Selective Aerobic Oxidation of P-Methoxytoluene by Co(II)-Promoted NHPI Incorporated into Cross-Linked Copolymer Structure

Abstract

A wide series of copolymer materials with various contents of 4-vinyl-diisopropyl-phtalate ester (10–90 mol%), divinylbenzene (1–11 mol%) and styrene, as monomers, were obtained by radical copolymerization. In the last steps of the synthesis, diisopropyl ester functionalities were converted into the form of N-hydroxyphthalimide (NHPI) rings. The obtained materials with the NHPI groups immobilized in the copolymer structure were studied by various physicochemical techniques, including FT-IR, UV-Vis-DR, XPS, elemental analysis, and tested as catalysts in aerobic oxidation of p-methoxytoluene in the presence of Co(II) acetate co-catalyst. Conversion of the aromatic substrate was correlated with the NHPI content and cross-linking degree. The best catalytic performance (conversions higher than 23%) was achieved for the copolymer catalysts containing 60% and 30% of 4-vinyl-diisopropyl-phtalate ester. At too high concentrations of NHPI and DVB, some of the NHPI groups were transformed into inactive (C=O)-N=O species or not available due to embedding inside the copolymer structure. The mechanism of the process involving both NHPI centers, forming phthalimide N-oxyl (PINO) radicals, and Co(II) cations was discussed. Stability of the developed catalysts was also tested. The opening of imide rings took place during the catalytic process, resulting in the formation of carboxyl groups and the release of hydroxylamine molecules. The deactivated catalyst could be easily regenerated by repeating two last steps of closing imide ring.

Covalent bonding of N‐hydroxyphthalimide on mesoporous silica for catalytic aerobic oxidation of p‐xylene at atmospheric pressure

“…In the range of 3600–3100 cm −1 , the bands corresponding to the −OH stretching vibrations coincide with the bands assigned to the vibrations of hydroxyls associated with the silica surface, which make their unambiguous identification impossible. It should also be noticed that the intensity of the bands originating from the outgoing asymmetric C(sp 3 )−H stretching isopropyl group vibrations decreased significantly [21a,18a] . The next modification step to the form of phthalic anhydride (A_SBA‐15_VTS_VPAn and B_SBA‐15_VTS_VPAn) resulted in the appearance of new bands at 1857 and 1780 cm −1 related to the C=O stretching vibrations [23] .…”

“…The catalytic activity of the developed NHPI‐immobilized catalysts was tested in the model reaction of p ‐xylene oxidation under oxygen atmosphere without the addition of solvent, only in the presence of the substrate, a Co 2+ salt (Co(OAc) 2 ⋅ 4H 2 O) acting as a co‐catalyst and azobisisobutyronitrile (AIBN) as an initiator [18a] . The reaction was carried out at temperatures not exceeding 80 °C, due to an enhanced probability of NHPI deactivation by the generation of PINO radicals and the formation of multi‐ring structures [6b,28] .…”

“…BrDPE was synthesized by the procedure described in ref. [18a]. The reaction was carried out at 100 °C for 24 h. The resulting solid (A_SBA‐15_VTS_VDPE) was filtered and washed with EtOH, dioxane, THF, EtOAc, cyclohexane, CH 2 Cl 2 , Et 2 O, and dried.…”

“…As a consequence, the most commonly used amide or ester bonds are easily hydrolysed, significantly reducing the possibility of using NHPI‐immobilized catalysts in practice. In recent papers, we described a novel approach to permanent anchoring NHPI groups in the poly(styrene‐ co ‐divinylbenzene) structure by copolymerization with 4‐vinyl‐diisopropyl‐phtalate ester (VDPE) monomer, followed by subsequent activation [18a,b] . Unfortunately, the resulting polymer structures showed low thermal stability and required a prolonged induction time needed to swell and open an access to the catalytically active NHPI groups located between the polymer chains.…”