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Greener and Sustainable Approach for the Synthesis of Commercially Important Epoxide Building Blocks Using Polymer-Supported Mo(VI) Complexes as Catalysts
Published in Arup K. Sengupta, Ion Exchange and Solvent Extraction, 2017
Misbahu Ladan Mohammed, Basudeb Saha
The conventional epoxidation methods in the fine chemical industries employ either stoichiometric peracids such as peracetic acid and m-chloroperbenzoic acid10 or chlorohydrin11 as oxidizing reagents in liquid-phase batch reactions. However, such processes are not environmentally benign as the former produces an equivalent amount of acid waste, while the latter yields chlorinated by-products and calcium chloride waste. In recent years, more and more attention has been focused on developing greener and more efficient epoxidation processes, employing environmentally benign oxidants such as tert-butyl hydroperoxide (TBHP) since it is atom efficient and safer to handle.12,13 A notable industrial implementation of alkene epoxidation with TBHP was the Halcon process described by Kollar,14 which employs homogeneous molybdenum (VI) as a catalyst for liquid-phase epoxidation of propylene to propylene oxide (PO). However, homogeneous-catalyzed epoxidation processes are not economically viable for industrial applications due to major requirements in terms of work-up, product isolation, and purification procedures. Therefore, researchers have focused on developing stable heterogeneous catalysts for epoxidation by immobilization of catalytically active metal species on organic or inorganic materials.15–17 Polymers have gained attention as suitable support for transition metal catalysts as they are inert, nontoxic, insoluble, and often recyclable.18 A number of polymer–supported molybdenum complexes have been prepared and used as catalysts for batch alkene epoxidation with TBHP as an oxidant and have shown good catalytic activity and product selectivity.19–31 However, despite numerous published works on polymer-supported Mo(VI) catalyzed alkene epoxidation with TBHP, there appears to have been no significant efforts to move the chemistry on from a small-scale laboratory batch reaction to a continuous flow process.
Review: Recent advances of one-dimensional coordination polymers as catalysts
Published in Journal of Coordination Chemistry, 2018
Edward Loukopoulos, George E. Kostakis
Additional examples of catalysts based on Schiff base ligands have been reported. Very recently Hazra [71] and co-workers utilized 2-[(2-hydroxy-3-methoxyphenyl)methylideneamino]benzenesulfonic acid (H2hmb), another sulfonated Schiff base ligand in order to generate Cu(II) compounds. Compared to the previously mentioned H3dmb, which derived from 2,3-dihydroxybenzaldehyde, in H2hmb o-vanillin is employed as the aldehyde component. Among other compounds, the authors report the synthesis of a 1-D framework when Cu(II) acetate and solvothermal conditions are used along with the secondary linker 4,4′-bipyridine (bipy). The resulting compound, [Cu2(hmb)2(bipy)]·nH2O·nDMF (9), contains dicopper Cu2(hmb)2 units which are linked to each other via bipy molecules, forming a 1-D hacksaw chain (Figure 8). Regarding its properties, 9 was investigated for its homogeneous activity toward the oxidation of primary and secondary alcohols to aldehydes and ketones; cyclohexanol, benzyl alcohol and 1-phenyl ethanol were tested as substrates (Scheme 6). The reaction also involves the use of microwave irradiation, aqueous tert-butyl hydroperoxide (TBHP) as oxidizing agent and 0.2 mol% of 9 at 60 °C and solvent-free medium. Corresponding products were afforded at moderate to excellent (49–90%) yields, much higher to the respective yields afforded when Cu(II) acetate is used as the catalyst.