Heteroaromatic compounds thiophene

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

A/-sulfonated ayiridines have also been used in Friedel-Crafts reactions (qv) (63). The successful C-alkylation of the heteroaromatic compounds indole (qv) [120-72-9] (64—66) and thiophene [110-02-1] (67) with a2itidines has also been reported ... 

The classical structures of pyrrole, furan and thiophene (31) suggest that these compounds might show chemical reactions similar to those of amines, ethers and thioethers (32) respectively. On this basis, the initial attack of the electrophile would be expected to take place at the heteroatom and lead to products such as quaternary ammonium and oxonium salts, sulfoxides and sulfones. Products of this type from the heteroaromatic compounds under consideration are relatively rare. 

Pyrroles, furans and thiophenes undergo photoinduced alkylation with diarylalkenes provided that the alkene and the heteroaromatic compound have similar oxidation potentials, indicating that alkylation can occur by a non-ionic mechanism (Scheme 20) (81JA5570). 

The validity of the Hammett relationship log K/Ko = pa- has been extensively investigated for five-membered heteroaromatic compounds and their benzo analogues. The ratio Pheterocycie/Pbenzene is closest to Unity for thiophene. Judged from work on the polarographic reduction of nitro compounds, the ability to transmit electronic effects is HC=CH = S < O < NH. 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

Small shift values for CH or CHr protons may indicate cyclopropane units. Proton shifts distinguish between alkyne CH (generally Sh = 2.5 - 3.2), alkene CH (generally 4, = 4.5-6) and aro-matic/heteroaromatic CH (Sh = 6 - 9.5), and also between rr-electron-rich (pyrrole, fiiran, thiophene, 4/ = d - 7) and Tt-electron-deficient heteroaromatic compounds (pyridine, Sh= 7.5 - 9.5). 

The heteroaromatic compounds can be divided into two broad groups, called n-excessive and n-deficient, depending on whether the heteroatom acts as an electron donor or an electron acceptor. Furan, pyrrole, thiophene, and other heterocyclics incorporating an oxygen, nitrogen, or sulfur atom that contributes two n electrons are in the rr-exeessive group. This classification is suggested by resonance structures and confirmed by various MO methods. ... 

The first group consists of monocyclic heteroaromatic compounds with one heteroatom and without strongly electron-donating substitutents (OH, NH2). Pyrrole, furan, and thiophene are better electron donors than benzene. The order of their reactivities in azo coupling is thiophene > pyrrole > furan > benzene. 

Reactions of tc-excessive heteroaromatic compounds such as pyrroles, thiophenes and furans with carbenoids have been known for several years 6-10>u>. Recent activities were directed towards further synthetic applications of already known reactions, evaluation of the efficacy of novel catalysts and towards mechanistic insights. 

The heteroaromatic compounds like furans, pyrroles or thiophenes cannot be generally used as dienes in Diels-Alder syntheses, because at the higher temperature required for the addition of less reactive dienophiles, the equilibrium is on the side of the starting materials due to the unfavorable T AS term comparable to the benzenoid aromatic compounds as mentioned. High pressure again shows the two effects already discussed the shift of the equilibrium toward the products and the enhancement of the rate of reaction which allows the temperature of reaction to be lowered. One... 

Alkyl aryl ketones are convenient intermediates for preparing other substituted aromatic and heteroaromatic compounds. Challenger and co-workers " acetylated thieno[2,3-h]thiophene (1) with acetyl chloride and stannic chloride [Eq. (59)]. The 2-acetylthieno[2,3-A]-thiophene obtained by this method was reduced to the 2-ethyl derivative (20) identical with the product of an independent synthesis. ... 

As described in the previous sections, a variety of nucleophiles attack the Cy atom of ruthenium-allenylidene intermediates. Aromatic compounds should also be suitable candidates and this was found to be the case [30]. Thus, reactions of propargylic alcohols with heteroaromatic compounds such as furans, thiophenes, pyrroles, and indoles in the presence of a diruthenium catalyst such as la proceeded smoothly to afford the corresponding propargylated heteroaromatic compounds in high yields with complete regioselectivity (Scheme 7.25). The reaction is considered to be an electrophilic aromatic substitution if viewed from the side of aromatic compounds. 

Homolytic substitution of thiophene as well as homolytic aromatic substitutions by thienyl radicals has been reviewed (73IJS295). In addition to this, a wider review on homolytic substitution of heteroaromatic compounds (74AHC(16)123) also briefly covers work done in the area of thiophenes. 

Semiempirical methods of calculation with consideration of all valence electrons have been used only recently but already have given results on the reactivities of some aromatic and heteroaromatic compounds.157,239 243 Thus, to analyze the reactivities of thiophene and the isomeric thienothiophenes 1-3 to electrophilic substitution,157 the semiempirical SCF LCAO MO method CNDO/2 was used, taking into account all valence electrons.244 The 3s, 3p, and 3d orbitals have been taken into account for the sulfur atom.245 The reactivities were estimated from the difference between bond energies of the initial and the protonated molecule (in a complex).157... 

The commonest five-membered heteroaromatic compounds containing one heteroatom are pyrrole (30), furan (31) and thiophene (32). Their substituted derivatives and the substituted derivatives of the corresponding benzo analogues [indole (33), benzofuran (34) and benzothiophene (35)] are widespread naturally-occurring compounds. 

Pyridine (C5H5N), pyrrole (C4H5N), furan (C4H40), and thiophene (C4HtS) are examples of heteroaromatic compounds. 

Pyrrole 2.1, thiophene 2.2, and furan 2.3, are five-membered ring heteroaromatic compounds containing one heteroatom. They derive their aromaticity from delocalisation of a lone pair of electrons from the... 

In 1958, Panov and Kocheshkov1216 found another route to the formation of the C—Pb bond, namely the interaction of tetraacyloxyplumbanes with aromatic and heteroaromatic compounds (the plumbylation reaction). They showed that the reaction of thiophene with Pb(OCOPr-i)4 at room temperature during 10 days led to unstable RPb(OCOR/)3 (R = 2-thienyl R = i-Pr), which was disproportionated to R2Pb(OCOR/)2 and Pb(OCOR/)4-... 

---