Propargylic substitution reactions with various nucleophilic compounds using efficient and recyclable mesoporous silica spheres embedded with FeCo/graphitic shell nanocrystals

Abstract

Phosphomolybdic acid (PMA, H3PMo12O40) functioned as a catalyst for reactions of secondary propargylic alcohols and nucleophiles. Highly stable and magnetically recyclable mesoporous silica spheres (MMS) embedded with FeCo-graphitic carbon shell nanocrystals (FeCo/GC@MSS) were fabricated by a modified Stöber process and chemical vapor deposition (CVD) method. The FeCo/GC@MSS were loaded with phosphomolybdic acid (PMA@FeCo/GC@MSS), and their catalytic activity was investigated. Propargylic reactions of 1,3-diphenyl-2-propyn-1-ol with a wide range of nucleophiles bearing activating substituents were catalyzed under mild conditions. It was found that the MMS possess mesoporosities and have enough inner space to load FeCo and phosphomolybdic acid. The FeCo/GC@MSS were found to be chemically stable against acid etching and oxidation. This suggests that the nanocrystals can be used as a support for an acid catalyst. Moreover, the magnetic property of the nanocrystals enabled the facile separation of catalysts from the products.

Keywords: FeCo/GC; Magnetic; Phosphomolybdic acid; Propargylic substitution; Recyclable.

Figure 1

Schematic diagram for the preparation of a PMA@FeCo/GC@MSS.

Figure 2

Morphology and structure of FeCo/GC@MSS and PMA@FeCo/GC@MSS. TEM images of (a) FeCo/GC@MSS and (b) PMA@FeCo/GC@MSS (Insets are higher magnification images.). (c) TEM image of FeCo/GC nanocrystals (Upper inset is the electron diffraction pattern. Lower inset is the EDX spectrum. Copper is from the TEM grids.) (d) X-ray diffraction patterns.

Figure 3

Suitability of FeCo/GC@MSS for use in the reaction system. (a) Field-dependent magnetization hysteresis of FeCo/GC@MSS at 300 K. (b) Nitrogen adsorption/desorption isotherm of MSS and FeCo/GC@MSS. (c) Photographs of 35% HCl solutions of (i, ii) FeCo/GC@MSS stored over a monitoring period of 2 months in air (i) and water (ii) and (iii) as-prepared FeCo@MSS. (d) A photograph of recycled PMA@FeCo/GC@MSS in acetonitrile in the presence of an external magnet. (e) TEM image of the PMA@FeCo/GC@MSS after the five sequential catalytic cycles.

Figure 4

Plausible mechanism for the PMA@FeCo/GC@MSS-catalyzed propargylic substitution reactions. (a) Protonation of hydroxyl group of propargylic alcohol. (b) Generation of propargylic carbenium ion by dehydration. (c) Donation of electron from electron-rich arene (d) removal of proton from the previous intermediate.