Pyrrole delocalization energy

A quite different base-catalyzed reaction was discovered by French workers in their studies of 2,3-disubstituted furans and pyrroles.224 They prepared a furooxepinone by standard methods and treated it with hot potassium ethoxide in ethanol to obtain a furoquinone. Scheme 55 shows one way in which this result can be rationalized the salient feature is that the benzene, and not the furan ring is disrupted, despite the large discrepancy in delocalization energies (150 vs 80 kJ mol ). 

Why is the delocalization energy of pyrrole (21 kcal/mol) greater than that of furan (16 kcal/mol) ... 

In Section 8.6, we saw a compound s delocalization energy increases as the resonance contributors become more stable and more nearly equivalent. The delocalizahon energies of pyrrole, furan, and thiophene are not as great as the delocalizahon energies of benzene or the cyclopentadienyl anion, each a compound for which the resonance contributors are all equivalent. 

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

Examine pyrrole s highest-occupied molecular orbital (HOMO) to see if your can predict the most favorable protonation site. Which of the pyrrole s conjugate acids (N protonated, C2 proto noted, C3 proto noted pyrrole) is lowest in energy Examine electrostatic potential maps to see if the lowest-energy form is also that in which the positive charged is best delocalized. Rationalize your result using resonance arguments. What should be the favored substitution product ... 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

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