Two new catalytic systems for hydrogen‐atom transfer (HAT) catalysis involving the N−H bonds of titanocene(III) complexes with pendant amide ligands are reported. In a monometallic system, a bifunctional catalyst for radical generation and reduction through HAT catalysis depending on the coordination of the amide ligand is employed. The pendant amide ligand is used to activate Crabtree's catalyst to
Reagent-Controlled Stereoselectivity in Titanocene-Catalyzed Epoxide Openings: Reductions and Intermolecular Additions to ,-Unsaturated Carbonyl Compounds
The generation and addition reactions of metal bound radicals derived from normal and meso epoxides by electrontransferfromtitanocene(III) reagents is described. The control of enantioselectivity and diastereoselectivity of these transformations is investigated by variation of the ligands of the metal complex. The reaction can lead to unprecedented and highly selective reactions, in which synthetically
Sustainable Radical Reduction through Catalytic Hydrogen Atom Transfer
作者:Andreas Gansäuer、Chun-An Fan、Frederik Piestert
DOI:10.1021/ja801232t
日期:2008.6.1
A system with coupled catalytic cycles is described that allows radicalreduction by hydrogenatom abstraction from rhodium hydrides. These intermediates are generated from H2 activation by Wilkinson's catalyst. Radical generation is carried out by titanocene-catalyzed electron transfer to epoxides.
A rationally designed titanium(III) catalyst allows the opening of epoxides with high enantioselectivity. This reaction [Eq. (1)] constitutes the first example of an enantioselective transitionmetalcatalyzedradicalreaction that proceeds by electrontransfer.
A comparison or different titanium catalysts in the enantioselective opening of meso-epoxides has been carried out. The best catalyst is readily available in both enantiomers from menthol and allows for a highly enantioselective reaction.