Remarkable Lewis acid catalytic performance of the scandium trimesate metal organic framework MIL-100(Sc) for C-C and C=N bond-forming reactions

The porous metal organic frameworks scandium trimesate MIL-100(Sc), scandium terephthalates MIL-101(Sc), MIL-88B(Sc) and MIL-68(Sc), scandium 4,4'-biphenyl-dicarboxylate MIL-88D(Sc) and the scandium 3,3',5,5-azobenzene- tetracarboxylate socMOF(Sc) have been compared as Lewis acid catalysts against Sc3+-exchanged zeolite Beta, MIL-100(Cr), MIL-101(Cr), MIL-100(Fe) and the divalent MOFs HKUST-1(Cu), CPO-27(Ni) and STA-12(Ni), each of which can be prepared with coordinatively unsaturated metal sites. The performance of these MOFs has been investigated in several Lewis acid-catalysed reactions that are of importance in organic synthesis but have rarely been studied using MOF catalysts. These reactions were (i) the intermolecular carbonyl ene reaction of nucleophilic alkenes and electron-poor aldehydes, (ii) a Friedel-Crafts type Michael addition between electron-rich heterocycles and electron-deficient alkenes and (iii) ketimine and aldimine formation. In each of these, MIL-100(Sc) is both active and selective and significantly outperforms the other catalysts. Filtration and recycle tests indicate that catalysis over MIL-100(Sc) is heterogeneous. The study of Michael addition reactions carried out over scandium-bearing MOFs with different window sizes on indole-based substrates of varying molecular dimensions indicates that most of the catalysis that involves molecules small enough to enter the pores occurs within the internal pore space. These results indicate MIL-100(Sc) is an exceptional Lewis acidic MOF catalyst, and suggest that MIL-100(Sc) and new derivatives of it could find application as recyclable solid catalysts in synthetic chemistry.