Asymmetrische Synthese von cyclischen allylischen S-tert-Butylsulfonen : Pd-katalysierte kinetische Racematspaltung racemischer Allylcarbonate und zur Struktur chiraler, cyclischer allylischer Alpha-tert-Butylsulfonyl-Carbanionen

摘要

Aim of this project was the asymmetric synthesis of cyclic allylic tert-butylsulfones by Pd-catalyzed asymmetric substitution in presence of a chiral bisamide-bisphosphane-Ligand. The sulfones were formed from the corresponding racemic carbonates and lithium-tert-butylsulfinate in a two phase system composed of dichlormethane and water at 0 °C containing THAB as a phase transfer catalyst and in presence of Pd(dibenzylidenacetone)3·CHCl3 as precatalyst and Pd(0)source and the chiral catalyst. Cyclopentenyl-tert-butylsulfone, cyclohexenyl-tert-butylsulfone and cycloheptenyl-tert-butylsulfon were isolated with high yields (76-94 %) and the cyclooct-enyl-tert-butylsulfone was isolated with moderate yields (30-50 %). All sulfones were obtained with excellent enantioselectivities (89-98 %). The absolute configuration of the sulfones was determined to be 'S' by crystal analysis. Another aim of the project was the investigation of the kinetic resolution occurring during the Pd-catalyzed sulfonylation of the racemic carbonates. Therefore the sulfonylation reactions were terminated at approximately 50 % conversion. The sulfones were isolated with very good enantioselectivities (94-98 %) and good yields (82-94 %). The unreacted carbonates were isolated with excellent enantioselectivities (97-99 %) and with good yields (82-85 %) under optimized working up conditions. During the course of the Pd-catalyzed kinetic resolution and up to complete conversion of the sulfonylation the enantiomeric excess of the product sulfone remained practically constant. The selectivity factor was estimated to be S > 40 by method of internal standard and Enantiomer-Labelling. The faster reacting enantiomers of the carbonates and the preferentially formed sulfones have the same absolute configuration ('S'). The obtained cyclic sulfones were deprotonated with BuLi at low temperatures giving the corresponding Lithiosulfones. These Lithiosulfones were afterwards investigated by NMR-spectroscopy. Deprotonation in PMDETA gave crystals of the Lithiosulfones, which were investigated by crystal analysis. Reactions of the obtained Lithiosulfones with electrophiles could not approve the expected configurative stability of the carbanions.