Molecular oxygen, oxidation ionic

Aromatic aldehydes themselves are oxidized to acid by molecular oxygen in ionic liquid under the influence of Ni(acac)2. Excellent yields and mild conditions are characteristic of the VOClj-catalyzed oxidation of a-hydroxy carbonyl compounds (to the dicarbonyl compounds). "... 

A normal oxide is a binary (two element) compound containing oxygen in the -2 oxidation state. BaO is an example of an ionic oxide and S02 is an example of a molecular (covalent) oxide. 

AUTOXIDATION. A word used to describe those spontaneous oxidations, which take place with molecular oxygen or air at moderate temperatures (usually below 150°C) without visible combustion. Autoxidation may proceed through an ionic mechanism, although in most cases the reaction follows a free radical-induced chain mechanism. The reaction is usually autocatalytic and may be initiated thermally, photoehemically, or by addition of either free radical generators or metallic catalysts. Being a chain reaction, the rate of autoxidation may be greatly increased of decreased by traces of foreign material. 

Metals can (a) directly oxidize the substrate trough a radical or ionic mechanism (b) activate molecular oxygen by complexation (c) decompose peroxides, as discussed in the previous section. A review of these mechanisms is beyond the scope of the present book and can be found in the literature (10). An interesting example of the effect of heavy metal salts on the stability of drugs is the oxidation of hydrocortisone, which was catalyzed by copper (II) ions at a concentration as low as 3 x 10-3 M (47). 

The formation of some organic hydroperoxides by oxidation with molecular oxygen is catalytically promoted by metals like silver or copper 171). A dissociative chemisorption of oxygen cannot be active in these processes they probably proceed via the chemisorption of O7 ions (or O2 molecules forming a covalent bond resonating with an ionic bond). 

Oxygen forms binary compounds with nearly all elements. Most may be prepared by direct reaction, although other methods (such as the thermal decomposition at carbonates or hydroxides) are sometimes more convenient. Oxides may be broadly classified as molecular, polymeric or ionic. 

It is of course essential that the ionic liquid is stable in the presence of the oxidant, which excludes ionic liquids with metallic anions such as chlorocuprates. In many oxidation reactions water is present as co-solvent, reagent or it is produced in the course of the reaction, which further eliminates the use of chloroaluminates. However, less care has to be taken with respect to drying the ionic liquid compared to other catalytic reactions when aqueous oxidants are used. Common imidazolium and ammonium based ionic liquids are neither water-nor oxygen sensitive and thus well suited and may even act as co-catalyst in the oxidation reaction. Some anions like [BF4] and [PF6] are, however, susceptible towards hydrolysis. Where the ionic liquid is highly viscous, a co-solvent such as dichloromethane might be necessary to afford acceptable reaction rates, especially when molecular oxygen is used as reagent. 

The first catalytic oxidation to be carried out in an ionic liquid was probably the oxidation of aromatic aldehydes with Ni(acac)2 and molecular oxygen to the corresponding carboxylic acids, reported by Howarth in 2000 (Scheme 5.20).181 Overall yields were only moderate, which was attributed to the low oxygen solubility in the ionic liquid, but the catalyst containing ionic liquid was successfully reused without deterioration in activity. 

With molecular oxygen as the oxidant, addition of perfluorinated solvents can help to increase the reaction rate as demonstrated in the Co(acac)2-catalysed oxidation of ethylbenzene to acetophenone. The function of the perfluorohexane solvent is to increase the otherwise low oxygen concentration in the ionic liquid phase. 

From an economic and environmental perspective, catalytic aerobic alcohol oxidation represents a promising protocol. The use of molecular oxygen as the primary oxidant has several benefits, including low cost, improved safety, abundance, and water as the sole by-product. In this way, many catalytic systems have been used for the aerobic oxidations in ionic hquids as green solvents. Different types of catalysts or catalytic systems useful for the oxidation of alcohols with as terminal oxidant in ionic liquids as solvent will be discussed below. 

In 2002, Ansari et al. reported the conversion of primary and secondary alcohols to corresponding aldehydes and ketones in the ionic liquid, l-butyl-3-methylimidazo-lium hexafluorophosphate ([bmim][PFg]). These conversions catalyzed by TEMPO/ CuCl system in the presence of molecular oxygen as a terminal oxidant at 65°C (Scheme 14.28) [27]. 

Another three-component catalytic system including CuCl, TEMPO, and a base was developed for the oxidation of alcohols with molecular oxygen in the ionic liquid [bmim][PFJ by Lin et al. (Scheme 14.31) [29]. 

Oxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although other methods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (see Topic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalent oxides are formed with nonmetals, and may contain terminal (E=0) or bridging (E-O-E) oxygen. Especially strong double bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formation of many polymeric structures such as Si02 (see Topics FT and F4). 

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