Catalytic Systems and Substrates

The application of Rh catalysts, which moved in the 1970s but in focus till today, allows operating the process under much milder conditions (20-120 bars, 70-160°C). Usually, the catalyst is produced from Rh(II)carboxylates, Rh(acac)(CO)2 (acac = acetylacetonate), or Rh(acac)(COD) (COD = 1,5-cyclo-octadiene). Commercial phosphorus ligands, such as monodentate phosphines (PPhg) or phosphites [P(OPh)3, P(0-o-tBuPh)3, Alkanox 240], significantly improve the activity. Simultaneously, isomerization of the starting olefin may occur. Commonly the ligand is used in an excess in comparison to the metal. 

With bidentate ligands (e.g., Xantphos, BIPHEPHOS), the regioselectivity toward the formation of the n-aldehyde can be advantageously influenced, even when internal olefins are used as substrates [21]. Trialkylphosphines enhance the hydrogenation activity of Rh catalysts, with the result that alcohols are formed immediately from the aldehydes, which maybe desirable. 

With the former, frequently a higher hydrogenation activity is associated [34]. 

In the following sections, typical hydroformylation protocols will be quoted that give the desired aldehydes in the most efficient and selective way reported up to now. If available, also different approaches will be discussed, thus allowing a comparison of these methods. Moreover, they will allow the reader to evaluate the progress of the methods over the years. 

When appropriate, also the source or method for the manufacture ofthe starting olefin will be detailed. In some cases, the tricky problem of the separation of catalyst and product will be likewise briefly addressed, which may become a critical issue in particular in the transformation of higher olefins. It can be expected that on the industrial scale, also other procedures are under operation, but usually this information is not available in the public domain. Subsequent transformations of aldehydes are considered only when they are related to the use in aroma industry. Thus, for example, aminomethylation of monoterpenes has not been accommodated in this summary [41]. 

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