Axial Tri-tert-butylphosphane Coordination to Rh2(OAc)4: Synthesis, Structure, and Catalytic Studies
摘要:
The introduction of strong sigma-donor axial ligands to the Rh-Rh metal bond has been utilized as an effective way to provide new chemical reactivities to bimetallic dirhodium(II) complexes. In this report, Rh-2(OAc)(4) complexes with axial bulky alkylphosphane ligands (PR3), in particular P(t- Bu)(3), were prepared and characterized. The net sigma-donation from the PR3 to the Rh-Rh bond is the result of the competition between the electron-donating ability of the R group and the steric profile of the PR3 at the Rh-2 core. Analysis of the crystal structure data showed that the strong sigma-donor P(t-Bu)(3) coordinates to the rhodium with an amount of sigma-donation to the rhodium similar to that of the aryl phosphane ligand PPh3, but has an unusually long Rh-P bond distance (2.663 angstrom). During catalytic trials to synthesize 3-aryl-3-hydroxy-2-oxindole by the addition of arylboronic acids to isatin derivatives, this longer Rh-P bond distance in Rh-2(OAc)(4)(P(t-Bu)(3))(2) (Cat-1) facilitates substitution of one of the axial phosphane ligands by the arylboronic acid. This sigma-donating effect greatly accelerated the arylation reaction in comparison to alternative catalysts. Additionally, Rh-2(OAc)(4) was easily recovered after completion of the reaction.