The Pivotal Role of Hydrido Complexes

There are very many investigations in the literature concerning the evaluation of different metals and associated organic ligands in hydroformylation. In 2013, Franke and Seller [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. 

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. 

Hydrcformylation Fundamentals, Processes, and Applications in Organic Synthes, First Edition. 

Noteworthy, the instability of HCo(CO)4 under the formation of Co2(CO)g 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 HCofCO) 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 HCofCO) or complexes with Lewis acid properties, such as Rh2Cl2(CO)4, [Ru(MeCN)3(triphos)](CF3S03)2, [Pt(H20)2(dppe)](CF3S03)2, [Pd(H20)2(dppe)](CF3S03)2, or [Ir(MeCN)3(tri-phos)] (CF3S03)3, are able to act in alcohols as acetahzation catalysts, which means they can mediate the transformation of the newly formed aldehydes into acetals (see Section 5.3). 

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