Selenium dioxide

Selenium dioxide (SeO₂)

Selenium dioxide (SeO₂) is an important reagent in organic syntheses, as it is both an oxidant and weakly acidic.  Selenium dioxide is the chemical compound with the formula SeO₂. This colorless solid is one of the most frequently encountered compounds of selenium.  The solid sublimes readily. The vapor has an odor resembling horseradish sauce and can burn the nose and throat on inhalation.  SeO₂ is considered an acidic oxide: it dissolves in water to form selenous (selenious) acid. Often the terms selenous acid and selenium dioxide are used interchangeably. It reacts with base to form selenite salts containing the SeO2−3 anion. For example, reaction with sodium hydroxide produces sodium selenite:

SeO₂ + 2 NaOH → Na₂SeO₃ + H2O

Uses in Organic synthesis:

SeO₂ is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO₂ gives glyoxal and the oxidation of cyclohexanone gives cyclohexane-1,2-dione. The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off. This type of reaction is called a Riley oxidation. It is also renowned as a reagent for "allylic" oxidation, a reaction that entails the conversion

R₂C=CR'-CHR"₂ + [O] → R₂C=CR'-C(OH)R"₂

(where R, R', R" are alkyl or aryl).

Mechanism of oxidation

Procedures:

A 500-ml three-necked, round-bottomed flask is fitted with a mechanical stirrer, a thermometer, a dropping funnel, and a reflux condenser. A solution of 0.74 g. (0.0067 mole) of selenium dioxide in 150 ml. of tert-butyl alcohol is introduced into the flask, followed by 68 g. (0.50 mole) of β-pinene. The resulting mixture is warmed to 40 °C with a hot water bath before 35 ml. (0.62 mole) of 50% aqueous hydrogen peroxide is added drop wise over 90 minutes, during which time the mixture is maintained at 40–50 °C by occasional immersion in a cold water bath. After stirring for an additional 2 hours, the reaction mixture is diluted with 50 ml. of benzene, washed with three 50-ml. portions of saturated aqueous ammonium sulfate, and dried over sodium sulfate. A small amount of hydroquinone is added, and the solvents are removed on a rotary evaporator. trans-Pinocarveol is isolated by simple distillation under reduced pressure, yielding 37–42 g. (49–55%), b.p. 60–70° (1 mm.).

Procedure was taken from Organic Syntheses, Coll. Vol. 6, p.946 (1988); Vol. 56, p.25 (1977).

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1,2-Cyclohexanedione. A 3-l. round-bottomed flask, fitted with stirrer and dropping funnel, is placed in a water bath containing a coppercoil through which cooling water may be circulated. In the 3-l. flask is placed 1708 g. (17.4 moles, 1.8 l) of cyclohexanone. Tap water is circulated through the cooling coil, and a solution containing 387 g. (3 moles) of selenious acid (H₂SeO₃), 500 ml. of 1,4-dioxane, and 100 ml. of water is added drop wise and with stirring to the cyclohexanone over a period of 3 hours. The reaction mixture immediately turns yellow, and red amorphous selenium gradually appears. Stirring is continued for 5 additional hours at water-bath temperatures and then for 6 more hours at room temperature. Removal of the bulky, amorphous selenium is accomplished with the aid of a 6-in. Buchner funnel. The selenium is returned to the reaction flask and extracted with 300 ml. of boiling 95% ethanol for 1 hour. The solution, obtained by decantation from the compact gray selenium, is combined with the above filtrate in a 4-l. distilling flask. Distillation under reduced pressure gives two fractions. The lower-boiling fraction (25–60°/16 mm.) consists mainly of ethanol, water, dioxane, and cyclohexanone; the higher-boiling one (60–90°/16 mm.) contains cyclohexanone and 1,2-cyclohexanedione with traces of water and dioxane. The yield of crude product is approximately 322 g.

Procedure was taken from Organic Syntheses, Coll. Vol. 4, p.229 (1963); Vol. 32, p.35 (1952).

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Microwave-assisted selenium dioxide mediated selective oxidation of 1-tetralones to 1,2-naphthoquinones

This is a microwave reaction reported in Tetrahedron Letters Volume 50, Issue 1, 7 January 2009, Pages 39-40

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Tips: 

1. Since only catalytic quantities of selenium dioxide are required, the danger of handling large quantities of this material is avoided.
2. Formation of selenium and organoselenides commonly arise in oxidations using molar quantities of selenium dioxide, are not encountered.
3. If too much selenous acid is added at once, or the cooling discontinued, the solution will heat up and the reaction will become extremely vigorous with subsequent decrease in yield.
4. Selenium compounds are exceedingly toxic