Promotion effect of water on hydroformylation of styrene and its derivatives with presence of amphiphilic zwitterionic phosphines

Two kinds of novel zwitterionic phosphines of L1 and L2 have been synthesized in which the sulfonate groups (-SO₃^-) are incorporated to render them amphiphilic. Both of them are strong π-acceptor ligands in which the positive-charged imidazoliums with intensive electron-withdrawing effect are vicinal to the coordinating phosphorous atoms. The amino-group of piperidyl is additionally incorporated in L2 to make it behave as a hybrid ligand with potential hemilability. The comparison of the ligands of L1 and L2 in Rh-catalyzed hydroformylation of styrene (and its derivatives) and the effect of water on their behaviors were discussed. Without the involvement of water, the catalytic system of Rh(acac)(CO)₂-L2 exhibited lower activity than that of Rh(acac)(CO)₂-L1 under the same conditions. However, the dramatic promotion effect of water as a suspension was observed especially for Rh(acac)(CO)₂-L2 system with TOF spurred up from 195 to 470 h^-1. The corresponding in situ high pressure FT-IR spectral information showed that the presence of water in Rh(acac)(CO)₂-L2 system greatly facilitated the formation and stability of the active Rh-H species (ν_Rh-H, 2047 cm^-1), which was in charge of the efficient hydroformylation of styrene and its derivatives.