RESUMO
Chiral alcohols are important building blocks in the pharmaceutical and fine chemical industries. The enantioselective reduction of prochiral ketones catalyzed by transition metal complexes, especially asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH), is one of the most efficient and practical methods for producing chiral alcohols. In both academic laboratories and industrial operations, catalysts based on noble metals such as ruthenium, rhodium, and iridium dominated the asymmetric reduction of ketones. However, the limited availability, high price, and toxicity of these critical metals demand their replacement with abundant, nonprecious, and biocommon metals. In this respect, the reactions catalyzed by first-row transition metals, which are more abundant and benign, have attracted more and more attention. As one of the most abundant metals on earth, iron is inexpensive, environmentally benign, and of low toxicity, and as such it is a fascinating alternative to the precious metals for catalysis and sustainable chemical manufacturing. However, iron catalysts have been undeveloped compared to other transition metals. Compared with the examples of iron-catalyzed asymmetric reduction, cobalt- and nickel-catalyzed ATH and AH of ketones are even seldom reported. In early 2004, we reported the first ATH of ketones with catalysts generated in situ from iron cluster complex and chiral PNNP ligand. Since then, we have devoted ourselves to the development of ATH and AH of ketones with iron, cobalt, and nickel catalysts containing novel chiral aminophosphine ligands. In our study, the iron catalyst containing chiral aminophosphine ligands, which are expected to control the stereochemistry at the metal atom, restrict the number of possible diastereoisomers, and effectively transfer chiral information, are successful catalysts for enantioselective reduction of ketones. Among these novel chiral aminophosphine ligands, 22-membered macrocycle P2N4 exhibited extraordinary enantioselectivities when combined with iron(0) cluster Fe3(CO)12. A broad scope of ketones including aromatic, heteroaromatic, and ß-ketoesters can be reduced smoothly with excellent enantioselectivities (up to 99% ee) approaching or exceeding those achievable with the noble metal catalysts. Notably, the chiral iron-based catalyst proved to be highly efficient for both ATH as well as AH of various ketones. Until now, such "universal" catalyst is very rare. Preliminary studies suggest that the AH reaction most likely involved iron particles as the active catalytic species. These research results point to a new direction in developing viable effective nonprecious metal catalysts for asymmetric reduction and probably for other asymmetric catalytic reactions as well.
RESUMO
Sodium tertitanate whisker is a new material for preconcentration. The adsorption properties of sodiium trititanate whisker for Cd(II) were studied and a new method for preconcentration and separation of Cd(II) was proposed. The adsorption rate of Cd(II) by sodium trititanate whisker was 98% at pH 5.0 and Cd(II) could be eluted from sodium trititanate whisker with hydrochloric acid (C: 0.1 mol x L(-1)). The Cd(II) in environmental water was preconcentrated with sodium trititanate whisker and determined by flame atomic absorption spectrometry (FAAS). The detection limit (3sigma, n = 9) was 3.1 ng x L(-1), and the relative standard deviation (RSD) was 2.6%. The response of proposed method is linear in the concentration range of 0.01-1.0 microg x mL(-1) of Cd(II). The method was applied to the determination of analytes in real samples, such as Changjiang River water, Canal water, Yudai River water etc. Good results were obtained (relative standard deviations were 1.5%-4.7%, while recoveries were 98%-102.0%).
