Selenophenes, acyl

Benzoselenolo[2,3-ii]benzoselenophenes synthesis, 4, 116, 1078 Benzoselenolopyrylium cations oxidative ring contraction, 4, 969 [l]Benzoselenolo[3,4-ii][l,2,3]selenadiazoles synthesis, 6, 354 Benzo[h]selenophene, 2-acetyl-synthesis, 4, 966 Benzo[h]selenophene, 2-bromo-acylation, 4, 948... 

Substrates 47 and 48 have also been used to study the relative reactivities of selenophene and thiophene and of selenophene and benzene.71 The higher reactivity of the selenophene ring was demonstrated by the fact that upon formylation 59% of compound 49a was formed and upon acylation 63% of derivative 49b. Acylation of 48 gave exclusively 2-acetyl-5-benzylselenophene. Structures of the products were determined by H NMR. 

Acyl azides 268, derived from furan, thiophene and selenophene, add slowly at room temperature to the strained double bond of 5-methylenebicyclo[2.2.1]hept-2-ene. Two regioisomeric triazolines, 269 and 270, which form in the first step, are unstable and decompose with elimination of nitrogen to provide aziridine derivatives 271. Products 271 are isolated in good yield (73-85%). It is worthy to note that not only the terminal, unstrained double bond in the starting material, 5-methylenebicyclo[2.2.1]hept-2-ene, is unaffected, but also the typical dipolarophiles like esters of crotonic, propiolic and byt-2-ynoic acids do not react with azides 268 under these conditions (Scheme 39) <2002J(P1)1420>. 

Acetylbenzo[6]tellurophene is also formed in isolable quantities. Benzo[6]selenophene is converted into 2-acyl derivatives by reaction with acid chlorides in the presence of aluminum chloride, whereas similar Friedel-Crafts acylation of benzo[6]thiophene yields 3-substituted products. When the 2-position is blocked, formylation can be directed into the 3-position. Thus treatment of 2-bromobenzo[6]selenophene with dichloromethyl butyl ether (Cl2CHOBu) and titanium tetrachloride yields the 3-formyl derivative (72BSF3955). If the 2- and 3-positions of benzo[6]selenophene are blocked with methyl groups, acylation under Friedel-Crafts conditions occurs in the 6-position (78CR(C)(287)333>. 

Selenienyllithium, for the most part, undergoes a completely similar series of transformations (75CS(7)lll) because of the great tendency of 3-selenienyllithium to ring open, it is necessary, however, to work at low temperatures (-70 °C). 3-Selenienyllithium can be converted into 3-formylselenophene by reaction with iV,N-dimethylformamide, and 3-acetylselenophene by reaction with N,N-dimethylacetamide <75CS(7)lll). These latter conversions illustrate the utility of 3-selenienyllithium since it will be recalled that acylation of selenophene occurs preferentially in the 2-position. 

The syntheses of 2-ketobenzo[ >]selenophene (134) (76JCS(P1)2452) and its benzo[c] counterpart (135) (64BSB491, 81JHC343) both depend on intramolecular acylation of an intermediary selenol, while in the benzo[Z>]selenophene (137) preparation from (136) the ring closure depends upon the intramolecular addition of the intermediary selenol to the carbonyl group (70BSB235). 

Selenophene undergoes various electrophilic substitutions nitration, sulfonation, halogenation, mercuration, chloromethylation, aminomethylation, acylaminomethylation, acylation, formylation, and hydrogen exchange. 

Acyl selenophenes are chloromethylated at the free a. position (as in sulfonation and halogenation). A second chloromethyl group can be introduced ortho to the first and meta to the acyl group.61 5-Methyl-2-acetylselenophene is chloromethylated in the 4-position.61 When reduced by stannous chloride in hydrochloric acid, 5-chloro-methyl-2-acetylselenophene simply undergoes reduction of the chloromethyl group, and 2-methyl-5-acetylselenophene is formed61 (Scheme 2). 

The selenophene ring may be acylated by acyl chlorides under Friedel-Crafts conditions7 63 by acid anhydrides in the presence of 85% phosphoric acid64 and by organic silicoanhydrides (tetraacyloxy-silanes) in the presence of stannic chloride.65-67 When acylated by silicoanhydrides of dibasic organic acids (succinic, adipic, azelaic, or... 

Thus, ketones of the selenophenes series are most readily obtained from selenophene by acylation with either tetraacyloxysilanes or acid anhydrides, the acyl group always entering the a position of the ring. 

No diacylated selenophene can be obtained by direct acylation however, 2,5-diacyl-3,4-dihydroxyselenophenes (3) were obtained from 1,3,4,6-tetraketones and selenium dioxide in dioxane.69... 

Since acylation of the selenophene nucleus failed to produce fi-acylated selenophenes, 3-acetylselenophene is synthesized from the corresponding acid chloride (Scheme 3).56... 

Many reactions of the acyl group of ketones of the selenophene series have been studied. Acyl selenophenes have been shown to be brominated, oxidized, and cyanoethylated, and to undergo the Kishner reduction, Mannich reactions, and Claisen condensations. 

Cyanoethylation of acyl selenophenes leads to the formation of S-ketonitriles which are easily converted by alkaline hydrolysis into... 

Ketones of the selenophene series are aminomethylated to give aminoketone hydrochlorides, which with phenylhydrazine form N-phenylpyrazolines.115 Kishner reduction of acyl selenophenes has led to various 2-alkylselenophenes the Clemmensen technique proved inapplicable for this purpose.116... 

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