Furan resonance effect

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... 

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... 

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. 

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). 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

An unexpected deshielding is found in 2-halogenoaryl furans such as (112) where the adjacent proton is noticeably affected (Table 6). No other substituent behaves like this, and halogens in other positions are likewise devoid of effect. Perhaps for some reason the conformation is preferred that has the halogen atom close to the proton. This would help to explain the fact that a second halogen atom at the aryl 6-position restores the resonance to its normal position, and that the effect is temperature dependent (71NKK1206). 

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