Journal
JOURNAL OF CATALYSIS
Volume 377, Issue -, Pages 41-50Publisher
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jcat.2019.07.003
Keywords
Metal-organic framework; Zirconium; Syn-selective aldol reaction; Chirality; L-proline
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Funding
- Vietnamese Government
- DFG [BU-911-27-1]
- Alexander von Humboldt Foundation
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A chiral zirconium-based catalyst, DUT-67-Pro containing 8-connected Zr-6-clusters is obtained by post synthetic functionalization of Zr6O6(OH)(2)(TDC)(4)(HCOO)(2) (DUT-67, TDC = 2,5-thiophenedicarboxylate) with the chiral monocarboxylic acid, L-proline. C-13 and N-15 solid state MAS and DNP NMR studies of DUT-67-Pro confirm the integration of L-proline into the porous framework. The chiral MOF catalyst exhibits an excellent catalytic activity at low temperature (298 K) with an unprecedented syn-(S,S)-product selectivity in an asymmetric aldol addition reaction of cyclohexanone to 4-nitrobenzaldehyde (yield = 95%, ee = 96%). Comparative catalytic studies using a molecular Zr-6-cluster model compound indicate the Zr-6-moiety to be responsible for this inverse diastereoselectivity compared to well-established L-proline organocatalysis and a mechanism is proposed to explain the Zr-6-cluster-mediated syn-selectivity. Masking residual acidic active sites in the cluster of the framework was found to be a key prerequisite to achieve the high enantioselectivity. The purely heterogeneous catalytic system based on DUT-67-Pro is highly stable and can be recycled several times. (C) 2019 Published by Elsevier Inc.
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