Selective oxidation of ethylbenzene by dioxygen in the presence of complexes of nickel and cobalt with macrocyclic polyethers

Abstract

The oxidation of alkylarenes by dioxygen in the presence of complexes of nickel and cobalt with macrocyclic ethers 18-crown-6 and 15-crown-5 was studied. The conditions for selective catalytic oxidation of ethylbenzene to ct-phenylethyl hydroperoxide were determined. The kinetics of the accumulation of all oxidation products was studied. The order o( the fom~ation of the products at different stages of chain oxidation was determined. The activity of the complexes at the elementary stages of the chain oxidation of ethytbenzene is discussed.Key words: oxidation, dioxygen, homogeneous catalysis, macrocyclic polyethers, nickel and cobalt complexes, ethylbenzene.The efficiency of oxidation of alkylarenes by dioxygen to the corresponding hydroperoxides in the presence of metal bisacetylacetonates can be substantially increased by using two-component catalytic systems M(acac)2--L (M = Ni n, Coil; L is N-methylpyrrolidone-2, HMPA, DMF, or another electron-donating ligand capable of coordinating to the fifth coordination site of the metal).t-4We have previously established 3 that the mechanism of controlling the catalytic activity of M(acac) 2 by monodentate ligands changes in the course of oxidation. At the initial stages, the activation of dioxygen by the M(acac) 2 9 L complexes and the homolytic decomposition of ROOH under the action of the complexes play the most important role. Later in the process, the selectivity of oxidation increases due to transformation of the complex promoted by the L ligand. This increase in selectivity is achieved by the participation of the transformed form of the complex in chain initiation and by a considerable decrease in the rates of both homolytic and heterolytic decomposition of the hydroperoxides. The mechanism of transformation of the catalyst in the course of oxidation depends on the nature of the metal.[n the case of Ni(acac)2, this transformation involves dioxygen. The limiting stage of the process is the regioselective addition of O, to the u atom of the acac-ligand. This addition is controlled by the L ligand and results in the oxidation of the acetylacetonate ion to the acetate iota. It is assumed that for the Co(acac)2--L system the central metal ion rather than the acaciigand is oxidized and, most likely, free peroxide radicals RO 2' act as tile oxidant. 4When the monodentate L ligand is replaced by macrocyclic polyether 18-crown-6 (18C6), the initial oxidation rate (w ~ and selectivity (S) increase substantially due to the fact that the activity of the "primary" M(acac)2-18C6 complexes in chain initiation is higher than that of M(acac) 2 activated by monodentate ligands and their activity in reactions of branching and propagation of the oxidation chain is lower, sAs in the presence of monodentate ligands, in the course of oxidation, the M(acac) 2 9 18C6 adduct is transformed into a more catalytically active form, and the selectivity of the oxidation increases.In the present work, the oxidation of ethylbenzene in the presence of Ni 11 and CO Il complexes with macrocyclic pol...

Synthesis and characterization of cobalt(iii) complexes containing 2-pyridinecarboxamide ligands and their application in catalytic oxidation of ethylbenzene with dioxygenElectronic supplementary information (ESI) available: full experimental and spectroscopic data for all compounds. See http://www.rsc.org/suppdata/cc/b3/b301963b/

“…Recently there has been an increased interest in the development of clean and economical processes for the selective oxidation of ethylbenzene to higher-value added product acetophenone. [8][9][10][11][12][13] The current industrial production of acetophenone is via the oxidation of ethylbenzene with molecular oxygen using cobalt cycloalkanecarboxylate or cobalt acetate as catalyst in acetic acid solvent. 12 This method suffers from its corrosive and environmentally unfriendly nature.…”

Polymer-anchored vanadium(IV), molybdenum(VI) and copper(II) complexes of bidentate ligand as catalyst for the liquid phase oxidation of organic substrates

“…Oxidation of ethylbenzene has mostly been carried out using molecular oxygen as oxidant and there are only limited reports where TBHP/H 2 O 2 has been used as oxidant [24][25][26][27][28][29][30][31][32][33][34][35]. With mild oxidant usually acetophenone is obtained as major products while strong oxidant gives several oxidized products.…”

“…[22,23]. For Scheme 1. the oxidation of ethylbenzene, homogeneous catalysts such as metal acetyleacetonates [24,25], [Co(2-pyridinecarboxamide) 3 ] [26], metalloporphyrins [27,28], macrocyclic complexes [29] etc. as well as heterogeneous catalysts such as metal complex supported on alumina/silica [30], heteropolyacids [31], M-APO-11 (M = Co, Mn and V) [32], Mn-MCM-41 [33], polymer-anchored complexes [34], zeolite encapsulated metal complexes [35], etc.…”

Selective oxidation of ethylbenzene by dioxygen. The effect of chelate center on catalysis by bicyclic nickel complexes

“…3) and Ni(acac)2-L complexes, w~ ~ [Cat]. 3,7 As follows from the data in Table 2, the w0Ap+MpC rates determined experimentally exceed the calculated values of Wterm. This implies that along with the square chain termination, there is another route of formation of AP and MPC, independent of the decomposition of PEH.…”