Synthesis, Properties, and Catalytic Applications of Caged, Compact Trialkylphosphine 4-Phenyl-1-phospha-4-silabicyclo[2.2.2]octane

Synthesis, properties, and catalytic applications of a caged trialkylphosphine ligand with Me3P-like steric and electronic characters, 4-phenyl-1-phospha-4-silabicyclo[2.2.2]octane (Ph-SMAP), are reported. Given a phenyl group at the silicon atom, the Ph-SMAP ligand displayed nonvolatility with retention of Me3P-like steric and electronic properties. The new ligand is air-stable, crystalline, and easy to handle. Single-crystal X-ray diffraction analyses of Ph-SMAP and its coordination compounds such as borane, rhodium(I), and Pt(II) complexes revealed a rigid, linear structural feature of the Ph-SMAP framework. DFT calculations [B3LYP/6-31G(d,p)] indicated that the electron-donating ability of Ph-SMAP is slightly stronger than that of Me3P and that replacement of Si atom of Ph-SMAP with a carbon atom drastically decreases the donor power. The Ph-SMAP ligand markedly accelerated the rhodium-catalyzed hydrosilylation and hydrogenation of ketones as compared with the effect of conventional phosphine ligands such as Me3P, Bu3P, (t-Bu)3P, and PPh3, when it was used in combination with [{RhCl(C2H4)2}2] and [Rh(OMe)(cod)], respectively, with P/Rh ratio of 1:1.