Selenoacetic acid

When magnesium bromohydroselenide reacts with ethyl chloro-formate or acetyl chloride it gives, respectively, ethoxyseleno-formic acid, C2H5O.COSeH, and selenoacetic acid, CH3.COSeH, products in which one atom of oxygen in the carboxyl group is replaced by selenium. 

Selenoacetic acid, CH3COSeH, from magnesium bromohydroselenide and acetyl chloride, has a penetrating, irritating odour. The ammonium salt is unstable and decomposes into selenium and ammonium acetate. With salts of the heavy metals the ammonium salt gives coloured precipitates, which decompose, forming the corresponding black selenides. 

Tetranitrodiphenyl selenide, [C6H3(N02)2]2Se.1— This occurs as a by-product in the preparation of 2 4-dinitrophenyl-selenoacetic acid, p. 11, and is probably formed as shown below. 

This occurs when one or more oxygen atoms in a functional group are notionally replaced by other heteroatoms. Depending on the hierarchy, this may lead to the use of functional replacement prefixes (e.g., seleno in selenoacetic acid, H3C-C(=Se)OH), infixes, or suffixes (e.g., in benzenecar-bodithioic acid, Ph-C(=S)-SH). 

Phenyheleno acetals. This borane converts both aldehydes and ketones into diphenyl diselenoacetals. Addition of TFA is usually advantageous. An alternative route, which sometimes is more efficient, is acid-catalyzed exchange from oxygen acetals. The selenoacetals are precursors to useful selenium-stabilized carbanions. 

Selenoacetals are readOy available compounds which have been prepared (i) by selenoacetalization of carbonyl compounds with selenols - in acidic media, with tris seleno)boranes > in neutral or acidic media or with trimethylsilyl selenides and aluminum trichloride (Schemes 69 and... 

The direct selenoacetalization of carbonyl compounds by selenols is by far the shortest and most convenient route to selenoacetals. The reaction is usually carried out at 20 C with zinc chloride (0.5 equiv. versus the carbonyl con x>und) and delivers rapidly (<3 h) and in reasonably good yields methyl and phenyl selenoacetals derived from aliphatic aldehydes and ketones and cyclic ketones (Scheme 69). Selenoacetalization is more difficult to achieve with hindered ketones, such as adamantanone and diisopropyl ketone, and with hindered aromatic carbtmyl compounds. In these cases the reaction is best achieved with titanium tetrachloride instead of zinc chloride and is often limited to the methylseleno derivatives (Scheme 78). Tris(methylseleno)borane offers the advantage of not requiring an acid catalyst and is particularly useful for the selenoacetalization of acid labQe aldehydes such as citronellal (Scheme 70, e). 

Although unable to metallate selenides, dialkyl amides are sufficiently strong to metallate phenylselenoacetals39,46 51) as well as methyl48,52) and phenyl 46,47,52) selenoorthoesters. They are also able to metallate selenoxides 4 9,11 53 55) and selenones 14). Finally selenoacetals are readily available 4,711,12 S6) from carbonyl compounds and selenols in the presence of a Lewis acid and selenoorthoesters have been prepared from orthoesters, selenols, and boron trifluoride etherate47,48,52). 

Homoallyl selenides. Selenoacetals are converted to homoallyl selenides in a Lewis acid-promoted reaction. 

Allylic alcohols. Two laboratories have reported a novel synthesis of allyUc alcohols from two carbonyl compounds. One carbonyl compound (1) is treated with benzeneselenol (2 eq.) under acid catalysis to give a selenoacetal (2). This product can be cleaved to the carbanion (a) by n-butyllithium in THE at -78. The carbanion reacts with a second aldehyde or ketone to form a /3-hydroxyselenide (3). The final step involves oxidation and selenoxide fragmentation to the aUyUc alcohol (4). j3-Hydroxyselenides have been obtained by... 

In the presence of trifluoroacetic acid, aldehydes and ketones react with the tris-selenoboranes (89) to give the corresponding bis-selenoacetals the reagents... 

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