Ligand XANTPHOS

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

Figure 7.8. Unmodified xantphos (a), sulfoxantphos (b) and guanidinium-modified xantphos ligand (c) as used in the biphasic, Rh-catalysed hydroformylation of 1-octene...

We applied the QM/MM IMOMM method [41] to Rh-diphosphine catalyzed hydroformylation, to provide a quantitative theoretical characterization of the origin of regioselectivity in Rh-diphosphine systems. We focused on the experimentally characterized xantphos ligands, for which variation in electronic properties is minimal. Using the IMOMM method, which only accounts for the steric properties of ligands, was fully justified. 

We therefore evaluated how the bite angle affected regioselectivity, and studied the counterbalance of non-bonding and orbital effects. We choose two diphosphine ligands (benzoxantphos and homoxantphos) which among the series of xantphos ligands represent the extreme cases of natural bite angle, and used propene as a model for terminal aliphatic alkenes and styrene. 

Table 6.1 shows data for rhodium complexes containing xantphos ligands in which the electronic properties were systematically varied [13]. With few exceptions, we see that the rate of the hydroformylation reaction increases when the rhodium center is more electron deficient. This is as expected, since dissociation of... 

The decay of the carbonyl bands of the HRh(diphosphine)("CO)2 complexes with time follows simple first-order kinetics in all experiments. Plots of ln[HRh (diphosphine)("CO)2] vs. time are linear for at least two half-lives. Comparison of the rate constants, kj, obtained for ligands 32 and 33 [54] with those obtained for other xantphos ligands [52] shows that the CO dissociation rate for ligand 32 is in the same range as other ligands. The CO dissociation rate for ligand 33, however, proves to be four to six times higher. 

A rhodium catalyst based on the glycine-urea functionalized Xantphos ligand was subsequently used in combination vdth the same support material, as a catalyst for the hydroformylation of 1-octene. In eleven consecutive reactions the catalyst did not... 

Even alkyl Grignard reagents can be coupled with alkyl halides 1 using iron catalysis with the xantphos ligand 11 (entry 16) [56]. The yields are acceptable for primary alkyl bromides. Secondary alkyl halides reacted only in low yield. 

The POP-Xantphos ligand, which at the present affords the best turnover frequencies and l b ratios is considerably more expensive than the rather simple TPPTS ligand employed in aqueous biphasic hydroformylation. Compared to the classical rhodium-phosphine process, manufacturing costs in the aqueous biphasic process are about 10% lower. Accordingly, requirements for a fairly expensive catalyst will be exceptional if it should be considered in an industrial process, especially if one bears the additional costs for the required ionic liquid in mind. Nevertheless, the prospect of biphasic ionic liquid hydroformylation looks very promising. 

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