stronger acid than H2Ru(CO)4, H2Fe(CO)4, H2Os(CO)4, or HMn(CO)5 [5]. Moreover, anionic hydrido complexes, such as [HRu(CO)4]-, behave as strong bases [6]. The conversion of the latter into H2Ru(CO)4 is probably a precondition for the success of the hydroformylation and one explanation why Ru3(CO)12 is more active than [HRu(CO)4]-. The former reacts with H2 to form H2Ru(CO)4 [7]. Low activity was likewise observed for [HOs3(CO)11]- associated with a low thermal stability [8]. Also, [Co(CO)4]- is a poor hydroformylation catalyst [9]. However, with the addition of strong acids, the active species HCo(CO)4 can be generated.
Noteworthy, the instability of HCo(CO)4 under the formation of Co2(CO)8 can be attributed in part to the fast intermolecular elimination of H2. In this manner, also the formation of alkanes can be explained as a key step in the hydrogenation of olefins. On the other hand, the acidic properties of HCo(CO)4 allow the convenient separation of product and catalyst after hydroformylation by conversion into water-soluble Co salts ("decobalting") [[10]].
Strong acidic metal hydrido complexes such as HCo(CO)4 or complexes with Lewis acid properties, such as Rh2Cl2(CO)4, [Ru(MeCN)3(triphos)](CF3SO3)2, [Pt(H2O)2(dppe)](CF3SO3)2, [Pd(H2O)2(dppe)](CF3SO3)2, or [Ir(MeCN)3(triphos)](CF3SO3)3, are able to act in alcohols as acetalization catalysts, which means they can mediate the transformation of the newly formed aldehydes into acetals (see Section 5.3).
The number of CO ligated to the same metal may affect the catalytic properties (Scheme 1.3) [11]. With cobalt (but also with rhodium) both the tetra and tricarbonyl complexes are considered as catalysts (Scheme 1.2). It is thought that the coordinatively unsaturated complex HCo(CO)3 is more active than HCo(CO)4. Moreover, because of different steric congestions of the metal center, it is assumed that both complexes have different regiodiscriminating propensities for the formation of transient alkyl complexes and, consequently, for the formation of isomeric aldehydes. Therefore, the effects that have been observed at different CO partial pressures can be best explained by assuming the formation of HCo3(CO)9 in a solution containing HCo(CO)4 and its precursor Co2(CO)8 under hydrogen [[12]]. HCo3(CO)9 reacts with hydrogen to form HCo(CO)3 [13]. The latter is more active in isomerization and, consequently, forms more isomeric aldehyde as a final product.
Scheme 1.2 Competition between isomerization and hydroformylation in relation to CO pressure.
In comparison to HCo(CO)4, the rhodium congener has a greater tendency to liberate one CO ligand [14]. In other words, the equilibrium in Scheme 1.3 is less markedly displaced to the left-hand side in comparison to the cobalt-based system.
Scheme 1.3 Equilibrium of catalytically active hydrido carbonyl complexes.
Bearing in mind the greater atomic radius of Rh, it becomes apparent why an unmodified rhodium catalyst generates a greater amount of branched aldehydes in comparison to the cobalt congener. For example, in the hydroformylation of 1-pentene, an l/b ratio of only 1.6 : 1 was found, while with the cobalt complex a ratio of 4 : 1 resulted. A similar correlation has been qualitatively deduced from reactions mediated by the metal clusters Ru3(CO)12, Os3(CO)12, and Ir4(CO)12. Because of the larger atomic radii of the metals, in hydroformylation these catalysts produce more branched aldehydes than observed in the reaction with Co2(CO)8. Unfortunately, most of these results were achieved under different reaction conditions or are difficult to interpret because of low reaction rates and are therefore not strictly comparable.
Polynuclear metal clusters may behave differently in catalysis in comparison to their mononuclear species [15]. Thus, the catalytic activity of [HRu(CO)4]- is superior to that of [HRu3(CO)11]- [6]. Noteworthy, H4Ru4(CO)12 is particularly active in hydroformylation with CO2 [16].
Currently, with unmodified metal carbonyl complexes, the following trend of hydroformylation activity is accepted (ordered by decreasing activity) [17]:
In subsequent chapters, only hydroformylations with Co, Rh, Ru, Pd, Pt, Ir, and Fe will be discussed in detail. Occasionally also molybdenum complexes (e.g., mer-Mo(CO)3(p-C5H4N-CN)3) [18] or osmium complexes (e.g., HOs(?3-O2CR)(PPh3)2) have been investigated [19]. Only recently, HOs(CO)(PPh3)3Br was evaluated for the hydroformylation of several olefins [20]. A main concern was the high isomerization tendency (up to 39%) noted.
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