Thiophene, 3-iodo-, iodination

Iodination and chlorination have been considerably less studied. Data on iodination are limited to those of Challenger and co-workers, who showed that thienothiophenes 119 and 220 with iodine and mercuric oxide give rather unstable 2-iodo derivatives, the structures of which were confirmed by their conversion into the corresponding 2-carboxylic acids.20 The formation of 2,5-diiodothieno[3,2-6]thiophene by iodination of 2 was also observed.22... 

Iodination of 4 with molecular iodine in the presence of mercuric oxide formed the 3-iodo derivative (52JA4951 66CJC2283). Iodine in tetrahydro-furan oxidatively cyclized /3-(3-benzo[b]thienyl)-a-mercaptoacrylic acids rather than iodinating the thiophene ring [70JCS(C)2431]. 

Although chlorination, bromination and iodination of thiophenes by polyhalide salts require forcing conditions with the addition of zinc chloride [52], halogenation of acridine and acridone has been recorded to yield both 3-halo and 3,7-dihalo derivatives under relatively mild reaction conditions [53], However, whereas chloro-, bromo- and iodo-compounds are readily obtained from acridone, acridine only forms the bromo derivatives, as it produces stable complexes with the dichloroiodate and tetrachloroiodate salts [53]. 

Halobenzo[6]thiophenes are readily obtained from the corresponding 2-benzo[6]thienyllithium by the reactions described in Section VII. 2-Iodo-3-methylbenzo[6]thiophene is obtained by treatment of the 2-acetoxymercury compound with iodine.496... 

Thienylzincates are available using lithium di-zW-butyltetramethylpiperidinozincate (TMP-zincate) <1999JA3539> thus, ethyl thiophene-3-carboxylate is easily metallated at C(2) at room temperature, subsequent reaction with iodine giving ethyl 2-iodothiophene-3-carboxylate in 89% yield. Similarly ethyl thiophene-2-carboxylate gives the 5-iodo derivative in 62% yield. 

Oxidation of N-aromatic methanesulfonamides 321 with (diacetoxyiodo)benzene in the presence of thiophene in trifluoroethanol or hexafluoroisopropanol (HFIP) affords the respective coupling products 322 in good yield (Scheme 3.131) [380]. The head-to-tail thiophene dimers 324 can be selectively prepared by the hypervalent iodine oxidation of 3-substituted thiophenes 323 [381,382] and bipyrroles 326 can be regios-electively synthesized by oxidative dimerization of pyrroles 325 with [bis(trifluoroacetoxy)iodo]benzene in the presence of bromotrimethylsilane [383]. Likewise, bithiophenes 328 have been synthesized from 3,4-disubstituted thiophenes 327 using [hydroxy(tosyloxy)iodo]benzene in the presence of bromotrimethylsilane in hexafluoroisopropanol [384]. 

Boron Reagents. lodination of 3,4-bis(trimethylsilyl)thiophene followed by a Suzuki phenylation gives the monophenylated product 3-phenyl-4-trimethylsilylthiophene 45 (Scheme 23). After another ipso-iodination using iodine and silver trifluoroactetate and Suzuki coupling at the iodo carbon, 3,4-diarylated thiophenes are produced.The same reactions have been effected in furans. ... 

The first example of alkylthiophene bromination was also demonstrated at that time. Methylthiophene which was obtained from coal tar as a mixture with toluene was reacted with bromine, and dibromo(methyl)thiophene separated from toluene. Its further bromination led to the synthesis of fully substituted tribromo(methyl) thiophene. 2-Iodo-5-methylthiophene was obtained by the same iodination procedure as iodothiophene. Deiodination with sodium allowed for the isolation of pure methylthiophene [80]. 

The preparation of (alk-l-ynyl)thiophenes 154 is possible via palladium-catalyzed Sonogashira cross-coupling reactions between iodo- or bromothiophenes 152 and terminal alkynes 153 (Scheme 60, Table 38) [309, 360, 363, 364], Alkynylated thiophenes represent an important structural motif found in many ir-electronic systems such as molecular rods or conjugated macrocycles [163, 362, 365, 366], In the case of bromoiodo-substituted thiophenes, the more reactive iodine atom is replaced selectively [367]. Some coupling methods require stoichiometric amotmts of paUadium(0) [360, 368], whereas other methods proceed successfully tmder photochemical conditions without the need of a catalyst [369]. Recent... 

Reactions.—All the mono- and poly-bromo-derivatives, as well as iodo-derivatives, of thieno[2,3-h]thiophen have been prepared by direct bromination or iodination, or from the lithium compounds, and characterized by their n.m.r. spectra. Boronic acids were obtained from several alkyl- and aryl-thienothiophens via lithiation and were converted by hydrogen peroxide oxidation into the tautomeric hydroxy-derivatives. It was shown by n.m.r. that all thieno[2,3-h]-thiophen systems exist as thieno[2,3-h]thiophen-2(3 )-ones, while in the case of... 

A soln. of 0.24 mole of p-xylene in acetic acid treated at 80° with water until the soln. turns turbid, coned. H2SO4, 0.15 g.-atom iodine, 0.04 mole of iodic acid, and CGI4 added, vigorously stirred ca. 4 hrs. until the iodine color disappears 2-iodo-p-xylene. Y 85%.—Similarly at 40° Veratrole 4-iodoveratrole. Y 88%.—Thiophene 2-iodothiophene. Y 75%, F. e. and discussion of the... 

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