Selenium, dioxide

Selenium dioxide is probably the most common reagent used to selectively oxidize an alkyl fragment, and is particularly effective for converting allylic or benzylic C—H fragments to the corresponding allylic alcohol.522 In the most common applications, carbonyl derivatives are oxidized to 1,2-dicarbonyl compounds and allylic hydrocarbons to alcohols or esters. 23 

The reaction of Se02 and allylic molecules proceeds via an initial ene reaction (sec. 11.13) to give 374. A 2,3-sigmatropic shift (sec. 11.12.A) affords 375, which is hydrolyzed to the allylic alcohol 376.524 jg important to note that the reaction shows an overall transformation that appears to be a direct conversion of 

C—H to C—OH, but in fact the reaction is accompanied by two migrations of the jr bond, as shown. An illustration is the preparation of fran -pinocarveol (377) from 3-pinene (378) in 55% yield.525 when the reaction in done is the presence of acetic anhydride, the allylic alcohol product is converted to the corresponding acetate.526 

Guillemonat assembled a set of rules that predict the oxidation products for different alkenes.527 Trachtenberg528 expanded the discussion of these rules to include general statements concerning oxidation with selenium dioxide and these are listed below. 

Oxidation always occurs on the disubstituted side of a double bond if there is a nonbridged alkenyl hydrogen available there. 

Selenium reacts with oxygen, but not readily, when heated in air forming selenium dioxide. Under standard conditions the compound is a solid, which sublimes. 

The paper also tabulates the values of the thermodynamic functions of Se02(cr) between 0 and 305 K. 

The enthalpy of formation of Se02(cr) is obtained from the three accepted measurements of the heat of combustion of Se(trigonal) listed in Table V-22 (references to earlier experimental results or selected values in compilations can be found in [62GAT]). Selenium reacts only partly with oxygen in the bomb calorimeter unless mixed with a combustion aid. Benzoic acid was used for this purpose and the heat evolved by the combustion aid was about 50% in the static bomb and about 90% in the rotating bomb experiments. 

The review selects the weighted mean, with suitable uncertainty limits  

In [92GRE/FUG], Af//°(Se02, cr, 298.15 K) = -(225.1 +2.1) kJ-mol was adopted from Mills [74M1L]. With the selected data the Gibbs energy of formation is 

In a reinvestigation of earlier work, Mock and McCausland established that the products of cycloaddition of dienes and selenium dioxide have six-membered ring seleninic ester structures [Eq. (40)].  

However, little additional information is available on this interesting Diels-Alder reaction. 

Previous reviews a. Kresze, G. (1967). In 1,4-Cycloaddition Reactions, the Diels-Alder Reaction in Heterocyclic Syntheses (J. Hamer, ed.), p. 453. Academic Press. 

van Woeiden, H. F., and BijI-Vieger, S. H. (1974). Reel. Trav. Chim. Pays-Bas93, 85. 

Hanson, P., and Stone, T. W. (1984). J. Chem. Soc., Perkin Trans. I, 2429, and references cited therein. 

Hydrolysis proceeds at low temperatures. Light decomposes the chloride with the production of chlorine. Chlorides of organic acids can be produced by the action of this substance upon salts of organic acids, 

Selenium dioxide, unlike sulfur dioxide, is not readily prepared by direct combination of the elements. The best methods depend upon combustion of selenium in the presence of nitrogen dioxide1 or oxidation of selenium by nitric acid.2 The first method includes purification by sublimation the second gives a pure product only if the selenium is pure or if the product is sublimed. 

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Selenium dioxide is a volatile solid obtained when selenium is burnt in air or oxygen. It is very soluble in water, forming a solution of... 

Selenium dioxide, SeO, is ery poisonous (cf. p. I47), but is valuable particu larly for the oxidation of methylene ( CHs) groups to carbonyl ( CO) groups. 

The following oxidation of camphor to camphor-quinone illustrates the oxidising action of selenium dioxide, and readily gives a crystalline product. 

CAUTION. The vapour of selenium dioxide is poisonous, and all operations involving the hot material, alone or in solution, should be performed in a fume-cupboard. If lumps of selenium dioxide have to be powdered in a mortar, the latter should also be in a fume-cupboard, with the window lowered as far as possible, to avoid inhaling the fine dust. (cf. p. 191)... 

When camphor (I) is heated with selenium dioxide in acetic acid, the methylene group next to the carbonyl group is oxidised also to a carbonyl group, to form camphorquinone (II). Note that the compound (II) is not a true quinone but a 1,2-diketone ... 

Place a mixture of 5 g. of camphor, 6 g. of powdered selenium dioxide and 5 ml. of acetic anhydride in flask fitted with a reflux water-condenser. Heat the flask in an oil-bath for 3 hours at 140-150 so that gentle boiling occurs shake the mixture from time to time. 

