Cyclodextrins that are indiscriminately carboxymethylated at the 2-, 3-, and 6-positions are used as chiral NMR solvating agents for cationic substrates with phenyl, naphthyl, pyridyl, indoline, and indole rings. Enantiodifferentiation with the alpha-, beta-, and gamma-cyclodextrin derivatives is compared. The carboxymethylated derivatives are almost always more effective as chiral NMR solvating agents for cationic substrates than native cyclodextrins. The most effective carboxymethylated cyclodextrin varies for different substrates, and at times even different resonances of the substrate. Addition of paramagnetic praseodymium(III) or ytterbium(III) to mixtures of the carboxymethylated cyclodextrin and substrate often causes enhancements in enantiomeric discrimination and facilitates measurements of enantiomeric purity. The lanthanide ion bonds to the carboxymethyl groups and causes perturbations in the chemical shifts in the NMR spectra of substrate molecules in the cyclodextrin cavity.
Abstract:Described are studies of the dehydrogenation of 1,2-and 1,3-diols in homogenous solutions catalyzed by {[2,5-diphenyl-3,4-ditoluyl-(η 5 -C4CO)]2H}Ru2(CO)4(μ-H) (otherwise known as the Casey/Shvo catalyst). Both in the presence and absence of a dihydrogen acceptor, these reactions led to the analogous α-hydroxyketone as the only organic product. Isotopic labeling studies indicate that this product arises from reversible dehydrogenation/hydrogenation reactions, resulting in formation of the thermodynamically favored α-hydroxyketone. When this catalytic dehydrogenation was carried out in the presence of the rhodium decarbonylation catalyst Rh(dppp)2Cl (dppp = 1,3-bis(diphenylphosphino)propane), modest amounts of carbon monoxide result, suggesting that the dehydrogenation does generate at least some aldehydes that are intercepted by this catalyst. However, the efficiency of the latter reaction is poor.
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