Mercuration thiophenes

Procedure. The thiophene, mercuric oxide, and glacial acetic acid are mixed tc ether and heated quickly to reflux and the iodine is added in small portions to the mixture at such a rate that the color of iodine has disappeared before the next portion is added. After the last portion of the iodine has been introduced, the mixture is cooled and the contents of the reaction flask are diluted with 2 liters of water. The insoluble product and mercuric iodide are collected by suction filtration. The mixture is digested with carbon disulfide to remove the tetraiodothiophene. The latter is recrystallized once from dioxane, m.p. 198-199°. 

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

Pd(II) salts promote the carbonylation of organomercury compounds. Reaction of phenylmercury chloride and PdCh under CO pressure affords benzophenone (429)[387]. Both esters and ketones are obtained by the carbonylation of furylmercury(Il) chloride in alcohol[388]. Although the yields are not satisfactory, esters are obtained by the carbonylation of aryl- and alkylmercuryfll) chlorides[389,390]. One-pot catalytic carbonylation of thiophene, furan, and pyrrole (430) takes place at the 2-position via mercuration and transmetallation by the use of PdCb, Hg(N03), and CuCl2[391]. 

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). 

H NMR, 4, 1042 ionization potentials, 4, 1046 synthesis, 4, 1066 UV spectra, 4, 1044 Selenolo[2,3 -cjthiophenes H NMR, 4, 1042 synthesis, 4, 1067 UV spectra, 4, 1044 Selenolo[3,2-6]thiophenes dipole moments, 4, 1049 H NMR, 4, 1042 ionization potentials, 4, 1046 synthesis, 4, 1066 UV spectra, 4, 1044 Selenolo[3,4-6]thiophenes synthesis, 4, 1067 Selenolo[3,4-c]thiophenes addition reactions, 4, 1062 synthesis, 4, 1076 Selenomethionine applications, 4, 970 Selenophene, 3-acetamido-reactions, 4, 953 Selenophene, 2-acetyl-mercuration, 4, 946 nitration, 4, 947 Selenophene, 2-alkyl-reactions, 4, 45 synthesis, 4, 135, 967 Selenophene, 3-alkyl-synthesis, 4, 135, 967 Selenophene, 3-aryl-synthesis, 4, 963 Selenophene, 2-benzyl-reactivity, 4, 946 Selenophene, 2-benzyl-5-ethyl-reduction, 4, 950... 

Further substitution of 2,4-disubstituted and most 2,3-disubstituted thiophenes occurs in the free a-position, except when a - -M-substituent in the 3-position strengthens the 4-directing power of a —1—M-sub-stituent in the 2-position. Thus methyl 3-methyl-2-thiophenecar-boxylate is brominated in the 4-position and 3-brorao-2-thiophene-aldehyde is nitrated in the 4-position. Recent investigations on the chloromethylation, sulfonation, mercuration, and nitration of 2,4-di-chlorothiophene, which without proof are assumed to occur in the 5-position serves as examples of the reactivity of a 2,4-disubstituted thiophene. Formylation with iV,A-dimethylformamide and... 

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

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]. 

By shaking the recovered benzene-thiophene mixture with a solution of 5.5 g. of mercuric chloride, 10 g. of sodium acetate, and ro cc. of alcohol in 80 cc. of water, the unchanged thiophene is converted to the 2-chloromercurithiophene (containing a small amount of the dimercurichloride) from this the free thiophene can be obtained by treatment with hydrochloric acid. The recovered thiophene amounts to 2-2.5 g-... 

The mercuration of 2-substituted thiophenes by mercuric acetate at temperatures between 16.9 and 50.0 °C has been shown to be second-order over the first 20 % of reaction449. The activation energies for thiophen and its 2-acetyl, 2-methyl,... 

Thiophenes 81a and 81b, which bear carboxylic acid functionalities, have been mercurated using a variety of mercury salts, including Hg(OAc)2 and Hg(OCOCF3)2. When Hg(OAc)2 is employed, these reactions afford a mixture of monomercurated 82a and 82b and dimercurated products 83a and 83b (Equation (32)).101 Dimercuration is also observed when 3,4-ethylenedioxythiophene 84 is treated with Hg(OAc)2 to form 85 (Equation (33)).102... 

Propionylation (EtCOCl/SnCl4/CS2) of thienothiophene 2 readily gives (in 88% yield) 2-propionylthieno[3,2-i]thiophene. With acetic anhydride and traces of iodine, 2 yields 2-acetylthieno[3,2-6]thiophene (50%). The latter was also prepared by acetyl chloride treatment of the monomercury compound produced by mercuration (HgClj) of thienothiophene 2. ... 

