Cyclophane-type imidazolium salts with planar chirality as a new class of N-heterocyclic carbene precursors |
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Authors: | Matsuoka Yuki Ishida Yasuhiro Sasaki Daisuke Saigo Kazuhiko |
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Institution: | Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. |
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Abstract: | Cyclophane-type imidazolium salts with planar chirality were synthesized from enantiopure 2-amino alcohols, of which the N(1) and N(3) positions were connected with a bridge. The structural profiles of the imidazolium salts and their derivative N-heterocyclic carbenes (NHCs) were investigated by means of several analyses. The chiral NHCs derived from these imidazolium salts were found to catalyze the asymmetric cross-annulation of an alpha,beta-unsaturated aldehyde with a ketone by means of the "conjugated" umpolung of the enal to give the target gamma-lactone with good to excellent enantioselectivity (up to 94% ee). Based on the expected structure of the NHCs and their intermediates, together with the absolute configuration of the products, a plausible mechanism for the stereocontrol was proposed. |
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Keywords: | carbenes chirality cyclophanes heterocycles umpolung |
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