Mercury compounds thiophenes

Upon distilling the mercury compound with concentrated hydrochloric acid, it is readily decomposed into mercuric chloride and pure thiophene. 

Photolysis of the mercury compound 286 in benzene leads to 3-phenyl-thiophene (291) and 3-iodothiophene (287). The former could arise from an insertion reaction of the aryne (4) with benzene if it is assumed that the expected but missing 2-phenylthiophene 292 completely isomerizes to the 3-isomer (291) under the reaction conditions. The argument is not convincing, however, since the 3-iodothiophene (287) formed as a coproduct, presumably by homolysis of the C-Hg bond, also yields 3-phenylthiophene (291) on photolysis in benzene, albeit in very low yield. Furthermore, by analogy with the reaction of benzyne the major products should be those of cycloaddition, which, in the case of 4, might lead, after loss of acetylene, to thianaphthene (293). No such products were reported from the photolysis of the mercury compound (286), but they have been observed from other possible sources of the aryne (4) (Section III.3.A.f). 

Although research on the FVT of the thiophene anhydride 229 has been primarily directed at obtaining evidence for the intermediacy of the aryne 4, it has also yielded some information on the chemistry of this species. The most studied reaction of thiophyne (4) thus far is with dienes 146 to give thianaphthenes 293, presumably via the Diels-Alder adducts 390. The ubiquity of this reaction, as discussed above, appears to make it virtually diagnostic for the presence of the aryne (4). Thus the formation of thianaphthene (293) when benzene is the diene has been interpreted to involve a retro-Diels-Alder loss of acetylene from the initially formed thiophyne-ben-zene adduct (390c). Conversely the absence of thianaphthene (293) in the photolysis of the mercury compound 286 in benzene (Section III.3.A.a) strongly suggests that the aryne 4 is not produced in this reaction. 

Thermolysis of the mercury compound 444 in the presence of tetracyclone (151) did give a tetraphenylthianaphthene which proved to be the same one (285) as that obtained from the isomeric precursor 286 and not the one (445) expected if the 3,4-aryne 25 was an intermediate. Since it was demonstrated that the mercury compounds 286 and 444 did not interconvert under the themolysis conditions, it was postulated that an initially formed 3,4-aryne (25) rearranged to the apparently more stable 2,3-aryne (4). Because of the unprecedented nature of this proposed aryne rearrangement, doubts as to the validity of this interpretation were soon raised. A semiempirical scf-mo calculation of the isomeric didehydrothiophenes predicted that, in contrast to the above suggestion, the 3,4-isomer 25 was more stable than the 2,3-isomer 4. The parallel behavior of the two mercury compounds 286 and 444 with respect to the formation of 3-iodothiophene (287) only when thermolysis is carried out in the absence of tetracyclone (151) suggests that, as in the former case (Section III.3.A.a), the adduct 285 probably forms by reaction of tetracyclone (151) with 3-iodothiophene (287) and not the aryne (4). Support for this hypothesis comes from the demonstration of this last conversion, 151H-287->285 (Section III.3.A.a) and the isolation of 2,5-diphenyl-3-iodo-thiophene (446) from thermolysis of the diphenylmercury precursor 447. In... 

Mercury(II) acetate tends to mercurate all the free nuclear positions in pyrrole, furan and thiophene to give derivatives of type (74). The acetoxymercuration of thiophene has been estimated to proceed ca. 10 times faster than that of benzene. Mercuration of rings with deactivating substituents such as ethoxycarbonyl and nitro is still possible with this reagent, as shown by the formation of compounds (75) and (76). Mercury(II) chloride is a milder mercurating agent, as illustrated by the chloromercuration of thiophene to give either the 2- or 2,5-disubstituted product (Scheme 25). 

Derivatives of 5-alkyl-2-(l,3,4-oxadiazol-2-yl)thiophenes 168 were synthesized and their photochromic and fluorescent properties studied. A solution of the photochrome was subjected to irradiation over a wide range, including the lines of the mercury spectrum at 313, 365, 405, 436, 546, and 578 nm. It was discovered that the open form of compounds 168 showed strong fluorescence <2002CHE165>. 

A new synthetic route to alkyl-substituted quinones has relied on the photochemical reaction of 2,3-dichloro-l,4-naphthoquinone with a thiophene derivative (77CL851). Irradiation of a benzene solution of the quinone and thiophene by a high pressure mercury lamp gave photoadduct (295) in 56% yield. Desulfurization of this compound over Raney nickel (W-7) gave the 2-butyl-1,4-naphthoquinone derivative (296 Scheme 62). Alkyl-substituted quinones such as coenzyme Q and vitamin K, compounds of important biological activity, could possibly be prepared through such methodology. 

