National Center for Nanoscience and Technology, China

Chiral-metal organic frameworks advance the selective C-C establishment of carbonyl compounds

Data：2022-03-03 | 【 A A A 】 | 【Print】【Close】

Chiral molecules, mainly synthesized from asymmetric catalysis, are valuable in agriculture, medicine, and daily necessity. Therefore, it is highly expected that they can be produced on a large scale from cheap reactants with recyclable catalysts. C-C establishment based on naturally abundant carbonyl compounds is an efficient and widely investigated strategy for significant transformations, but it is still largely unmet in terms of achieving high regio- and stereo- selectivity at the same time.

Recently, Prof. TANG Zhiyong’s team (from the National Center for Nanoscience and Technology) reported the successful regio- and stereo-selective functionalization of a wide range of carbonyls compounds by well-designed chiral metal-organic frameworks (MOFs) catalysts. This study was published in Chem.

A scheme showing that chiral MOFs are tuned in the aspect of light absorption and catalytic center for regio- and stereo-selective functionalization of aldehydes and ketones via three synthesis routes (Imaged by TANG Zhiyong et al.).

Specifically, inspired by the biological catalysts, porous crystalline MOFs assembled from metal ion and organic ligand are selected as a good platform for integrating multifunction. As designed chiral MOFs were prepared from the coordination of Cd²⁺(Zn²⁺) and N-(tert-butyloxycarbonyl)pyrrolidine-S(R)-2-(3,5-bis(4'-(pyridin-4-yl)-[1,1'-biphenyl]-3-yl)-4H-1,2,4-triazol-4-yl) carbamoyl (denoted as S(R)-Boc-L, Boc = tert-butyloxycarbonyl, and L = ligand). They were named as S(R)-Boc-Cd-MOFs and S(R)-Boc-Zn-MOFs, respectively. S-Boc-Cd-MOF, and S-Boc-Zn-MOF in solid-state exhibit broad absorption band in the UV-vis light region centered at 345 nm and 330 nm, respectively, which benefits the generation of carbonyl radicals from aldehydes adsorbed in MOFs under light illumination. Meanwhile, the removal of Boc groups in S(R)-Boc-Cd(Zn)-MOF could introduce one extra catalytic center.

By fully exploiting the properties of S(R)-Boc-Cd(Zn)-MOF, the asymmetric cross-coupling of aldehyde radical and aryl radical was implemented with high performance in both yields and enantiomeric excess (ee) value. The capture of respective aldehyde radical and aryl radical in experiments represents supportive evidence of reaction proceeding in a radical-radical cross-coupling route. In addition, the lifetime of aldehyde and aryl radical is different, dividing them into persistent radical and transient radical, respectively. Furthermore, the Gibbs free energy calculation of the reaction process also supports the cross-coupling route, which is in well accord with the persistent radical effect.

Interestingly, S(R)-Cd(Zn)-MOF obtained from the removal of Boc groups in S(R)-Boc-Cd(Zn)-MOF could switch on efficient transformation of aldehydes and ketones via photoredox activation for asymmetric β-arylation and asymmetric Aldol reaction for α-arylation.

Taken together, by fully taking the advantages of the structure and function of chiral catalysts, chiral MOFs designed and prepared in our study serve as highly efficient recyclable catalysts for regio- and stereo-selective functionalization of carbonyl compounds under mild conditions.

This work was supported by the Strategic Priority Research Program of Chinese Academy of Sciences, National Key Basic Research Program of China, National Natural Science Foundation of China, and Frontier Science Key Project of Chinese Academy of Sciences.

Contact:

TANG Zhiyong

National Center for Nanoscience and Technology

E-mail:zytang@nanoctr.cn