PL1
Microporous materials absorbing the mechanisms of homogeneous catalysis for C-H functionalisation of arene compounds Dirk De Vos KU Leuven, Belgium. Email: dirk.devos@kuleuven.be
In the search for safe, atom-economic reactions that fit in synthetic routes with improved step economy, microporous materials, and in particular zeolites and MOFs can play a key role. The pore environments of MOFs and zeolites can control the redox chemistry of embedded transition metals; in appropriate coordination environments, stability and TON of the catalysts can be highly improved, and unexpected shape selectivities induced. The lecture will illustrate these general ideas with particular focus on catalytic centres that can activate C-H sp2 bonds. Classically, in homogeneous catalysis, arenes are functionalized using cross-coupling reactions of the Heck or Suzuki type, requiring prefunctionalized, e.g. halogenated reactants. In a more atom-economic approach, the metal centre can directly activate a C-H bond, but this requires that an oxidant is used in the overall reaction, as illustrated for the Pd-catalyzed Fujiwara alkenylation of arenes with olefins to form styrenes. We will highlight several designs of porous catalysts which bring significant benefits to such C-H activating reactions, beyond merely providing immobilization. Starting from the wellknown MOF-808, we docked a S-containing carboxylic ligand to the Zr6 clusters in the structure. This provides an excellent environment for Pd2+/Pd0 to affect the Fujiwara reaction, giving direct access to alkenylated arenes.1 For synthesizing biaryl motives, Pdzeolites stand out. We proved that in the pores of zeolite Beta, toluene is homocoupled to produce with high selectivity the p,p’-bitolyl isomer, out of 6 possible isomers.2 These reactions are now extended to selective heterocouplings, e.g. of phenyl rings and heteroaromatics. In both cases, the involved active species were studied in detail, with a combination of XAS, NMR and theoretical calculations.
Using similar designs of zeolite-entrapped transition metals, we also present (i) a Rh catalyst for the selective carboxylation of indoles using CO and O2, and (i) a Ru catalyst for the photocatalytic trifluoromethylation of arenes. In the final section, we will reveal an unexpected role of zeolites as equilibrium shifting agents in the transfer hydrocyanation, allowing a much safer introduction of HCN than when using HCN. References [1] Van Velthoven, N et. al. ACS Catal. 2020, 10, 5077–5085. [2] Vercammen, J. et al., Nature Catalysis 2020, 3, 1002–1009.
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