Selenophenes halogenation

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). 

In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as is illustrated by the preparation of 3-bromothiophene... 

Halogen can be removed by catalytic hydrogenation and so it is possible to use halogen as a blocking group in pyrrole chemistry. In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as illustrated by the preparation of 3-bromothiophene (390) from 2,3,5-tribromothiophene (389) (81SC25). 

Dibenzoselenophene is readily alkylated to the selenienyl salts (62), which are powerful methylation and trifluoromethylation agents [98, 99], Halogens preferentially attack a-carbon in selenophenes but the 1,2-dibromo-l-benzoselenophenium bromide (63) was found among the bromination products of 1-benzoselenophene (Fig. 6) [100],... 

Yur ev et al. 2 prepared deuteriated selenophenes from the respective halogenated compounds by halogen-deuterium replacement. Thus, 2-deuterio-, 2,5-dideuterio-, tetradeuterioselenophene, 3-methyl-, and 5-methyl-2-deuterioselenophene were obtained from the corresponding iodoselenophenes on reduction by zinc in deuteriated acetic acid. 3-Deuterioselenophene was obtained from 3-selenienyl-lithium hydrolyzed with deuteriated acetic acid at — 7 0°C the lithium derivative was obtained from 3-bromoselenophene and ethyllithium, also at — 70°C. The deuterium content was determined by combustion, the water being analyzed by the drop method.43... 

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

Bromo and chloro derivatives of selenophene are obtained by direct halogenation, whereas iodo derivatives are obtained either... 

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