Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany.
J Am Chem Soc. 2011 Sep 14;133(36):14349-58. doi: 10.1021/ja2034377. Epub 2011 Aug 18.
Selective hydrogenation of biogenic carboxylic acids is an important transformation for biorefinery concepts based on platform chemicals. We herein report a mechanistic study on the homogeneously ruthenium/phosphine catalyzed transformations of levulinic acid (LA) and itaconic acid (IA) to the corresponding lactones, diols, and cyclic ethers. A density functional theory (DFT) study was performed and corroborated with experimental data from catalytic processes and NMR investigations. For Ru(TriPhos)H as the catalytically active unit, a common mechanistic pathway for the reduction of the C═O functionality in aldehydes, ketones, lactones, and even free carboxylic acids could be identified. Hydride transfer from the Ru-H group to the carbonyl or carboxyl carbon is followed by protonation of the resulting Ru-O unit via σ-bond metathesis from a coordinated dihydrogen molecule. The energetic spans for the reduction of the different functional groups increase in the order aldehyde < ketone < lactone ≈ carboxylic acid. This reactivity pattern as well as the absolute values are in full agreement with experimentally observed activities and selectivities, forming a rational basis for further catalyst development.
生物源羧酸的选择性加氢是基于平台化学品的生物炼制概念的重要转化。本文报道了均相钌/膦催化的生物源羧酸（如乙酰丙酸（LA）和衣康酸（IA））转化为相应内酯、二醇和环醚的机理研究。通过密度泛函理论（DFT）研究并结合催化过程和 NMR 研究的实验数据进行了验证。对于作为催化活性单元的[Ru（TriPhos）H]（+），可以确定醛、酮、内酯甚至游离羧酸中 C═O 官能团还原的常见机理途径。Ru-H 基团向羰基或羧基碳的氢化物转移，随后通过来自配位氢气分子的σ键复分解，对生成的 Ru-O 单元进行质子化。不同官能团还原的能量跨度按醛＜酮＜内酯≈羧酸的顺序增加。这种反应性模式以及绝对值与实验观察到的活性和选择性完全一致，为进一步的催化剂开发提供了合理的基础。
