ABSTRACT
We report the synthesis, spectroscopic characterization, and reactivity of Eu(3+) in the presence of a new set of chiral ligands designed for the aqueous, enantioselective Mukaiyama aldol reaction. Luminescence and NMR measurements were used to characterize the coordination environments of the Eu(3+)-based precatalysts, and this data is compared with yields and stereoselectivities. In addition to structure-function relationships, we found that, in the presence of excess hexadentate ligands, Eu(3+) is coordinatively saturated, and subsequently, the reactivity of the precatalysts is reduced. These findings are helpful for the design of new ligands that bind Eu(3+) without saturating the Eu(3+) coordination sphere.
Subject(s)
Aldehydes/chemistry , Europium/chemistry , Catalysis , Cations/chemistry , Ligands , Luminescence , Magnetic Resonance Spectroscopy , Spectrum Analysis , Water/chemistryABSTRACT
The development of efficient methods for the asymmetric Mukaiyama aldol reaction in aqueous solution has received great attention. We have developed a new series of chiral lanthanide-containing complexes that produce Mukaiyama aldol products with outstanding enantioselectivities. In this paper, we describe an optimized ligand synthesis, trends in stereoselectivity that result from changing lanthanide ions, and an exploration of substrate scope that includes aromatic and aliphatic aldehydes and silyl enol ethers derived from aromatic and aliphatic ketones.
Subject(s)
Aldehydes/chemical synthesis , Lanthanoid Series Elements/chemistry , Organometallic Compounds/chemistry , Aldehydes/chemistry , Catalysis , Molecular Structure , Stereoisomerism , Water/chemistryABSTRACT
Luminescence-decay measurements in combination with high-performance liquid chromatography analyses were used to study the relationship between rates of catalysis and water-coordination numbers of europium-based precatalysts in the aqueous Mukaiyama aldol reaction. A correlation between reactivity and water-coordination number was observed and is reported here.
Subject(s)
Carbon/chemistry , Lanthanoid Series Elements/chemistry , Water/chemistry , Catalysis , Chromatography, High Pressure Liquid/methods , Europium/chemistryABSTRACT
Polyaminopolycarboxylate-based ligands are commonly used to chelate lanthanide ions, and the resulting complexes are useful as contrast agents for magnetic resonance imaging (MRI). Many commercially available ligands are especially useful because they contain functional groups that allow for fast, high-purity, and high-yielding conjugation to macromolecules and biomolecules via amine-reactive activated esters and isothiocyanate groups or thiol-reactive maleimides. While metalation of these ligands is considered common knowledge in the field of bioconjugation chemistry, subtle differences in metalation procedures must be taken into account when selecting metal starting materials. Furthermore, multiple options for purification and characterization exist, and selection of the most effective procedure partially depends on the selection of starting materials. These subtle differences are often neglected in published protocols. Here, our goal is to demonstrate common methods for metalation, purification, and characterization of lanthanide complexes that can be used as contrast agents for MRI (Figure 1). We expect that this publication will enable biomedical scientists to incorporate lanthanide complexation reactions into their repertoire of commonly used reactions by easing the selection of starting materials and purification methods.
