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Catalytic oxidation of organic compounds is an extremely important field of chemistry, spanning the range from biological oxidations to large scale industrial production of commodity chemicals. However, many of these transformations can hardly be classified as organometallic reactions, since the catalysts (often simple metal salts) and the intermediates can be rather regarded as coordination complexes than organometallic compounds. Therefore our discussion will be limited to a few specific examples, despite the fact that oxidations have an inherent connection to aqueous systems after all in many cases (except e.g. epoxidations or hydrogen transfer oxidations) water is produced as byproduct. . | Chapter 8 Catalytic oxidations in aqueous media - recent developments Catalytic oxidation of organic compounds is an extremely important field of chemistry spanning the range from biological oxidations to large scale industrial production of commodity chemicals. However many of these transformations can hardly be classified as organometallic reactions since the catalysts often simple metal salts and the intermediates can be rather regarded as coordination complexes than organometallic compounds. Therefore our discussion will be limited to a few specific examples despite the fact that oxidations have an inherent connection to aqueous systems -after all in many cases except e.g. epoxidations or hydrogen transfer oxidations water is produced as byproduct. Even the truly organometallic activation of hydrocarbons by platinum complexes is excluded from this discussion the simple reason being in that a monumental treatise 1 of this fundamentally important problem has appeared quite recently. Other books and reviews describe the field from the aspects of industry 2 3 basic catalysis research 4 5 6 activation of dioxygen 7 or hydrogen peroxide 8 and from that of organic synthesis 9 - and the list is far from being complete. 8.1 Wacker-type oxidations This is a genuine organometallic reaction in which ethene is oxidized by Pd II to yield acetaldehyde eq. 8.1 3 C2H4 H2O PdCl2 CH3CHO Pd 2 HC1 8.1 Similar oxidations of longer chain olefins provide methyl ketones however the reaction is accompanied by olefin isomerization and 257 258 Chapter 8 subsequent oxidation so usually a rather complex product mixture is formed. Pd is prone to aggregate into palladium black however this can be prevented by reoxidation by CuCl2 eq. 8.2 followed by aerobic oxidation of Cu2Cl2 to CuCl2 in an excess of HC1 eq. 8.3 . With ethene as substrate the overall process is described by eq. 8.4. Pd 2 CuCl2 4 CP PdCl4 2 2 CuCl2 2 CuCl2 2 H 2 O2 2 CuCl2 H2O C2H4 l 2 O2 CH3CHO 8-2 8-3 8-4 The reaction has .
