Mechanistic insight into the [4 + 2] Diels-Alder cycloaddition over first row d-block cation-exchanged faujasites

Roderigh Y. Rohling, Ionut C. Tranca, Emiel J.M. Hensen, Evgeny A. Pidko

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Abstract

The Diels-Alder cycloaddition (DAC) is a powerful tool to construct C-C bonds. The DAC reaction can be accelerated in several ways, one of which is reactant confinement as observed in supramolecular complexes and Diels-Alderases. Another method is altering the frontier molecular orbitals (FMOs) of the reactants by using homogeneous transition-metal complexes whose active sites exhibit d-orbitals suitable for net-bonding orbital interactions with the substrates. Both features can be combined in first row d-block (TM) exchanged faujasite catalysts where the zeolite framework acts as a stabilizing ligand for the active site while confining the reactants. Herein, we report on a mechanistic and periodic DFT study on TM-(Cu(I), Cu(II), Zn(II), Ni(II), Cr(III), Sc(III), V(V))exchanged faujasites to elucidate the effect of d-shell filling on the DAC reaction between 2,5-dimethylfuran and ethylene. Two pathways were found: one being the concerted one-step and the other being the stepwise two-step pathway. A decrease in d-shell filling results in a concomitant increase in reactant activation as evidenced by increasingly narrow energy gaps and lower activation barriers. For models holding relatively small d-block cations, the zeolite framework was found to bias the DAC reaction toward an asynchronous one-step pathway instead of the two-step pathway. This work is an example of how the active site properties and the surrounding chemical environment influence the reaction mechanism of chemical transformations. ©

Original languageEnglish
Pages (from-to)376-391
Number of pages16
JournalACS Catalysis
Volume9
Issue number1
DOIs
Publication statusPublished - 4 Jan 2019

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Keywords

  • 2,5-dimethylfuran
  • DFT calculations
  • ethylene
  • property-activity relations
  • zeolite catalysis

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