Selenoureas

The replacement of selenoamide by selenourea in the Hantzsch s synthesis. (1st method) leads to 2-aminoselenazoles 2, 14. 15). This series of compounds has been well developed, mainlv because selenourea is much more easily accessible than the selenoamides, but also because a wide variety of a-halogenated carbonyl compounds are available for the Hantzsch s evdization reaction (Scheme 5). 2-Aminoselenazole itself was prepared from commercially available chloroacetaldehyde semihydrate... 

With a-halogenated carbonyl derivatives such as a.a -dibromobiacetyl. Br-CH -C-C-CHi-Br. and selenourea, 2.2 -diamino-4,4 -biselenazole... 

DiaminoseIenazole was obtained by the action of selenourea on chloracetonitrile (15). Its salts are fairly unstable and decompose to give metallic selenium. All attempts to obtain the free base or 2,4-dihydroxyselenazole from 2,4-diaminoseIenazole hydrochloride were unsuccessful and led to decomposition of the heterocycle. 

As in the case of the thiazoles, a variation on the Hantzsch s method has been used. This consists of using a nonhalogenated carbonyl derivative directly in the presence of iodine in the reaction with selenourea (Scheme 7) (20). However, in this case the reaction with selenourea is slower than with thiourea, and normally an excess of carbonyl compound is used. 

N-substituted selenoureas give the same condensation reaction (Scheme 9. Table X-3). 

Mono (H N-C-NHRi) or disubstituted (H N-C-NRiR i) selenoureas can be used. The group R, R( can be hydrogen, alkyl, aryl, acyl, or benzoyl. 

The selenoureas needed for condensation can usually be prepared by addition of hydrogen selenide to a solution of the corresponding cyanamides or carbodiimides. They can be less dangerously and quite... 

The hydrolysis of 2-benzamidoselenazoles in aqueous phosphoric or sulfuric acid gives 2-aminoselenazOles identical to those prepared from selenourea (Scheme 12). 

The advantage of this method is that it avoids the use of hydrogen selenide, necessary for the preparation of selenourea from cyanamide (14). Benzoylselenourea is synthesized by the method of Douglass (24) by the action of potassium selenocyanate On benzoyl chloride in acetone solution. 

The condensation of a-haloketones with monosubstituted alkyl or aryl-selenoureas (25) leads to 2-alkylamino- (26. 27) or 2-arylaminOselana-zoles (28) while disubstituted selenoureas give 2-(dialkylamino) selenazoles (26. 27) (Table X-3a). 

Amino-4-(2-thienyI)selenazoIe (in.p. 132°C. yield = 82%) also has been prepared by condensation of selenourea on 2 bromoacetyithiophene (Scheme 26). In a first step the bromhydrate is obtained (105). 

Starting from 2-bromoethylamine hydrobromide and selenourea, Chu and Mautner (60) prepared the salt of 2-(aminoethyl)-selenopseudourea, which cyclizes upon heating in aqueous solution to give 2-aminoselenazoline (Scheme 58). 

Oxoselenazolines with a 2-(disubstituted amino) group are obtained by the action of a-halocarboxylic acid or rather their esters on N-disubstituted selenoureas (Scheme 63, Table X-14) (27. 67. 68). 

Comrie et al. (68) have thus prepared several compounds of this type by condensation of various selenoureas with a-halocarboxylic acids and have assigned them the imino structure (Table X-15a). 

Selenourea [630-10-4] ]5k.e urea and thiomea can form channel inclusion compounds (87) with a variety of hydrocarbons. Though the difference in channel diameter between thiourea and selenourea is small, selenourea seems to be much more selective for the inclusion of certain guest molecules (eg, cis/trans isomers). 

Detector elements are prepared either by sublimation in the presence of a small partial pressure of O2 or by chemical deposition from alkaline solution containing a lead salt and thiourea or selenourea (63). Lead sulfide and lead selenide deposit from solutions as mirror-like coatings made up of cubic crystallites 0.2—1 p.m on a side. The reaction may nominally be represented by the following ... 

Reaction of a-chlorocyclohexanone (93) with selenourea has been reported to yield 2-amino-4,5,6,7-tetrahydrobenzoselenazole (99) (43RTC580, 5UA1864). 

Selenourea [630-10-4] M 123.0, m 214-215 (dec). Recrystd from water under nitrogen. 

Davies et al. have described the synthesis of 2,4-diamino-selena-zole (4), by the reaction of selenourea with chloroacetonitrile. The... 

A variation of the general method for the synthesis of 2-amino-selenazoles is to avoid the use of the free a-halogenocarbonyl compound and in its place react the corresponding ketone and iodine with selenourea.This procedure is also taken from thiazole chemistry. By contrast with thiourea, the reaction with selenourea needs a longer reaction time and the work up of the reaction mixture is somewhat more difficult. Usually an excess of the ketone is used. In the preparation of 2-amino-4-( n-nitrophenyl)selenazole, a very high yield, calculated on the amount of iodine used, was obtained. To explain this peculiar result, the oxidative action of the nitro group was invoked. This liberates free iodine from some of the hydrogen iodide eliminated in the condensation reaction, and the free iodine then re-enters into the reaction. 

Reaction of A -substituted selenoureas with a-halogenocarbonyl compounds yields the corresponding 2-amino-selenazole substituted in... 

There are amines such as A-methylnitroamine, that are too weakly nucleophilic to be able to form covalent adducts with arenediazonium ions. The products of the latter appear to be those of salts ArNJ N(N02)CH3, as found by Baranchik et al. (1957). Amides also appear not to be sufficiently nucleophilic, but thioamides are, as is shown by the reaction of A-phenylthiourea in the presence of NaOH (Scheme 13-12 Nesynov et al., 1970). First a (probably homolytic) phenylation-de-diazoniation takes place, followed by A-coupling. Selenourea also reacts a mixture of products is formed, which indicates a reaction of the same type as with thiourea (Nesynov and Aldokhina, 1976). 

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