Phosphines cyclodextrin-modified

A variety of rhodium complexes with cyclodextrins modified by various phosphine groups have been synthesized for performing hydroformylation and hydrogenation (Table 11-3). 

In catalysis the excess of a phosphine ligand is often necessary because it preserves the active species in the medium [2a]. However, it retards to some extent the co-ordination of the alkene to the metal center. Recent studies, performed by Monflier and coworkers, have shown that the water-soluble TPPTS ligand could reduce the rate of the reaction by another effect. Indeed, TPPTS can be included partially in the cyclodextrin hydrophobic cavity [53,54] NMR measurements, observation by UV-visible spectroscopy and circular dichroism, as well as scanning tunneling microscopy are consistent with a 1 1 inclusion complex in which the phosphorus atom would be incorporated into the torus of the /S-CD. NMR investigations carried out on (m-sulfonatophenyl)diphenylphosphine have shown that a phenyl group is incorporated [55]. Thus, the phosphorus ligand could modify the association constant of the alkene with the cyclodextrin so that the mass transfer between the two phases could be decreased. 

Cyclodextrins can be modified by thiol substituents and further attachment to phosphine or aminophosphine ligands (see 9) allows the coordination to rhodium catalysts [30], The activity in the hydroformylation reaction of 1-octene is interesting since turnover frequencies near to 180 h can be reached with a selectivity in Cg-aldehydes higher than 99% (31). 

Efficient biphasic catalysis relies on the rapid mass transfer across the aqueous and organic phases. As indicated, this poses a problem for higher olefins because of their insolubility in water. To tackle the issue and thus to increase the hydroformylation rates, additives, such as co-solvents, surfactants, or modified cyclodextrins, have been explored. A water-miscible organic cosolvent such as an alcohol could increase the solubility of alkenes in the aqueous phase or the catalyst in the organic phase. For example, using [Rh(p-S Bu)(CO)(m-TPPTS)]2 as a catalyst, hydroformylation of 1-octene gave less than 24% conversion after 15 h in water at 80 °C but it reached 90% conversion in 10 h in water/methanol (3 1) [28]. Using Rh-1, 1-dodecene was hydroformylated with 42% conversion to aldehydes in a mixture solvent of water/propanol, while no hydroformylation was observed at all in water alone under identical conditions [29]. The same trend was observed in the reaction of 1-octene catalyzed by Co/BiphTS (BiphTS, trisulfonated tris(biphenyl)phosphine) [30]. 

---