Heteronuclear bidentate Lewis acids: Synthesis, binding properties, and synthetic applications

Ferrocene-based planar chiral bidentate Lewis acids were obtained as racemates through electrophilic substitution of 1,1'-bis(trimethylstannyl)ferrocene with various haloboranes (BCl3, RBCl2, R2BCl). The binding of the resulting bidentate Lewis acids to different nucleophiles was studied by means of multinuclear NMR analysis, UV/Vis spectroscopy, and single-crystal X-ray diffraction. Cooperative effects involving the adjacent Lewis acid centers are found to result in the stereoselective binding of pyridine to Lewis acid 1-Cl with the formation of a B-chiral pyridine complex. Competition experiments with the respective monodentate Lewis acid for the binding of pyridine showed significant enhancement of the Lewis acidity of the boron center.;Fluoride complexes of ferrocene-based bidentate Lewis acids 1-Cl and 2-Ph were synthesized by treatment with KF in the presence of 18-crown-6. Multinuclear NMR studies and single-crystal X-ray diffraction confirmed the simultaneous coordination of fluoride to both boron and tin in the solid state as well as in solution. The fluoride complexes derived from compound 2-Ph were found to exist as two distinct diastereomers in solution. Rapid fluoride exchange between the two isomers was observed by 19F-19F 2D-EXSY NMR spectroscopy.;When the reaction of the aryl and allyl derivatives of 1-stannyl-2-borylferrocene with KF was performed in the absence of 18-crown-6, an unusual fluoride-promoted migration of the aryl and allyl group, respectively, from the boron to the tin center was observed. The products were studied by multinuclear NMR analysis and single-crystal X-ray diffraction. Preliminary mechanistic studies were carried out and the intermediacy of a transient cationic triorganotin species is proposed.;Chiral resolution of compound 1-Cl was accomplished in high yields with optical purities exceeding 97% via treatment with (+) and (-)- N-methylpseudoephedrine as chiral resolving agent. The absolute configuration of the complexes was established by single-crystal X-ray diffraction. The respective planar chiral allylboranes were synthesized and characterized by multinuclear NMR analysis. The stereoselectivity of the allylboranes was examined in allylation reactions of various carbonyl compounds. Reactions with ketones resulted in the facile formation of the respective homoallylic tertiary alcohols with up to 80% ee.*;*Please refer to dissertation for diagrams.