Thiophene resonance effect

Gronowitz et al. have discussed the effects of substituents on chemical reactivity and on ultraviolet (XJV), infrared (IR), and nuclear magnetic resonance (NMR) spectra in terms of simple resonance theory,They assume resonance structures (1-5) to contribute to a —I—M (Ingold s terminology) 2-substituted thiophene, resonance forms (6-10) to the structure of a drI-fM 2-substituted thiophene, forms (11-16) to a —I—M 3-substituted thiophene, and forms (17-22) to a I -M 3-substituted thiophene. 

Note that the dipoles of furan and thiophene are opposite in direction to that in pyrrole. In furan and thiophene, there is a greater inductive effect opposing the resonance effect, whereas in pyrrole the resonance... 

The effects of substituents on the H NMR spectra of benzo[6]thiophenes are in line with expectations for the operation of normal inductive and resonance effects. A discussion of such effects is available and need not be repeated here (70AHCtll)177). 

The 13C NMR spectra of some parent A,B-diheteropentalenes and T(C,H) coupling constants are summarized in Tables 4 and 5. The sensitivity of C-3 to substituent effects is the same in both thieno[2,3-/>]thiophene (33) and thieno[3,2-6]thiophene (12) <76ACS(B)417>. The substituted carbon atoms also show comparable sensitivity. Differences were observed for the substituent transmittance effects to similar positions in these two systems. For example the resonance effect is much more efficiently transmitted over two sulfur atoms in derivatives of compound (12). On the other hand, for the 6a position the system (33) shows a great similarity with 2-substituted thiophenes. 

In spite of the limited data, there are good reasons to believe that the above sequence of increase in the transmission of substituent effects can be applied to the 2,4-disubstituted five-membered heterocycles (76JOC2350). In contrast to the 2,5-disubstituted heterocycles, however, the resonance effect transmission increases more dramatically. This increase is reflected in the value of the relative resonance effect contribution in various heteroaromatic systems 3,1-benzene and 4,2-thiophene (33%) < 4,2-furan (39%) < 2,4-furan... 

The assignment of thiophene resonances based on chemical shifts is complicated by the fact that the chemical shift of C-2 and C-5 is very similar to that of C-3 and C-4 (125.0 vs. 126.7 ppm). In general, it is found that there is a rough correlation between a substituent s shielding effect on benzene carbon atoms and its effect on the corresponding thiophene ring positions. 

As a consequence, pyrrole has a resonance hybrid that places the partial positive charge on the nitrogen atom and the partial negative charges on the four carbon atoms. For pyrrole, the resonance effect overpowers the inductive effect exerted by the nitrogen atom, whereas the inductive effect for furan and thiophene was a stronger force than their resonance effect. 

As far as furan and thiophene are concerned, their resonance effect is not as strong as the inductive effect. Therefore, their dipole moments are in the same direction of pyrrolidine. 

Furan has a dipole moment of 0.70 D, while thiophene has a dipole moment of 0.51 D. The dipole moments of furan and thiophene are in the opposite direction of pyrrole due largely to the relatively strong inductive effect caused by the oxygen and sulfur in relation to weaker resonance effects. In the case of pyrrole, as described in Chapter 2, the resonance hybrids of the molecule result in the inversion of the dipole. 

Carbon-13 resonances for twelve monosubstituted thiophen derivatives have been observed at 15.085 MHz. In 2-substituted thiophens the C-4 shifts fall into a narrow range of 5.2 p.p.m. (as do the C-5 shifts of 3-sub-stituted compounds), whereas the C-5 shifts are spread over a range of 21.1 p.p.m., indicating the sensitivity of the chemical shifts to resonance effects. There has been active interest in the n.m.r. spectra of thienyl-substituted phosphorus and silicon derivatives. The H n.m.r. spectra of... 

One of the more useful predicative applications of the relatively crude Hiickel method has been to illustrate quantitatively the effect of benzenoid annelation on the resonance energies of furan and thiophene. The results are summarized in Figure 1. As expected, thiophenes are more stable than the corresponding furans and 3,4-fusion results in less stable compounds than 2,3-fusion (77CR(C)(285)42l). 

The foregoing resonance structures describe the influence of the substituents on the 7r-electron distribution in the thiophene ring. Besides this effect the inductive effect of the substituents on the a-electi on system must be considered when discussing physical and chemical properties of thiophenes. 

Physical and chemical evidence supporting the theory mentioned in the foregoing will be given in the appropriate sections. Some predic-. tions may be mentioned here. From resonance structures (2, 3, 7, 8) it is inferred that the substituent effect in 2-substituted thiophenes should be parallel to that in the corresponding benzenes, the 3- and 5-positions may be considered as ortho and para positions and the 4-position as a meta position. It is, however, obvious that the effect of a —M- and a d-M-substituent are not simply reversed, as reso-... 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

It can be seen from resonance structures (2) to (4) that a — I — M-substituent deactivates the 3- and 5-position most strongly in electrophilic substitution. If this deactivation of the 5-position is strong enough to overcome the activating effects of the sulfur in the 5-position, substitution will be directed to the 4-position to an increasing extent. Tirouflet and Fournari studied the nitration of 2-substituted thiophenes of this type. The analysis was carried out polarographically, and the percentage of 4-isomer was as follows ... 

Electrophilic substitution in thieno[2,3-2>] and [3,2-6]thiophene systems is expected to proceed similarly to that in thiophene (see, e.g., Gronowitz and Marino ), a substitution occurring at position 2. Schomaker and Pauling were the first to discuss this effect in the case of thiophene. Challenger and Fishwick came to a similar conclusion about enothiophene 1 on the basis of the possible resonance forms. 

JOC2223>, as well as l,4-bis(/ rt-butylthio)-3,6-diphenyl- 185 and l,4-bis(/ r/-butylthio)-3,6-di-2-thie-nylthieno[3,4-c]thienophenes 186 <1998JOC163> were successfully synthesized as isolable compounds. These isolable thieno [3,4-f]thiophenes owe their stability to steric hindrance together with resonance and the electron-withdrawing effects of the substituents. 

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