Groups VII, VIII and IB as Catalysts

It has long been known that metal salts and complexes promote the reaction of olefins with oxygen in the liquid phase. Early work ([429] and references cited therein) established that during olefin oxidation in the presence of various copper, cobalt and manganese salts, free radicals arise via decomposition of a catalyst-hydroperoxide complex formed from allylic hydroperoxide generated in situ. Although the metal modifies the nature of the observed products in many cases, most homogeneous metal-catalyzed oxidations exhibit characteristics of free radical initiated autoxidations. 

The oxidation of cyclohexene in the presence of copper, cobalt and manganese carboxylates has continued to receive attention in recent years [430-441]. The stable monomeric products of reaction are largely 2-cyclohexene-l-one and 2-cyclo-hexene-l-ol with smaller amounts of cyclohexene oxide. Cyclohexenyl hydroperoxide formed by attack of dioxygen on the allylic radical produced by allylic-hydrogen abstraction, has been established to be the reaction intermediate. The product profile has been found to vary somewhat with the metal complex used. It was found [431] that with Co(II) or Mn(II) carboxylates reaction rate and selectivity to 2-cyclohexene-l-one were maximal at 46 °C. 

Cyclohexene oxidations in the presence of a variety of acetylacetonates [442] were found to be free radical chain reactions having the same homogeneous propagation steps and yielding as the principle primary product, cyclohexenyl hydroperoxide. The metal catalyzed decomposition of the primary product appeared to give rise to varying amounts of the principle stable monomeric products of oxidation 2-cyclohexene-l-one, 2-cyclohexene-l-ol and cyclohexene oxide. 

The use of cobalt and manganese carboxylates to initiate the oxidation of a large number of olefins such as the butenes [447, 448], propylene [449], oleic [450] linoleic [451], and stearic [452, 453] acids or their derivatives and a-methylstyrene [454, 455] is well known. The kinetics of oxidation of a-methylstyrene in the presence of cobaltous and manganous acetylacetonates as well as copper phthalocy-anine have been investigated [454, 455]. The results of this study led Kamiya to postulate a mechanism involving formation of radical species by a metal dioxygen complex, equation (270), concurrent with radical generation by hydroperoxide decomposition. 

Although the predominant products of oxidations of olefins in the presence of cobalt complexes are usually a, jS-unsaturated aldehydes and ketones, a report in the patent literature [456] asserts that cobalt di-(salicylal)-arylene-diimines such as XXXVIII, are efficient catalysts for the direct epoxidation of olefins with molecular 

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