The 2-ethyl derivatives of thienothiophenes 1 and 2 form in high yields the corresponding monoacetoxymercury compounds which considerably differ in their melting-points and may be used for identification of the isomeric thienothiophenes. Bisacetoxymercury compounds can be formed from 2,5-dialkyl thienothiophene 1. 2-Ethyl-5-methyl-thieno[2,3-6]thiophene with mercuric acetate in methanol produced the... 

Iodothiophene has been prepared by the action of iodine and mercuric oxide on thiophene. ... 

Ferrocene, like thiophene, furan, and other so-called superaromatic systems, reacts readily with mercuric acetate to form mercurated derivatives. Nes-meyanov and coworkers first reported that ferrocene could be mercurated under relatively mild conditions in either ethyl ether-alcohol or benzene-alcohol solution (63). The acetoxymercuriferrocenes formed in this manner are usually treated with an alcoholic solution of an alkali metal halide. The resulting products, chloro-mercuriferrocene (XXVII) and l,l -di(chloromercuri)ferrocene (XXVIII), can be conveniently separated by extraction with n-butyl alcohol. 

The mercuration of thiophene, which presumably goes by way of a coordination compound, gives an initial attack in the 2 position as expected from the reactions of thiophene itself (67). The bromination (or chlorination) of indazole or its silver salt also leads to the same products (82). 

In a glass-stoppered, wide-mouth bottle cooled by ice water are placed 35 g. (0.42 mole) of thiophene (p. 73) and 50 cc. of benzene (Note 1). With constant shaking (Note 2), and cooling when necessary, 75 g. (0.35 mole) of yellow mercuric oxide and 109 g. (0.43 mole) of iodine are added alternately in small amounts during a period of fifteen to twenty minutes. The yellow mercuric oxide changes to crimson mercuric iodide. The mixture is filtered, and the residue is washed with three 25-cc. portions of ether. The ether-benzene filtrate is shaken with a dilute solution of sodium thiosulfate to remove excess iodine and then dried over 5 g. of calcium chloride and filtered. The ether and benzene are removed by distillation on a steam bath (Note 3), and the residue is fractionally distilled under reduced pressure. Iodothiophene distils at 73°/15 mm. 8o-8i°/20 mm. 90-94°/34 38 mm. The yield is 63-66 g. (72-75 per cent of the theoretical amount) (Note 4). If the iodothiophene is still colored by traces of iodine, the color may be removed by shaking with a small amount of mercuric oxide. 

There are some exceptions to this general similarity of mechanism between thiophene and benzene substrates in electrophilic substitution. One of them is the mercuration of thiophene by mercury(II) acetate in acetic acid. It is very probable that the reason for the divergence is that a preliminary coordination of mercury with the sulfur atom is involved. Another reaction which deviates from the pattern in the benzenoid series is nitration in this case the complication may be due to the principal route being nitrosation, followed by oxidation of the nitroso group. 

The partial rate factors af and /3f for the a- and /3-positions of thiophene have been calculated for a wide range of electrophilic reactions these have been tabulated (71 AHC(13)235, 72IJS(C)(7)6l). Some side-chain reactions in which resonance-stabilized car-benium ions are formed in the transition states have also been included in this study. A correspondence between solvolytic reactivity and reactivity in electrophilic aromatic substitution is expected because of the similar electron-deficiency developed in the aromatic system in the two types of reactions. The plot of log a or log /3f against the p-values of the respective reaction determined for benzene derivatives, under the same reaction conditions, has shown a linear relationship. Only two major deviations are observed mercuration and protodemercuration. This is understandable since the mechanism of these two reactions might differ in the thiophene series from the benzene case. 

In all the reactions (except mercuration, for which the two reactions probably proceed by different mechanisms) the p-values for reactions at the thiophene nucleus are smaller (in absolute value) than those for the benzene reactions. For a critical evaluation of such data, consult (76AHC(20)l). The p-value for the solvolysis of l-(2-thienyl)ethyl p-nitroben-zoate and its nuclear substituted derivatives is -6.79 (the corresponding value for the benzene series being -5.7) (72JOC2615). Here [Pg.753]

Dihydrobenzo[6]thiophene 426 and its 2-methyl derivative182 are readily dehydrogenated with sulfur to the corresponding benzo[ ]-thiophene. 2,3-Dihydrobenzo[6]thiophenes are oxidized by peracetic acid to the corresponding sulfones (Section VI, P, 2), and give crystalline complexes with mercuric chloride. 

Tetrahydrobenzo[ ]thiophene behaves like thiophene in electrophilic substitution reactions. Thus, it is formylated with a mixture of vV-methylformanilide and phosphorus oxychloride,436 iodinated in the presence of mercuric oxide,193 and brominated by V-bromosuccinimide,193 all in the 2-position in Friedel-Crafts reactions with acetyl chloride,194-436 propionyl chloride,436 succinic... 

Iodination of benzo[6]thiophene in benzene in the presence of mercuric oxide gives exclusively 3-iodobenzo[6]thiophene.411-413... 

---