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

Selenium dioxide. The compound is conveniently prepared by the oxidation of selenium with nitric acid, followed by dehydration of the selenious acid thus formed by cautious heating ... 

In view of the small difference in cost between selenium and selenium dioxide, the preparation of any quantity of the latter from selenium is hardly worth while. It is better to purify commercial selenium dioxide. 

Extreme care should be taken when working with selenium dioxide because of its poisonous properties. 

Oxidation of acetophenone with selenium dioxide iu the i)reseiice of dioxan or ethyl alcohol as solvent affords j.henylglyoxal ... 

CgHjCOCHj + SeOa —> CgHgCOCHO + Se + H O This is one example of the oxidation by selenium dioxide of compounds containing a methylene group adjacent to a carbonyl group to thecorresponding a-ketoaldehyde or a-diketone (see also Section VII,23). 

Ninhydrin (also named 1 2 3-triketoindane or 1 2 3-triketohydrindene hydrate) is prepared most simply from the inexpensive phthahc anhydride (I). The latter is condensed with acetic anhydride In the presence of potassium acetate to give phthalylacetlc acid (II) reaction of the latter with sodium methoxide in methanol yields 1 3-indanedionecarboxyhc acid, which is decomposed upon warming with dilute hydrochloric or sulphuric acid to indane-1 3-dione (or 1 3-diketohydrindene) (HI). Selenium dioxide oxidation of (III) afibrds indane-1 2 3-trione hydrate (ninhydrin) (IV). 

TTie reaction of 2.4-dimethylselenazole with an excess of benzaldehyde gives a monostyryl derivative exclusively (12). Oxidation of 2,4-dimethyl-selenazole with selenium dioxide yields 4-methylselenazole (Scheme 30) (4). 

Selenium dioxide oxidation of methyl derivatives (Scheme 30). 

Furalazine, Acetylfuratrizine, Panfuran-S. Heating nitrovin in butanol or dimethylformamide at 100—130°C affords furalazine, 6-[2-(5-nitro-2-furanyl)ethenyl]-l,2,4-triazine-3-amine (34). An improved synthesis originates with 5-nitro-2-furancarboxaldehyde and acetone, proceeds through 4-(5-nitro-2-furanyl)-3-buten-2-one followed by a selenium dioxide oxidation to the pymvaldehyde hydrate, and subsequent reaction with aininoguariidine (35). Furalazine, acetylfuratrizine (36), and the A[-A/-bis(hydroxymethyl) derivative, Panfuran-S, formed from the parent compound and formaldehyde (37), are systemic antibacterial agents. 

Mocimycin was prepared from dihydromocimycin (3) by oxidation with selenium dioxide (14,56). 4-Amino-4-dehydroxyefrotomycins were obtained by aminolysis of efrotomycin-4-0-phenylchlorophosphate (57). 

Both pyrrole and indole react with selenium dioxide in the presence of nitric acid to give a deep violet solution. Very small quantities (ca 4 of pyrrole can be detected by this method. 

Alkyl Isoquinolines. Coal tar contains small amounts of l-methylisoquinoline [1721-93-3] 3-methylisoquinoline [1125-80-0] and 1,3-dimetliylisoquinoline [1721-94-4J. The 1- and 3-methyl groups are more reactive than others in the isoquinoline nucleus and readily oxidize with selenium dioxide to form the corresponding isoquinoline aldehydes (174). These compounds can also be obtained by the hydrolysis of the dihalomethyl group. The 1- and 3-methyhsoquinolines condense with benzaldehyde in the presence of zinc chloride or acetic anhydride to produce 1- and 3-styryhsoquinolines. Radicals formed by decarboxylation of carboxyUc acids react to produce 1-aIkyhsoquinolines. 

In many reactions, selenium is an oxidant as well as a reductant. Strong oxidants convert selenium dioxide and its derivatives to the hexavalent state. Although hexavalent selenium compounds are oxidants, these are less active and difficult to reduce. Selenium salts resemble the corresponding sulfur and tellurium salts in behavior. 

It is generally unacceptable to emit sulfur dioxide, thus the scmbber effluent must be treated for sulfur dioxide removal. If the plant aheady possesses faciUties for the production of sulfuric acid, this rather concentrated sulfur dioxide stream can be easily fed into the wet gas cleaning circuit and disposed of in the sulfuric acid plant. The quantity is so small that it does not put any additional burden on the sulfuric acid plant. Because no tellurium is carried over with the selenium dioxide during roasting, it is possible to produce a selenium product which can be purified to commercial grade (99.5-99.7%). 

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