The Phosphorus Ligands and Selectivity

As has already been mentioned (Section 5.2.1), the catalyst precursor RhH(CO)L3 has to undergo ligand dissociation to generate an unsaturated, cat-alytically active intermediate. In the absence of phosphines CO is the main ligand of the various catalytically active intermediates. It is obvious, therefore, that in the presence of both CO and L there are several possible equilibria. Under mild conditions, the ones shown in Fig. 5.3 have been observed by multinuclear ( ll, 13C, 31P) temperature-variable NMR. 

In a typical industrial hydroformylation process, the rhodium to phosphorus molar ratio is between 1 50 to 1 100, while the partial pressure of CO is about 

25 bar. It is reasonable to assume that under these conditions all the complexes with the general formula HRh(CO) L4(n = 0-4) might be present with variable concentrations. Like HRh(CO)L3, the catalyst precursor in Fig. 5.1, all these species can undergo ligand dissociation and initiate catalytic cycles similar to the one already discussed. Indeed there are commercial hydroformylation processes where rhodium precatalysts are used without any phosphorous ligand (see Section 5.4). In such processes HRh(CO)4-derived catalytic intermediates take part in catalysis. 

The individual catalytic cycles generated by HRh(CO) L4 species will be coupled with each other by ligand substitution reactions. This is shown in Fig. 5.4, where only the possible equilibria between the catalyst precursors (i.e., analogues of 5.1), the dissociated species (i.e., analogues of 5.2), and the alkene coordinated species (i.e., analogues of 5.3) are shown 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

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