The reaction of 1-alkenes with carbon monoxide and hydrogen in the presence of a catalyst gives the corresponding homologous aldehydes. The discovery of this reaction was made by Roelen in 1938 using Co2(CO)8 as the catalyst at high temperature (120–170°) and high pressure of carbon monoxide/hydrogen (200–300 bar). This reaction has been called the “Oxo reaction”, “Roelen reaction” or “hydroformylation”. Hydroformylation is a general term indicating that both a hydrogen and a formyl group are introduced to unsaturated bonds, especially olefins.
Later this reaction was developed as an industrial process, i.e., the Oxo Process, for the production of alkanals from 1-alkenes using a cobalt or rhodium catalyst. Most noteworthy is the conversion of propene to butanal, which can be subsequently hydrogenated to 1-butanol or converted to 2-ethylhexanol by self-aldol condensation. 2-Ethylhexanol, a crucial intermediate for the production of ester-type plasticizers, is the most important bulk chemical produced by the Oxo Process. A variety of transition metal catalysts other than Co2(CO)8 have been investigated, including phosphine complexes of cobalt and hydridocobalt clusters. Platinum and ruthenium complexes show reasonably good catalytic activities, but modified cobalt catalysts are still much more advantageous. However, various rhodium complexes demonstrate higher catalytic activity (103–104 times) than the cobalt complexes. Although the price of rhodium is higher than cobalt, reactions using rhodium catalysts require lower temperature (50–80°) and pressure (10–50 atm).
Other important commercial applications of hydroformylation include the production of long-chain alcohols from C5–C17 isomeric linear alkenes. These long-chain alcohols serve as intermediates for lubricants, plasticizers and detergents. The hydroformylation of ethene to propanal is another important Oxo Process.
In this chapter, the authors put clear emphasis on the scope of the hydroformylation reaction in organic synthesis. In this context, there is a relevant review in 1987 of the hydroformylation of functionalized alkenes. The hydroformylation reaction now can be performed under very mild conditions using a variety of functionalized alkenes. Reactions in aqueous biphase, supercritical carbon dioxide or fluorous biphase have recently emerged in response to separation and environmental issues. In fact, a highly efficient Oxo Process using a water-soluble rhodium catalyst in aqueous biphasic conditions has been commercialized by Ruhrchemie/Rhône-Poulenc for the production of butanal. Asymmetric hydroformylation of prochiral olefins catalyzed by enantiopure rhodium complexes has been developed to the level that practical applications appear possible. Although the reactions of formaldehyde, oxiranes, and others with carbon monoxide and hydrogen in the presence of transition metal catalyst could be considered as variations of hydroformylation, this chapter only deals with hydroformylation of carbon-carbon multiple bonds.