Catalysis of a Diels-Alder Reaction between Azachalcones and Cyclopentadiene by a Recyclable Copper(II)-PEIP Metal-Organic Framework

Abstract

Metal-organic frameworks (MOFs) have attracted considerable interest as emerging heterogeneous catalysts for organic transformations of synthetic utility. Herein, a Lewis-acidic MOF, {[Cu₃(PEIP)₂(5-NH₂-mBDC)(DMF)]·7DMF}∞, denoted as Cu(ΙΙ)-PEIP, has been synthesized via a one-pot process and deployed as an efficient heterogeneous catalyst for a Diels-Alder cycloaddition. Specifically, the [4 + 2] cycloaddition of 13 substituted azachalcone dienophiles with cyclopentadiene has been investigated. MOF-catalyzed reaction conditions were optimized, leading to the selection of water as the solvent, in the presence of 10% mol sodium dodecyl sulfate (SDS) to address substrate solubility. The Cu(II)-PEIP catalyst showed excellent activity under these green and mild conditions, exhibiting comparable or, in some cases, superior efficiency to a homogeneous catalyst often employed in Diels-Alder reactions, namely, Cu(OTf)₂. The nature of the azachalcone substituent played a significant role in the reactivity of the dienophiles, with electron-withdrawing (EW) substituents enhancing conversion and electron-donating (ED) ones exhibiting the opposite effect. Coordinating substituents appeared to enhance the endo selectivity. Importantly, the Cu(II)-PEIP catalyst can be readily isolated from the reaction mixture and recycled up to four times without any significant reduction in conversion or selectivity.

Keywords: Diels–Alder cycloaddition; MOF; green solvent; heterogeneous catalysis; sustainability.

Figure 1

Structures of constituent components and in situ generated ligand (PEIPH₂) employed in the synthesis of Cu(II)-PEIP MOF.

Scheme 1

Model Diels–Alder cycloaddition employed for the evaluation of various solvent systems.

Figure 2

(a) Representation of the 3D Cu(II)-PEIP MOF material along the a axis. (b,c) Coordination modes of 5-NH₂-mBDC²⁻ and PEIP²⁻ ligands, respectively. (d) Dinuclear paddle-wheel secondary building unit [Cu₂(RCOO)₄(DMF)₂], believed to be the catalytic site. [H atoms are omitted for clarity].

Scheme 2

Parallel Diels–Alder reactions of azachalcone dienophiles 1a–m (1 equiv.) with diene 2 (10 equiv.), catalyzed by Cu(II)-PEIP MOF. A representative set of electron-deficient and electron-rich dienophiles were included to assess the scope of the MOF-catalyzed D-A (data in red). Control reactions were performed in solution, with homogeneous catalyst Cu(OTf)₂ for comparison (data in blue). Inset: Structures of obtained products (major = endo D-A adduct, racemic), reaction conversions, and endo:exo molar ratios, as determined by ¹H NMR. The Cu(II)-PEIP affords excellent conversions (~99%) for most substrates and remarkable endo selectivities, rivaling those of the homogeneous catalyst.

Figure 3

Comparison of the powder X-ray diffraction (PXRD) pattern of the pristine Cu(II)-PEIP catalyst (red) with the simulated from single crystal X-ray diffraction data pattern, and with the ones from each of the first four D-A reaction cycles.