The following compounds 1 are prepared in a similar manner to the corresponding thiophenes, fusion of sulphur or selenium with acetophenone anil giving rise respectively to 2 4-diphenylthiophene and 2 4-diphenylsdencpkene. The melting-points of these compounds and their derivatives are as follows 2 4-diphenylthiophene, M.pt. 122-5° C., 2 4-diphenylselenophene, M.pt. 112-3° C., 5-chloro-mercuri-2 4-diphenylthiophene, M.pt. 223° C., o-chloromercuri-2 4-diphenylselenophene, M.pt. 224° C. 

The photochemistry of trisilanes 16 and 19 has been investigated in some detail (Schemes 1 and 2)21. Upon irradiation of compound 16 only the Si—SiMe2Ph bond is broken and the initially formed silyl radical 17 undergoes a rearrangement to the more stable silacyclobutenyl radical 18 whose EPR spectrum has been recorded (Scheme l)21a. Irradiation of trisilane 19 with a medium pressure mercury lamp resulted in the formation of hexamethyldisilane, 2-(trimethylsilyl)thiophene and 20, with 20 dominating (Scheme 2)21b. In the presence of carbon tetrachloride, a significant yield (19.2%) of the... 

Another reaction of considerable preparative value for thiophenes is mercuriation (with mercuric chloride or mercuric acetate), the lower selectivity here (especially with the acetate) giving significant yields of (3-substituted derivatives. These arylmercurial derivatives are readily converted into other compounds. Mercuric chloride substitutes in the 2- and then the 5-position [1892LA(267)172 14LA(403)50], and the OR and SR substituents direct into the 5- and then the 3-position [32LA(495)166]. 

This leaves unexplained the anomalously high reactivities for both mercuriation and protiodemercuriation. For the latter, it may be relevant that with thiophene and substituted thiophenes kinetics were second order for only 20% of reaction, and there were marked differences in entropies of activation for the heterocyclic and phenyl derivatives. Even for benzenoid compounds the reaction is not well behaved as (1) the Hammett correlation shows marked curvature, suggesting that reactive compounds undergo a different mechanism to unreactive ones (2) the reaction is sensitive to oxygen and (3) it is sensitive to added chloride ion, due possibly to salt effects and reaction via hydrogen chloride ion pairs (65AJC1521). A reexamination of both mercuriation and protiodemercuriation of thiophene would seem desirable. 

Condensation of, J,iV-acetals 77 with 1,3-dicatbonyl compounds in the presence of mercury acetate leads to thiophenes 80. Mercury complexes 78 derived from 77 react with 1,3-dicarbonyl compounds to generate intermediates 79, which undergo cyclization and subsequent hydrolysis-deacylation to afford 80 <1998JOC6086, 2000JOC3690, 2000JHC363>. Thiophenes 82 <20020L873, 2004JOC4867> are also prepared by reaction of 77 with 2-diazo-3-trimethylsilyloxy-3-butenoate 81 (Scheme 18). 

Mercury 5 5 -di-n-propyl"2 2 dithienyl yields silvery crystals, melting at 57° to 58° C. It is formed from 5-chloromercuri-2"n-propyl-thiophene by the action of a little more than 2 molecules of sodium iodide in acetone solution. The corres 3onding di isoamyl compound forms shining crystals from alcohoh melting, not very sharply, at 55° to 57° C. 

As described in the previous section, the Dewar thiophene is a suitable dienophile and reacts with many kinds of dienesU9). If the sulfur atom of the thiirane part can be eliminated, the Dewar thiophene will be a suitable precursor of a C4(CF3)4 unit. An example is the synthesis of oxahomocubanes. Thus, the Diels-Alder adducts from the Dewar thiophene and furans were treated with triphenylphosphine and irradiated with a low-pressure mercury lamp to give oxahomocubanes (119). A peculiar point is the fact that exo-tricyclic compound is eyclized. Therefore, isomerization should have occurred prior to cyclization. This suggests that photocyclization cannot be used for the determination of such configurations110). 

The reaction of aryllead triacetates is not limited to substituted phenyllead derivatives. It has also been extended to the use of heteroaryllead compounds, derived from furan and thiophene.Due to their relative instability and moisture-sensitivity, they are best prepared by metal exchange, such as mercury-lead or tin-lead, and used in situ in the reactions of carbon nucleophiles. 

Oxidation of freshly liberated 1,2,3,4-tetrahydrophthalazine (42) by mercury(II) oxide in the presence of sulfur dioxide gave an (unseparated) mixture of 1,4-dihydro-2,3-benzoxathiin 3-oxide (43) and l,3-dihydrobenzo[c]thiophene 2,2-dioxide (44) (substrate, CH2CI2, -20X, S02i, then HgOj, 20°C, 12h 45% of a 9 1 mixture in which the compounds were clearly identified). ... 

Multiple substitution of reactive aromatic compounds may occur. Thiophene gives a 2, 5-disubstituted and furan a 2,3,4, 5-tetrasubstituted derivative with mercury (II) acetate in boiling ethanol. The preferential mercuration of thiophene has been used for its removal from commercial benzene. Mercuration of ferrocene (p. 284) also occurs readily. 

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