Thiophene relative aromaticity

Thiophene-1-oxide and 1 -substituted thiophenium salts present reduced aromaticity.144 A variety of aromaticity criteria were used in order to assess which of the 1,1-dioxide isomers of thiophene, thiazole, isothiazole, and thiadiazole was the most delocalized (Scheme 46).145 The relative aromaticity of those molecules is determined by the proximity of the nitrogen atoms to the sulfur, which actually accounts for its ability to participate in a push-pull system with the oxygen atoms of the sulfone moiety. The relative aromaticity decreases in the series isothiazole-1,1-dioxide (97) > thiazole-1,1 -dioxide (98) > thiophene-1-dioxide (99) then, one has the series 1,2,5 -thiadiazole-1,1 -dioxide (100) > 1, 2,4-thiadiaz-ole-1,1-dioxide (101) > 1,2,3-thiadiazole-1,1 -dioxide (102) > 1,3,4-thiadiazole-l,1-dioxide (103) in the order of decreasing aromaticity. As 1,2,5-thiadiazole-1,1-dioxide (100) was not synthesized, the approximations used extrapolations of data obtained for its 3,4-dimethyl-substituted analogue 104 (Scheme 46). 

The aromaticity of pyrrole, furan, and thiophene may also be assessed by considering the 7r-electron distribution in them (8UST163), which points to a greater aromaticity of pyrrole and thiophene relative to furan. 

Refining engineers and chemists are most interested in the ease of desulfurizing petroleum using thermal and thermocatalytic treatments. The sulfur is removed primarily as hydrogen sulfide. Thermal and thermocatalytic studies have established that non-thiophenic sulfur (aliphatic as in thiols, acyclic and cyclic sulfides) evolve H2S much more readily than thiophenic sulfur (aromatic heterocyclic compounds). Thus, the relative abundances of nonthiophenic (aliphatic) and thiophenic (aromatic) sulfur is a critical characteristic for all fuels with respect to ease of desulfurization. Analytical methods were developed in the 1960s for classifying the total sulfur in crude... 

Partial pyrolysis-gas chromatograms of representative immature kerogens or coals from the four sequences studied are shown in Figure 2. The abundance of thiophenes relative to aliphatic and aromatic hydrocarbons in the partial FID chromatograms differs markedly for the four samples shown. This is reflected by the ratio of the peak area of 2,3-dimethylthiophene relative to those due to 1,2-dimethylbenzene and n-non-l-ene (Le. TR = [2,3-dimethylthiophene]/[l,2-dimethylbenzene+n-non-... 

While there are no extensive reports on the relative aromaticity of the heterocycles covered in this chapter, the general reactivity of these systems can be predicted based on first principles. By assuming that these fused systems are comprised of a five-membered rc-excessive heterocyclic system and a five-membered -deficient heterocyclic system, electrophilic agents are expected to react on the n-excessive subunit. Ab initio calculations on the thienothiazoles and furothiazoles predicted that electrophilic substitutions should occur exclusively on the furan or thiophene subunit with the regioselectivity being a function of the resonance-stabilization of the reactive intermediates <76KGS1202>. A priori, C-H deprotonation by a nonnucleophilic base should occur preferentially on the -deficient heterocyclic component. 

It can be concluded that the B3LYP/6-31G will produce high-quality structural parameters for the five-membered rings and their benzo derivatives. Based on the structural uniformity principle and magnetic susceptibility anisotropies, the predicted relative aromaticity of these systems is found to be reliable. From the computed values, the relative stability of thiophene, benzo[ ]thiophene, and benzo[f]thiophene is accurately predicted. 

The classic photochemical reaction involving thiophenes is the isomerisation of 2-aryl-thiophenes to 3-aryl-thiophenes the aromatic substituent remains attached to the same carbon and the net effect involves interchange of C-2 and C-3, with C-4 and C-5 remaining in the same relative positions scrambling of deuterium labelling is, however, observed, complicating interpretation of the detailed mechaifism. 

Though both Suzuki and Stille reactions have been widely used to prepare conjugated polymers (including D-A copolymers), there are some subtle issues to consider when it comes to choose which reaction to use. For example, it is worth noting that the electron richness of stannyl aromatics decides whether these monomers are suitable for Stille-based polymerization or not. Mechanistically, relatively electron-rich thiophenes undergo the transmetalation step more readily than stannylbenzenes. Thus, stannylbenzenes experience low reactivity under Stille reaction conditions. Correspondingly, most thiophene-based aromatics are polymerized via Stille reactions, whereas a Suzuki reaction is a better option for benzene-based compounds. For example, fluorene and carbazole based polymers are usually prepared by Suzuki reaction, whereas polymers with cyclo-penta[2,l- ) 3,4-6 ]dithiophene, silolo[3,2- 4,5- ) ]dithiophene or benzo[l,2- 4,5-i Jdithiophene are often polymerized via Stille reaction. Due to its broader utilization over the Suzuki reaction in preparing D-A copolymers, Stille reaction-based polymerization will be the focus of this chapter, with a brief discussion on the Suzuki-based polymerization also included (Section 15.2.3). 

Furan is a good substrate for the Diels-Alder reaction, but thiophene reacts only under very forcing conditions. This reflects their relative aromatic stabilization energy. 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

Rate data are also available for the solvolysis of l-(2-heteroaryl)ethyl acetates in aqueous ethanol. Side-chain reactions such as this, in which a delocalizable positive charge is developed in the transition state, are frequently regarded as analogous to electrophilic aromatic substitution reactions. In solvolysis the relative order of reactivity is tellurienyl> furyl > selenienyl > thienyl whereas in electrophilic substitutions the reactivity sequence is furan > tellurophene > selenophene > thiophene. This discrepancy has been explained in terms of different charge distributions in the transition states of these two classes of reaction (77AHC(21)119>. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

Some theoretical aspects of thiophene reactivity and structure have also been discussed, for example the kinetics of proton transfer from 2,3-dihydrobenzo[6]thiophenc-2-onc <06JOC8203>, the configuration of imines derived from thiophenecarbaldehydes <06JOC7165>, and the relative stability of benzo[c]thiophene <06T12204>. The kinetics of nucleophilic aromatic substitution of some 2-substituted-5-nitrothiophenes in room temperature ionic liquids have also been investigated <06JOC5144>. 

NMR studies on 28 and 29 indicate that both the thiophene and furan rings rotate freely at room temperature (vide infra) and therefore, anomalies in the UV spectra of 28 and 29 should be attributed to the through-bond interaction between the Si-Si cr bonds and aromatic 77 bonds. This was further confirmed by photoelectron spectral studies. As shown in Table II, the lift of HOMO for 12 relative to the model compound was 0.4 eV, but those for 28 and 29 were 0.7 and 0.6 eV, respectively. Apparently, more effective through-bond interaction occurs for 28 and 29 (21). 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

Thiophene as a compound is not found in high concentration in crude oil. However, benzothiophenes and other aromatic thiophenes are critical components of high-sulfur crude oil. Also, thiophenes are present in relatively high concentrations in oils containing increased percentages of aromatics, asphaltenes, and resins. 

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