Oxidation of 1-methoxy-4-(1-methylethyl)benzene (1) with oxygen proceeds according to a widely known free-radical chain mechanism [1,2]. The obtained product contains 1-methyl-1-(4-methoxyphenyl)ethyl hydroperoxide (2) as well as 1-(4-methoxyphenyl) ethanone (3) and 2-(4-methoxyphenyl)-2-propanol (4) (Scheme 1) [3,4].The oxidation products of 1 have many applications. 4-Methoxyphenol widely used as a polymerisation inhibitor, e.g. in paints and as dental adhesives [5], antioxidants [6] or depigmenting agents in cosmetics [7] can be obtained by acidic decomposition of hydroperoxide 2 analogous to phenol synthesis from cumene. Ketone 3 is used in pharmaceutical and polymer synthesis and as fragrance components or repellents [8].The processes of 1 oxidation to hydroperoxide have been described in only a few papers [3,4,9,10]. The oxidisability of 1 was determined in the temperature range of 60-110°C and compared with that of cumene [3,4,9,10]. The maximum concentration of hydroperoxide 2 amounting to 40% was obtained when 1 was oxidised without a catalyst at 60°C for 18 h [4].Recently, it has been demonstrated that the use of N-hydroxyphthalimide (NHPI) as a catalyst in isopropylaromatic hydrocarbon oxidation processes significantly increases the hydrocarbon conversion and hydroperoxide yield [11][12][13][14][15][16]. The catalytic activity of NHPI results from phthalimido-N-oxyl radicals (PINO) formation in the propagation step of the oxidation process (Scheme 2). It has been observed experimentally that PINO radicals abstract hydrogen atoms from hydrocarbons much faster than peroxyl radicals. Moreover, PINO radicals terminate much more slowly than peroxyl radicals [17][18][19][20][21].When cumene [22], 2,6-diisopropylnaphthalene [23] or 1,3,5-triisopropylbenzene [24] were oxidised in the presence of NHPI (10 mol%) at 75°C using acetonitrile as the solvent, the respective hydroperoxides were obtained in yields of approximately >70% with selectivities >90%. The reported data on alkoxy derivatives of cumene oxidation have been controversial. The oxidation of 1 in the presence of NHPI, mentioned in literature, gave hydroperoxide 2 in a low yield of 15 mol% [25]. On the other hand oxidation of an analogous compound 4-propyloxycumene gave respective hydroperoxide in a good yield of 80% [26].
Central European Journal of Chemistry
Keywords: Catalytic oxidations • NHPI • 1-Methyl-1-(4-methoxyphenyl)ethyl hydroperoxide • 1-(4-Methoxyphenyl)ethanoneThe oxidation process of 1-methoxy-4-(1-methylethyl)benzene catalysed by N-hydroxyphthalimide (NHPI) or NHPI in combination with Cu(II), Co(II), Mn(II) and Fe(II) salts was studied. The effects of the amount of catalyst and the temperature were determined. 1-Methyl-1-(4-methoxyphenyl)ethyl hydroperoxide was obtained in a yield of 73 mol% when 1-methoxy-4-(1-methylethyl)benzene was oxidised for 3 h at 60°C in acetonitrile as a solvent in the presence of NHPI. 1-(4-Methoxyphenyl)ethanone with high selectivity up to 68 -75 mol%, but low yield amounting to 11 mol% was obtained w...
