Cationic NHC-Phosphine Iridium Complexes: Highly Active Catalysts for Base-Free Hydrogenation of Ketones |
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Authors: | Dr Xu Quan Dr Sutthichat Kerdphon Bram B C Peters Dr Janjira Rujirawanich Dr Suppachai Krajangsri Dr Jira Jongcharoenkamol Prof Pher G Andersson |
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Institution: | 1. Department of Organic Chemistry, Stockholm University, Stockholm, 10691 Sweden
These author contributed equally to this work.;2. Department of Organic Chemistry, Stockholm University, Stockholm, 10691 Sweden |
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Abstract: | Novel bidentate N-heterocyclic carbene-phosphine iridium complexes have been synthesized and evaluated in the hydrogenation of ketones. Reported catalytic systems require base additives and, if excluded, need elevated temperature or high pressure of hydrogen gas to achieve satisfactory reactivity. The developed catalysts showed extremely high reactivity and good enantioselectivity under base-free and mild conditions. In the presence of 1 mol % catalyst under 1 bar hydrogen pressure at room temperature, hydrogenation was complete in 30 minutes giving up to 96 % ee. Again, this high reactivity was achieved in additive-free conditions. Mechanistic experiments demonstrated that balloon pressure of hydrogen was sufficient to form the activate species by reducing and eliminating the 1,5-cyclooctadiene ligand. The pre-activated catalyst was able to hydrogenate acetophenone with 89 % conversion in 5 min. |
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Keywords: | asymmetric hydrogenation iridium catalysis ketones ligand development N-heterocyclic carbene |
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