Thiophenes typical reactivity

Typical Reactivity of Indoles, Benzo[ ] thiophenes, Benzo[ ]furans, Isoindoles, Benzo[c]thiophenes and Isobenzofurans... 

Benzisothiazole and 2,1-benzisoxazole seem not to display the tendency to act as aza-dienes that might have been expected on the basis of comparison with the typical reactivity of isoindoles, benzo[c]thiophenes and isobenzofurans (cf. 22.2). In a different sense, electron-deficient 2-alkenyl-benzothiazoles react as... 

Typical reactivity of indoles, benzo[b]thiophenes, benzo[b]furans, isoindoles, benzo[c]thiophenes and isobenzofurans... 

Nevertheless, we can interpret the reactions of furan and thiophene by logical consideration as we did for pyrrole. In electrophilic substitutions, there is again a preference for 2- rather than 3-substitution, and typical electrophilic reactions carried out under acidic conditions are difficult to control. However, because of lower reactivity compared with pyrrole, it is possible to exploit Friedel-Crafts acylations, though using less-reactive anhydrides rather than... 

Dibenzophospholes A and dithienophospholes B and C (Fig. 4.7) do not display the typical electronic properties and reactivity patterns of phospholes, since the dienic system is engaged in the delocalized benzene or thiophene sextet [7, 10a, 51]. In fact, these building blocks have to be regarded as nonflexible diaryl-phosphines or as P-bridged diphenyl or dithienyl moieties. 

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

Here, the parallels with benzenoid counterparts continue, for these compounds have no special properties - their reactivities are those typical of benzenoid aldehydes, ketones, acids and esters. For example, in contrast to the easy decarboxylation of a-acids observed for pyrrole and furan, thiophene-2-acids do not easily lose carbon dioxide nevertheless, high-temperatme decarboxylations are of preparative value (see also 17.12.1.2). "... 

Commercial heterogeneous HDS catalysts for refinery use consist, almost without exception, of nickel- and/or cobalt-promoted molybdenum oxide located on a high surface area (approx. 300 m g ) alumina or silica-alumina support. Cobalt and nickel promoters increase the catalytic activity, particularly towards thiophenes whether Co or Ni is used as a promoter depends on the specific function for which the catalyst should be optimal. The catalyst material is shaped into porous pellets, a few millimeters in size, and these pellets are loaded into the reactor, forming a catalyst bed of 30-200 m volume. During start-up of a freshly loaded reactor, the catalyst bed, which is in the oxidic form, is sulfided, typically by treatment with an oil feed which has been spiked with a reactive sulfur compound that readily generates H2S in situ. The oxidic precursor phases (non-stoichiometric CoMo or NiMo surface oxides) are thereby converted into sulfidic phases termed Co-Mo-S and Ni-Mo-S. The conversion from the oxidic phase to the sulfidic is accompanied by a reduction in Mo oxidation state from +6 to +4. 

It has been found that 3-lithiothiophene is stable even at room temperature in hexane <94TL36V3>. This makes it easy for the synthesis of 3-functionalized thiophenes, since even less reactive electrophiles can be used. A typical procedure involves reaction of 3-bromothiophene with Bu"Li at —40°C in hexane to which about 10% THE has been added. The solution is then warmed to room temperature and treated with electrophiles to give good yields of the products. C—C, C—S, C—hal and C—Si bonds have been created at position 3 by this method. 

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