Trivalent phosphorus ligands

Alexakis, employing various chiral trivalent phosphorus ligands, has recently described Cu(OTf)2-catalyzed 1,4-additions of Et2Zn to a number of nitroalkenes (Scheme 7.22) [77]. TADDOL-based phosphonite 82 gave the highest ees for ar-ylnitroalkenes (up to 86%), whereas phosphoramidite 18 is the ligand of choice for alkylnitroalkenes [ees of up to 94%). 

The discovery and use of fluorophosphites and chlorophosphites as trivalent phosphorus ligands in the rhodium catalyzed, low-pressure hydroformylation reaction are described. The hydroformylation reaction with halophosphite ligands has been demonstrated with terminal and internal olefins. For the hydroformylation of propylene, the linear to branched ratio of the butyraldehyde product shows a strong dependency on the ligand to rhodium molar ratios, the reaction temperature, and the carbon monoxide partial pressure. 

The effect of ligand modification depends not only on the electronic and steric properties but also on the number of organic ligands in the coordination sphere of the metal. Appropriate organic ligands can displace coordinated CO in a stepwise manner [15]. The whole complexity is shown by means of the best studied system, namely rhodium catalysts based on trivalent phosphorus ligands (Scheme 1.6). 

Typical examples of different behavior in relation to the metal are trivalent phosphorus ligands. Thus, trials to modify cobalt complexes with PPhg proved rather problematic, due to the shift of the equilibrium to the left-hand side, especially under increased CO pressure (Scheme 1.7). As a consequence, the hydroformylation is catalyzed by the unmodified Co complex. Diphosphines of the type Ph2PZPPh2 (Z = (CH2)2, (CH2)4, CH=CH) cause a dramatic decrease in reactivity [19]. Also, phosphites do not form active hydroformylation catalysts with cobalt. It seems that only basic trialkyl phosphines are suitable for the generation of stable Co phosphine hydroformylation catalysts. 

Indeed, these calculations correspond to the experimental results of Beller s group with monodentate phosphine ligands and may explain to some extent the lower reactivity for iridium in comparison to rhodium catalysts found in the past [10]. By comparing the efficiency of trivalent phosphorus ligands on the aldehyde yield, the following order was established (Figure 1.14). 

In the following chapters, only the synthesis of those trivalent phosphorus ligands will be reviewed in detail that have (i) relevance to technical application documented by a pronounced patent activity, (ii) a unique structure, or (iii) unique properties in a selected hydroformylation reaction. A particular chapter will be dedicated to the synthesis and properties of carbenes and their use in hydroformylation. 

Figure 7.10 Trivalent phosphorus ligands used in rhodium-catalyzed hydroformylation in...

---