Metal-dioxygen complexes oxygen activation

C. Oxygen Activation-Direct Oxygen Transfer from Metal-Dioxygen Complexes to Organic Substrates. 354... 

Further evidence against initiation by direct oxygen activation in the oxidation of olefins is provided by the following two observations.185 First, no reaction was observed between olefins (e.g., cyclohexene, 1-octene, and styrene) and metal-dioxygen complexes, such as I, II, and V, when they were heated in an inert atmosphere (nitrogen). Second, no catalysis was observed with these metal complexes in the autoxidation of olefins, such as styrene, that cannot form hydroperoxides. 

SO2. Consequently, the dioxygen molecule can be thou t of as undergoing activation toward this substrate by virtue of being coordinated to the metal. In like manner, many other small molecules are oxidized far more readily in the coordination sphere of a metal dioxygen complex than with molecular oxygen alone. 

Dioxygen is a cheap and ideal source of oxygen but it is very difficult to activate and there are relatively few examples of 02 oxidations catalyzed by zeolite-encapsulated complexes. Encapsulated CoPc is active for the oxidation of propene to aldehyde, whereas the free complex is inactive.104 A triple catalytic system, Pd(OAc)2, benzoquinone, and a metal macrocycle, was used to oxidize alkenes with molecular oxygen at room temperature.105 Zeolite-encapsulated FePc106-108 and CoSalophen (Scheme 7.5)107109 complexes were used as oxygen-activating catalysts. With the use of a Ru complex instead of Pd(OAc)2 in the triple catalytic system, primary alcohols can be oxidized selectively to aldehydes.110... 

It should be added that MS-02 is not necessarily a mono-nuclear complex. It could be shown in a few cases that the catalytic activity of the metal ion is due to the formation of dinuclear metal-substrate complexes. Presumably in these species each oxygen atom of dioxygen coordinates to a different metal center. Such systems were extensively used to model the reactivity patterns of various enzymes containing a bimetallic active center. 

The facile activation of dioxygen by these simple organometalHc complexes generates high-valent (V(V), Cr(V)) metal 0x0 complexes, which may undergo oxygen atom transfer reaction with organic substrates, and thus serve as catalysts for aerobic oxidations (Sect. 3.3). 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

The gas-phase oxidation of ethylene to ethylene oxide over a supported silver catalyst was discovered in 1933 and is a commercially important industrial process. Using either air or oxygen, the ethylene oxide is produced with 75% selectivity at elevated temperatures (ca. 250 °C). Low yields of epoxides are obtained with propylene and higher alkenes so that other metal-based catalysts are used. A silver-dioxygen complex of ethylene has been implicated as the active reagent.222... 

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

Electronic properties of intrazeolitic complexes of transition metal ions with oxygen are of Interest for elucidation of oxygen binding and its activation for oxidation reactions. In an earlier study, dioxygen and monooxygen chromium species were reported to be formed by specific interactions between the oxygen molecules and the Cr ions planted in the Type A zeolite (1). 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

Sawyer DT. The chemistry and activation of dioxygen species in biology. In Oxygen Complexes and Oxygen Activation by Transition Metals. 1988. Martell AE, Sayer DT, eds. Plenum, New York. 

A reoxidation of the catalytic amounts of hydroquinone (HQ) to benzoquinone (BQ) in Scheme 8-11 by molecular dioxygen was realized by the use of an oxygen-activating macrocyclic metal complex as cocatalyst [53,62-65]. This leads to a mild biomimetic aerobic oxidation which is now based on a triple catalytic system (Scheme 8-12). With this system cyclohexa-1,3-diene is oxidized to frans-l,4-diacetoxycyclohex-2-ene at room temperature in 85-89% (>91% tmns) [62]. With the use of 2-phenylsulfonyl-l,4-benzoquinone as quinone, the trans selectivity of this process was >97% [53]. 

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