Axial Tri-<i>tert</i>-butylphosphane Coordination to Rh₂(OAc)₄: Synthesis, Structure, and Catalytic Studies

The introduction of strong σ-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, Rh₂(OAc)₄ complexes with axial bulky alkylphosphane ligands (PR₃), in particular P­(<i>t</i>-Bu)₃, were prepared and characterized. The net σ-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 Rh₂ core. Analysis of the crystal structure data showed that the strong σ-donor P­(<i>t</i>-Bu)₃ coordinates to the rhodium with an amount of σ-donation to the rhodium similar to that of the aryl phosphane ligand PPh₃, but has an unusually long Rh–P bond distance (2.663 Å). 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 Rh₂(OAc)₄(P­(<i>t</i>-Bu)₃)₂ (<b>Cat-1</b>) facilitates substitution of one of the axial phosphane ligands by the arylboronic acid. This σ-donating effect greatly accelerated the arylation reaction in comparison to alternative catalysts. Additionally, Rh₂(OAc)₄ was easily recovered after completion of the reaction

Axial Tri-<i>tert</i>-butylphosphane Coordination to Rh₂(OAc)₄: Synthesis, Structure, and Catalytic Studies

The introduction of strong σ-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, Rh₂(OAc)₄ complexes with axial bulky alkylphosphane ligands (PR₃), in particular P­(<i>t</i>-Bu)₃, were prepared and characterized. The net σ-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 Rh₂ core. Analysis of the crystal structure data showed that the strong σ-donor P­(<i>t</i>-Bu)₃ coordinates to the rhodium with an amount of σ-donation to the rhodium similar to that of the aryl phosphane ligand PPh₃, but has an unusually long Rh–P bond distance (2.663 Å). 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 Rh₂(OAc)₄(P­(<i>t</i>-Bu)₃)₂ (<b>Cat-1</b>) facilitates substitution of one of the axial phosphane ligands by the arylboronic acid. This σ-donating effect greatly accelerated the arylation reaction in comparison to alternative catalysts. Additionally, Rh₂(OAc)₄ was easily recovered after completion of the reaction

Chiral Surfactant-Type Catalyst: Enantioselective Reduction of Long-Chain Aliphatic Ketoesters in Water

A series of amphiphilic ligands were designed and synthesized. The rhodium complexes with the ligands were applied to the asymmetric transfer hydrogenation of broad range of long-chained aliphatic ketoesters in neat water. Quantitative conversion and excellent enantioselectivity (up to 99% ee) was observed for α-, β-, γ-, δ- and ε-ketoesters as well as for α- and β-acyloxyketone using chiral surfactant-type catalyst <b>2</b>. The CH/π interaction and the strong hydrophobic interaction of long aliphatic chains between the catalyst and the substrate in the metallomicelle core played a key role in the catalytic transition state. Synergistic effects between the metal-catalyzed site and the hydrophobic microenvironment of the core in the micelle contributed to high stereoselectivity