Solvent‐free oxidation of straight‐chain aliphatic primary alcohols by polymer‐grafted vanadium complexes

Abstract

AbstractOxidovanadium(IV) complexes [VO(tertacac)2] (1), [VO(dipd)2] (2), and [VO(phbd)2] (3) were synthesized by reacting [VO(acac)2] with 2,2,6,6‐tetramethyl‐3,5‐hepatanedione, 1,3‐diphenyl‐1,3‐propanedione, and 1‐phenyl‐1,3‐butanedione, respectively. Imidazole‐modified Merrifield resin was used for the heterogenization of complexes 1–3. During the process of heterogenization, the V4+ center in complex 2 converts into V5+, whereas the other two complexes 1 and 3 remain in the oxidovanadium(IV) state in the polymer matrix. Theoretically, calculated IPA values of 1–3 suggest that 2 is prone to oxidation compared with 1 and 3, which was also supported by the absence of EPR lines in 5. Polymer‐supported complexes Ps‐Im‐[VIVO(tertacac)2] (4), Ps‐Im‐[VVO2(dipd)2] (5), and Ps‐Im‐[VIVO(phbd)2] (6) were applied for the solvent‐free heterogenous oxidation of a series of straight‐chain aliphatic alcohols in the presence of H2O2 at 60°C and showed excellent substrate conversion specially for the alcohols with fewer carbon atoms. Higher reaction temperature improves the substrate conversion significantly for the alcohols containing more carbon atoms such as 1‐pentanol, 1‐hexanol, and 1‐heptanol while using optimized reaction conditions. However, alcohols with fewer carbon atoms seem less affected by reaction temperatures higher than the optimized temperature. A decreasing trend in the selectivity(%) of carboxylic acid was observed with increasing carbon atoms among the examined alcohols, whereas the selectivity towards aldehydes increased. The order of efficiency of the supported catalysts is 4 > 6 > 5 in terms of turnover frequency (TOF) values and substrate conversion, further supported by theoretical calculations.