Abstract:
Micellar systems are known to catalyze various organic transformations under aqueous conditions.1 However, micelle-forming metal complexes are relatively less explored for such transformations.2 We are interested in developing surfactant-based oxomolybdenum complexes as mild catalysts for selective oxidation reactions. Earlier in our laboratory, a few molybdenum-based ionic and covalent metallomicellar complexes were prepared where coordination entity and hydrophobic long chain are either held by electrostatic interaction or attached by covalent bond. The covalently bonded metallomicellar systems were found to be more effective as catalysts.3,4 In this line, we have reproduced a single chain containing covalent catalyst,4 [Mo(O)2(L1)], (Mo1) and applied that for oxidation of activated alcohols, oxidative cleavage of lignin mimics and oxidative depolymerization of real lignin to their corresponding carboxylic acids in water using open air as oxidant under basic conditions (pH~12). Further, to enhance the catalytic ability, a double chain containing covalent catalyst [Mo(O)2(L2)], (Mo2) was synthesized and it was found that the new catalyst could oxidatively cleave lignin mimics to corresponding acids even in the absence of base. The optimized condition was further utilized for the oxidative depolymerization of real lignin. With the anticipation of better reactivity, we synthesized another new catalyst [Mo(O)2(L3)], (Mo3) by modifying Mo2 where an electron-withdrawing polar group like nitro is attached to ligand to have it near the coordination site of metal center. Thus, the metal center became electron deficient, resulting in its better interaction with electron-rich substrates. The complex Mo3 was found to be more reactive compared to Mo2 for the oxidative cleavage of lignin model compounds. The catalyst was further utilized for direct conversion of lignin model compounds to the corresponding imidazoles or oxazoles, in the presence of diamine or aminophenol. Earlier in our laboratory, the oxalate and picolinate ligated surfactant-based ionic oxodiperoxo-molybdenum complexes were used for the oxidation of sulfides into sulfoxides/sulfones in aqueous medium using H2O2 as oxidant.3,4 Our present aim is to carry out the sulfoxidation reactions under oxygen environment. In this line, we reproduced the molybdenum complex (CTA)[MoO(O2)2(L4)], (Mo4) where picolinic acid-N-oxide is the ligand moiety. In view of N-oxide coordination, the catalyst Mo4 is expected to show better reactivity for oxidation4 as compared to oxalate and picolinate ligated molybdenum complexes. Utilizing catalyst Mo4, the selective oxidation of sulfides into corresponding sulfoxides/sulfones in an aqueous medium and oxidative desulfurization of model diesel were achieved using oxygen as oxidant5 instead of H2O2. The chemoselectivity, recyclability, and multi-gram scale preparation for some of the above-mentioned reactions were tested, and the plausible mechanisms have been proposed with the help of a few experimental evidences and preliminary DFT calculations.
References:
1. Sorella, G. L.; Strukul, G.; Scarso, A. Green Chem. 2015, 17, 644.
2. Zhang, J.; Meng, X. -G.; Zeng, X. -C.; Yu, X. -Q. Coord. Chem. Rev. 2009, 253, 2166.
3. Chakravarthy, R. D. Sulfoxidation Reactions Catalyzed by Mononeuclear Oxomolybdenum Complexes, Ph.D. Thesis, 2014.
4. Thiruvengetam, P. Molybdenum Based Metallomicellar Catalysts for Conversion of Feedstock Materials to Platform Chemicals in Aqueous Medium, Ph.D. Thesis, 2022.
5. Sunani, P.; Thiruvengetam, P.; Chand, D. K. A metallomicellar catalyst for sulfoxidation reactions in aqueous medium and desulfurization of model diesel under aerobic conditions (Indian Patent Appl. No. 202341024825).