Selective oxidation of aromatic compounds with dioxygen and peroxides catalyzed by phthalocyanine supported catalysts

This article summarizes our research in catalytic oxidation on the design and study of supported metallophthalocyanine catalysts. The catalytic properties of these materials were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (Vitamin K-3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl- 1,4-benzoquinone tprecursor of Vitamin E) and in the epoxidation of olefins. Iron tetrasulfophthalocyanine (FePcS) covalently grafted in the dimeric form yielded catalyst more active and selective that those containing monomeric species but suffered from a lack of stability transforming into less selective monomer complexes during catalysis. The stabilization of supported dimer form by covalent link of two adjacent phthalocyanine molecule through appropriate diamine spacer provided more selective and stable catalysts. Trimethyl-1,4-benzoquinone was obtained with 87% yield at 97% conversion of TMP. More demanding oxidation of 2MN afforded 45% yield of VK3. particular emphasis is placed on the mechanistic aspects of these oxidations using two mechanistic probes, 2-methyl-1-phenylpropan-2-yl hydroperoxide (MPPH) to distinguish between homolytic versus heterolytic cleavage of O-O bond during the formation of active species and thianthrene 5-oxide (SSO) to evaluate nucleophilic versus electrophilic character of formed active species. To illustrate a versatility of the phthalocyanine-based supported catalysts we prepared a novel phthalocyanine complex with eight triethoxysylil substituents which can be directly anchored to the silica without any modification of the silica support. This new catalyst shows good catalytic activity in epoxidation of olefins by dioxygen in the presence of isobutyraldehyde. The same catalytic system was also active in the oxidation of phenols to biphenols with 86% yields. This catalytic system is complementary to previous one that selectively oxidizes phenols to quinones. An appropriate choice of the reaction conditions allows selective oxidation either to quinones or to biaryl compounds. (C) 2002 Elsevier Science B.V. All rights reserved.