# Homogeneous catalysis

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## Importance and outlook

In addition to the polymerization reactions of olefins and dienes, hydroformylation is the most important homogeneous catalysis in the chemical industry in terms of quantity and value. 1995 were worldwide over $6,6⋅106$$t$ (after Beller, M .; Bolm, C. (1998):Transition Metals for Organic Synthesis. WILEY-VCH ,) produced on aldehydes by hydroformylation. 70% on the hydroformylation of propene and approx. 20% on that of C4-C13-Olefins. From most of the butyraldehyde, butanol is produced by hydrogenation or 2-ethylhexan-1-ol via aldol condensation and subsequent hydrogenation.

The hydroformylation of terminal olefins has achieved technical importance almost exclusively. Hydroformylation catalysts can also catalyze double bond isomerizations. This is used to selectively produce n-aldehydes from mixtures of linear internal olefins.

Such a process step can be part of the Shell Higher Olefin Process (SHOP). The following two examples demonstrate further applications of hydroformylation in the chemical industry:

• Two-phase catalysis. Rhodium complexes with sulfonated arylphosphine ligands (e.g. (C6H4-m-SO3H)3) are water-soluble. This enables a two-phase process that is carried out continuously. The catalyst remains in the aqueous phase. The water-immiscible aldehyde forms the organic phase and is simply separated off by phase separation. This process (Ruhr-Chemie / Rhone-Poulenc) is widely used for the synthesis of butyraldehyde and other aldehydes.
• The technical synthesis of Vitamin A(Annual production approx. 3,000 $t$) the end 1,2-diacetoxy-but-3-ene (BASF) or from 1,4-diacetoxy-but-2-ene (Hoffmann-LaRoche) involves hydroformylation. Special reaction conditions and the use of an unmodified rhodium catalyst ensure the (normally undesirable) formation of the branched aldehydes.
• More recent developments aim to use water-soluble rhodium catalysts with sulfonated phosphine ligands. This enables the hydroformylation to be carried out in a two-phase system (water / organic phase), which considerably simplifies the separation of the catalyst from the product.
• The use of rhodium complexes with optically active phosphine ligands such as BINAPHOS enables enantioselective hydroformylation of prochiral olefins. This is shown in the following equation using the example of 3-methyl-4- (4-chlorophenyl) butanal, an intermediate stage in the synthesis of the insecticide(S, S) -Fenvalerateshown.