Synthesis from Selenourea

The most important 2-aminoseIenazoles, along with their physical properties are shown in Table X-2. 

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

The dibromohydrate is first produced, and treatment with ammonia gives the free base, which after recryslallization in nitrobenzene gives brown crystals (m.p. 215°C). The picrate decomposes around 254°C. 

4-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. 

For 2-amino-4- m-nitrophenyl) seienazole, the yield is particularly high. This has been explained by the oxidizing effect of the nitro group, which liberates iodine from the hydrogen iodide eliminated in the condensation reaction. 

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The naphthoselenazole derivative (42 R = NHj) may be prepared by a Hantzsch-type synthesis from selenourea and 2-bromo-3,4-dihydronaphthalen-l(2 )-one-3-sulphonic acid followed by fusion of the product with sodium hydroxide. This amine (42 R = NHj) is ring-opened by sequential treatment with sodium borohydride and acetic anhydride to give compound (43), which may be cyclized to give the naphthoselenazole (42 R = Me). 

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... 

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. 

The synthesis of 1,3-selenazoles from A -phenylimidoyl isoselenocyanates has been reported. N-phenylimidoyl isoselenocyanates 94 are prepared from N-phenylbenzamides 92. Treatment of 92 with thionyl chloride affords N-phenylbenzimidoyl chlorides 93, which yield imidoyl isoselenocyanates 94 on reaction with potassium isoselenocyanate. The imidoyl isoselenocyanates 94 were transformed into selenoureas 95 with either ammonia or primary or secondary amines. Reaction of 95 with an activated bromomethylene compound such as bromoacetophenone in the prescence of a base gave the 1,3-selenazole 97 via the salt 96 <00HCA1576>. 

This chapter deals with the methods of synthesis, characterization, and growth mechanisms of well-defined uniform particles of metal sufides and selenides formed by direct reaction of metal ions with the chalcogenide ions, released from thioacetamide or selenourea in dilute solutions, or supplied continuously from outside in the form of a high concentration of sulfide ions. 

Another variation of this synthesis involves cyclization of 4-substituted 1-acyl-selenosemicarbazides (Scheme 80) (73JPR164). 2,5-Diamino-l,3,4-selenadiazoles of type (187) are prepared from bis(selenourea) (186 Scheme 81) (73JPR155). 

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