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At Cardiff University, in Wales, the research group of chemistry professor Graham J. Hutchings also has immobilized homogeneous chiral catalysts on the interior walls of pores and demonstrated that confining the catalysts improves enantioselectivity in aziridination reactions. Aziridines are three-membered heterocycles that contain nitrogen. Hutchings points out that chiral aziridines are important intermediates to drug precursors because they contain an asymmetric carbon center bonded to nitrogen. To carry out aziridination of alkenes, the Cardiff group used electrostatic immobilization techniques to modify zeolite HY with copper-bis(oxazoline) complexes pioneered by Harvard University chemistry professor David A. Evans. Then they compared the solution-phase and surface-bound forms of the catalysts and found that the heterogeneous version provides greater enantioselectivity than its homogeneous analog. Further investigation of the Cu-bis(oxazoline) catalysts led to a curious result: The Cardiff chemists, including graduate students John Gullick, Sophia Taylor, and others, and their coworkers at other institutions, were surprised to find that enantioselectivity in the aziridination reaction increases with conversion of the alkene. The result was observed in homogeneous and heterogeneous reactions. The group determined that the unusual behavior is caused by the reaction product--aziridines--and sulfonamide by-products reacting with the catalyst's active center [Chem. Commun., 2003, 2808]. "The work shows that the catalyst's active site is much more complex than was originally envisaged," Hutchings remarks. "More important, we see that the aziridine product is not necessarily stable under reaction conditions and can be reacted to form the opposite enantiomer," he adds. In related work, Hutchings and coworkers Neil A. Caplan, Frederick E. Hancock, and Philip C. Bulman Page recently demonstrated that catalysts consisting of CuH zeolite Y modified with bis(oxazoline) are very effective in facilitating asymmetric reactions of carbonyl and imino-ene compounds. The group points out that the catalysts can be recovered readily and reused without degrading the catalytic performance. The work will be published in a forthcoming issue of Angewandte Chemie International Edition. |
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