Benzo 6 selenophene bromination

Halogens attack the ring heteroatom in selenophene and tellurophene. Thus the selenienyl bromide (89) is among the bromination products of benzo[6]selenophene. Tellurophene reacts with halogens to give 1,1-dihalogeno derivatives e.g. 90). 

Treatment with NCS in carbon tetrachloride converted the parent into the 2,3-dichloro derivative. The 2,3-dibromo compound was made similarly with NBS (74BSF2239), or with bromine in chloroform in the presence of sodium acetate (72CHE13). Monobromination is possible, but generally mixtures form with 2- and 3-bromo products in ratios of the order of 1 3 (72CHE13). It was possible to prepare 3-bromobenzo[h]sele-nophene by reaction of the 2,3-dibromo derivative with butyl lithium followed by hydrolysis. Four moles of bromine gave the 2,3,6-tribromo derivative from benzo[h]selenophene (74BSF2239). 

Dihydrobenzo[c]selenophene (117) forms a 1,1-adduct with bromine and an oxide on treatment with hydrogen peroxide. Both derivatives are transformed into benzo[c]selenophene (5) on suitable treatment with base the latter unstable compound forms a stable Diels-Alder adduct with tetracyanoethylene (76JA867). More vigorous oxidation of (117) with peracetic acid yields the diselenide (118). 

Substituted 1,2-benzoseIenazoles are synthesized by the reaction of 6H, 1H-benzo[fc]selenophene-6,7-dione (31) with bromine followed by ammonia. The amide (14 Scheme 10) may be used for the preparation of the nitrile (15) or the amino compound (16) (75JHC1091). 

Bromination of benzo[b]selenophene occurs at both the 2- and the 3-positions (72CHE13). 

The bromination of dihydrobenzo[( ]selenophene 20 led to the formation of the corresponding 1,1-dibromo derivative 21 (Scheme 1) <20030L2519>. Treatment of the latter with lithium hexamethyldisilazane (LiHMDS) produced benzo[f]selenophene 3 which was subsequently converted to the stable diester 23 via dilithiation. The structure of 23 was confirmed by X-ray crystallographic analysis. 

An approach amenable to the preparation of various fused selenophenes and benzo[3]selenophenes involves the treatment of 1,4-dilithiated intermediates with bis(phenylsulfonyl)selenide 112 <1994CPB1437>. The synthetic sequence to the selenolo[2,3-/ ]selenophene 113 from 3-ethynylselenophene 110 is shown in Scheme 11 <1997H(45)1891>. A stereoselective hydroalumination of 110 with diisobutylaluminium hydride (DIBAL-H) followed by bromination with NBS gave dibromo compound 111. Dilithiation of 111 followed by treatment with selenium electrophile 112 gave 113. 

Benzo[c]selenophene 32 has been prepared by the aromatization of dihydrobenzo[c]selenophene 31 following the bromination of the selenium using molecular bromine and a subsequent elimination in the presence of an aqueous or non-aqueous base <03OL2519>. Furthermore, the dialdehyde and dicarboxylic acids on the a positions of benzo[c]selenophenes have been synthesized. 

Kinetic studies on the bromination of selenophene and benzo-thiophene have also been reported. 

The bromination, acetylation, and mercuration of benzo[6]selenophen have been studied by Russian workers. 2-Methylbenzo[6]selenophen is acylated and brominated in the 3-position, whereas the 3-methyl isomer reacts in the 3-position. The nitrosation of (672) is claimed to yield (673), which was reduced and acetylated to (674). From this compound, the... 

Attempts to prepare benzo[c]selenophen from (688) have been made. The reaction of (688) with peracetic acid gave (689), and this reacted with bromine to give (690). 

Contrary to benzo[b]thiophen, benzo[b]selenophen is acylated at position 2. 2-Bromobenzo[b]selenophen, prepared from the 2-lithium derivative and bromine, is acylated at position 3, and from this derivative the 3-isomers could be synthesized. A large number of 2- and 3-aroylbenzo[b]seleno-phens were synthesized, either by Friedel-Crafts reaction of benzo-[bjselenophencarboxylic acid chlorides and benzenes or from benzol bjselenophens and benzoic acid chlorides. Also the reaction between Grignard reagents and nitriles was used for the synthesis of aroylbenzo[b]selenophens. 4,S-Diaminobenzo[b]selenophen has been synthesized via nitration of 5-aminobenzo[b]selenophen or via the reaction of the diazonium salt derived from the S-amino-derivative with p-sulphanilic acid. From the diamino-derivative, several fused benzo[b]selenophens, e.g. (525), (526), and (527), were prepared. Reaction of the phenyl-... 

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