Synthesis of P-Stereogenic Bisphosphine Ligands: Application as Hemi-labile Ligands in the Asymmetric Pauson-Khand Reaction

Abstract

The synthesis of new P-stereogenic ligands and investigation of their use in a Co-catalysed asymmetric Pauson-Khand reaction has been carried out. The ligands were synthesised by asymmetric lithiation of a phosphine sulfide or a phosphine borane using alkyllithium/()-sparteine complexes, trapping with Ph2PCl or t-Bu2PCl and treatment with BH3•Me2S or sulfur. This gave P-stereogenic bisphosphines. To afford highly enantioenriched ligands, the bisphosphines were recrystallised to increase the enantiomeric ratio up to 99:1 er. After borane deprotection of the bisphosphines using DABCO, hemi-labile ligands A and B were obtained. These hemi-labile ligands contain a phosphorus lone pair as a strong coordinating group and a sulfur lone pair on the P=S group as a weak coordinating group. The sensitivity after deboronation to oxidation to the phosphine oxide was studied through 1H and 31P NMR spectra. To evaluate the new hemi-labile ligands, they were used as a coordinating ligand with Co2(CO)8 in a catalytic asymmetric intramolecular Pauson-Khand reaction of a 1,6-enyne to a cyclopentenone. The synthesis of new P-stereogenic ligands and investigation of their use in a Co-catalysed asymmetric Pauson-Khand reaction has been carried out. The ligands were synthesised by asymmetric lithiation of a phosphine sulfide or a phosphine borane using alkyllithium/()-sparteine complexes, trapping with Ph2PCl or t-Bu2PCl and treatment with BH3•Me2S or sulfur. This gave P-stereogenic bisphosphines. To afford highly enantioenriched ligands, the bisphosphines were recrystallised to increase the enantiomeric ratio up to 99:1 er. After borane deprotection of the bisphosphines using DABCO, hemi-labile ligands A and B were obtained. These hemi-labile ligands contain a phosphorus lone pair as a strong coordinating group and a sulfur lone pair on the P=S group as a weak coordinating group. The sensitivity after deboronation to oxidation to the phosphine oxide was studied through 1H and 31P NMR spectra. To evaluate the new hemi-labile ligands, they were used as a coordinating ligand with Co2(CO)8 in a catalytic asymmetric intramolecular Pauson-Khand reaction of a 1,6-enyne to a cyclopentenone