Proline Functionalization of the Mesoporous Metal−Organic Framework DUT-32

The linker functionalization strategy was applied to incorporate proline moieties into a metal–organic framework (MOF). When 4,4′-biphenyldicarboxylic acid was replaced with a Boc-protected proline-functionalized linker (H₂<b>L</b>) in the synthesis of DUT-32 (DUT = Dresden University of Technology), a highly porous enantiomerically pure MOF (DUT-32-NHProBoc) was obtained, as could be confirmed by enantioselective high-performance liquid chromatography (HPLC) measurements and solid-state NMR experiments. Isotope labeling of the chiral side group proline enabled highly sensitive one- and two-dimensional solid-state ¹³C NMR experiments. For samples loaded with (<i>S</i>)-1-phenyl-2,2,2-trifluoroethanol [(<i>S</i>)-TFPE], the proline groups are shown to exhibit a lower mobility than that for (<i>R</i>)-TFPE-loaded samples. This indicates a preferred interaction of the shift agent (<i>S</i>)-TFPE with the chiral moieties. The high porosity of the compound is reflected by an exceptionally high ethyl cinnamate adsorption capacity. However, postsynthetic thermal deprotection of Boc−proline in the MOF leads to racemization of the chiral center, which was verified by stereoselective HPLC experiments and asymmetric catalysis of aldol addition