Sulfonated Xantphos

In line with the above mechanism, catalyst deactivation by formation of palladium black can be retarded by increasing the [P]/[Pd] ratio, however, only on the expense of the reaction rate. Bidentate phosphines form stronger chelate complexes than TPPMS which may allow at working with lower phosphine to palladium ratios. Indeed, the palladium complex of sulfonated XANTPHOS (51) proved to be an effective and selective catalyst for hydroxycarbonylation of propene, although at [51]/[Pd] < 2 formation of palladium black was still observed. The catalyst was selective towards the formation of butyric acid, with 1/b = 65/35 [41]. 

Figure 17 Xantphos 14 and its sulfonated derivatives for normaZ-selective hydroformylation in aqueous/organic diphasic media.

As described by Paganelli et al, hydroformylation of 1,1-diarylallyl alcohols such as l,l-bis(p-iluorophenyl)-2-propenol can be earned out conveniently in a biphasic system using a water-soluble complex formed by a mixture of [Rh(COD)Cl]2 and TPPTS [13, 14] or sulfonated Xantphos [14]. Interestingly, the retroaldoHzation reaction leading to byproducts seems to take place with more difficulty in a biphasic medium. 

The characteristics of the xantphos-type ligands in terms of activity and selectivity were later exploited by Dupont et al. (37). For a better solubility in ILs, sulfonated groups were grafted onto the xanthene backbone (9). Because of the poor natural solubility of this Kgand in I Ls, catalyst formation was achieved by mixing the ligand with (Rh(CO)2(acac)] in a mixture of (BMIM](PFg] and methanol. After evaporation of the methanol under vacuum, a yellow homogeneous IL phase was obtained. 

Similar results were recently published for the continuous-flow gas-phase hydroformylation of propene [51]. This time, the supported system was prepared by direct impregnation of an unmodified silica gel with methanol solution containing [Rh(CO)2(acac)j, the ligand, and the IL. The sulfonated xantphos used in this investigation must be added in large excess (10-20 equiv. per Rh) to afford the expected selectivity for the linear aldehyde n/i = 16.9). The performance of the optimized catalytic support remains stable up to 5 h before a decrease in activity and selectivity was observed. This work was also completed using other ligands derived from the series developed in classical biphasic IL systems, and applied to the continuous hydroformylation of propene and 1-octene [52]. 

These diphosphine ligands are often suitable or can be modified for use in biphasic systems. Sulfonation of xantphos led to a suitable water-soluble ligand [2,7-bis(S03Na)-xantphos], which was employed for hydroformylation of propylene, 1-hexene, and 4-styrene sulfonate. " HP-NMR confirmed that the diphosphine adopted the desired ee configuration. A rhodium-sulfoxantphos complex was employed for hydroformylation of 1-octene in [bmim]PF6, where bmim= l-butyl-3-methyl-imidazolium. " High pressure studies showed that the ee-ea ratio was sensitive to temperature and syngas pressure variations, but not to hydrogen partial pressure. 

Several new water-soluble phosphines were prepared for hydroformylation catalyzed by rhodium(I) complexes. These include the sulfonated derivatives BISBIS (16), BINAS (16,139), BIPHLOPHOS (143), sulfonated XANTPHOS (144), and others (Scheme 15). Rh-BINAS showed an enormously high activity in propene hydroformylation at 125°C and 5.2 MPa syngas, TOF = 10710 h , which was about 12 times higher than that of Rh-TPPTS under the same conditions (143). At the same time, the selectivity also increased to n/iso 98/2 from 95/5. Despite these improvements, the low price and availability still favors the industrial use of [HRh(CO)(TPPTS)3] -I- excess TPPTS. 

The reaction of pyrrolidinone with o-anisyl bromide as an electron-rich aryl halide also gave satisfactory yields of the N-arylation product Buchwald et al. [40] revealed that XantPhos (ll)/[Pd2(dba)3] and CSjCOj as base in 1,4-dioxane was a more efficient and more versatile catalyst system. The amidation of aryl bromides and sulfonates was readily achieved under mild reaction conditions (45-100°C) and displayed high functional group tolerance (Scheme 13.73). 

The reaction was conducted either in toluene or in an aqueous solution. In the reaction in water, a sulfonated phosphine was utilized in order to enhance the solubility of the catalyst. For the improvement of the -regioselectivity, the application of Xantphos as ligand proved to be advantageous [69]. It was exemplarily shown that after alkylation of the remaining hydroxy group with propylene and... 

The palladium-catalyzed coupling of aryl halides with a sodium aryl sulfinate afforded symmetrical and unsymmetrical diaryl sulfones in good to excellent yield (Scheme 5.47) [69]. A common palladium(O) compound was used as the metal source, and XantPhos served as the supporting/stabilizing ligand. A base was also needed for this chemistry, and... 

The combination of rhodium with sulfonated Xantphos (3) gave rise to a selective catalyst. In the hydroformylation of propene, Rh-3 afforded a higher n/iso ratio of 30 1 than Rh-(m-TPPTS) (16 1) under a low pressure (10bar CO/H2). The catalyst could be recycled up to five times without loss of its activity [20]. The Xantphos catalyst was also active for the hydroformylation of 1-hexene in water, showing modest activity (turnover frequency (TOP) of 24 h ), but again a high n/iso ratio (34.7 1) as well as 100% selectivity for aldehyde formation at 120 °C and 19bar CO/H2 [20]. 

Subsequently, a series of modified cyclodextrins were demonstrated to be effective for the hydroformylation of higher olefins [39], For example, combined with the sulfonated Xantphos 3, randomly methylated a- or p-cyclodextrin (RAME-a-CD or RAME-P-CD) provided an enhanced rate and n/iso ratio for the hydroformylation of 1-octene, allowing for a 74% conversion (99% selectivity) and n/iso ratio of 33 when using RAME-a-CD and 90% conversion (>99% selectivity) and an n/iso ratio of 20 when using RAME-P-CD. In contrast, Rh-3 alone led to 19% conversion (94% selectivity) and an n/iso ratio of 14 [40]. 

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