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. ...
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Chapter 8: Catalytic oxidations in aqueous media - recent developments Chapter 8Catalytic oxidations in aqueous media - recentdevelopments Catalytic oxidation of organic compounds is an extremely important fieldof chemistry, spanning the range from biological oxidations to large scaleindustrial production of commodity chemicals. However, many of thesetransformations can hardly be classified as organometallic reactions, sincethe catalysts (often simple metal salts) and the intermediates can be ratherregarded as coordination complexes than organometallic compounds.Therefore our discussion will be limited to a few specific examples, despitethe fact that oxidations have an inherent connection to aqueous systems -after all in many cases (except e.g. epoxidations or hydrogen transferoxidations) water is produced as byproduct. Even the truly organometallicactivation of hydrocarbons by platinum complexes is excluded from thisdiscussion, the simple reason being in that a monumental treatise [1] of thisfundamentally important problem has appeared quite recently. Other booksand reviews describe the field from the aspects of industry [2,3], basiccatalysis 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 beingcomplete.8.1 Wacker-type oxidations This is a genuine organometallic reaction in which ethene is oxidized byPd(II) to yield acetaldehyde (eq. 8.1) [3]: Similar oxidations of longer chain olefins provide methyl ketones,however, the reaction is accompanied by olefin isomerization and 257258 Chapter 8subsequent oxidation so usually a rather complex product mixture is formed. is prone to aggregate into palladium black, however, this can beprevented by reoxidation by (eq. 8.2) followed by aerobic oxidationof to in an excess of HC1 (eq. 8.3). With ethene assubstrate the overall process is described by eq. 8.4. The reaction has been developed into an industrial process which hasbeen in production for about 40 years now. Although eq. 8.4 does not tellabout it, the process suffers from the need of a highly corrosive reactionmixture containing large amounts of copper chlorides - a rather nastysituation from environmental aspects. In a quest for a more environment-friendly process it has been found thatreaction 8.4 can be catalyzed by Pd(II) complexes of various nitrogen-donorligands (Scheme 8.1) under not too harsh conditions (100 °C, air) withoutthe need of copper chlorides [10, 11]. Of the investigated ligands, sulfonatedbatophenanthroline proved to be the best. Higher olefins, such as 1-hexeneor cyclooctene were similarly transformed by this catalyst. Veryimportantly, there was no isomerization to internal olefins and 2-hexanonewas formed with higher than 99 % selectivity. This outstanding selectivity isprobably due to the absence of acid and Cu-chlorides. In contrast to the usual Wacker-conditions, optimum rates and catalyststability in the Pd/batophenanthroline-catalyzed olefin oxidations wasobserved in the presence of Under such conditions, thecatalyst-containing aqueous phase could be recycled with about 2-3 % lossof activity in each cycle. In the absence of NaOAc precipitation of Pd-blackwas observed after the second and third cycles. Nevertheless, kinetic datarefer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallicpalladium. Poly(ethylene oxide) polymers and poly(ethylene oxide/propylene oxide)copolymers with iminodipropionitrile (139) or iminodiacetonitrile endgroups were used as ligands in the palladium-catalyzed oxidation of higherolefins (1-octene to 1-hexadecene) at 50-70 °C with atmospheric air or 1-3bar In an ethanol/water mixture 88 % yield of 2-hexanone and 92 %yield of 2-hexadecanone was obtained in 4 and 2 h, respectively, with aCatalytic oxidations in aqueous media - recent developments 259substrate/catalyst ratio of 65. The aqueous-alcoholic catalyst solutions couldbe recycled with no loss of activity after phase separation [12]. It is known of the Wacker reaction, that at low chloride concentration(< 1 M) it yields exclusively acetaldehyde. However, atchloroethanol is produced in appreciable quantities. In a detailed kineticstudy it was established, that when a chloride ligand in is replacedby pyridine, the intermediate hydroxyethylpalladium complex is stableenough to undergo reaction with with the formation ofchloroethanol up to a yield of 98 % in 8 M chloride solutions (Scheme 8.2)[13]. With olefins other than ethene two isomeric chlorohydrins can beobtained, one of them being chiral. When pyridine was replaced bymonodentate chiral amines in the enantioselectivitieswere low (8-12%) (Scheme 8.3) [14]. The mononuclearcomplexes performed better providing thechiral chlorohydrin in 46-76% e.e. Even better activities and260 Chapter 8enantioselectivities were achived with the dinuclear, mixed (bisphosphine or diamine) catalysts (Scheme 8.3) whichallowed enantioselective production of chlorohydrins with several olefins.The highest optical purities were 94% e.e. for propene and 93 % e.e. forallylphenyl ether [15]. The reactions can be conducted under mildconditions, although the environmental concerns with regard to the use ofconcentrated solutions still prevail.8.2 Oxidations with and An important trend in oxidations is the use of or in place ofinorganic or org ...