Furan, resonance structures

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. ... 

When the resonance is between purely homopolar valence bond structures, the resonance energy is less than when ionic structures arc involved. This general rule may be illustrated by considering the resonance structures of furan and r/( pentadiene. In furan, /, the resonance involves... 

The role of heteroatoms in ground- and excited-state electronic distribution in saturated and aromatic heterocyclic compounds is easily demonstrated by a comparison of a number of heteroaromatic systems with their perhydro counterparts. In Jt-excessive heteroaromatic systems, because of their resonance structures, their dipole moments are less in the direction of the heteroatom than in the corresponding saturated heterocycles furan (1, 0.71 D) vs. tetrahydrofliran (2, 1.68 D), thiophene (3, 0.52 D) vs. tetrahydrothiophene (4, 1.87 D), and selenophene (5, 0.40 D) vs. tetrahydroselenophene (6, 1.97 D). In the case of pyrrole (7, 1.80 D), the dipole moment is reversed and is actually higher than that of pyrrolidine (8, 1.57 D) due to the acidic nature of the pyrrole ring (the N-H bond) In contrast, the dipole moment of n-deficient pyridine (9, 2.22 D) is higher than that of piperidine (10, 1.17 D). In all these compounds, with the exception of pyrrole (7), the direction of the dipole moment is from the ring towards the heteroatom [32-34]. 

The chemical reactivity of the ring is linked with the aromaticity. The preferred protonation site in furan was determined at 6-31G //3-2lG level <86JCS(P2)147>. The protonation of the oxygen atom is less favored and occurs at the Q, in agreement with experimental evidence. This result was also examined in terms of simple resonance structure arguments supported by MCSCF calculations. 

In furan chemistry, substitution reactions, such as alkylation, halogenation, sulphonation and nitration, occur regioselectively at C2 and/or C5 when these positions are not substituted, suggesting that la is the dominant resonance structure. Compared with its classical homologues 2 and 3, furan displays the lowest aromatic and the highest dienic character, as illustrated in Scheme 6.1 which also gives the two limiting structures of benzene and cyclopentadiene, respectively. 

Furan undergoes electrophilic aromatic substitution. Use resonance structures for possible arenium ion intermediates to predict whether furan is likely to undergo bromination more rapidly at C2 or at C3. 

There are more resonance structures for substitution at the 2 position of furan. The pathway for EAS should be lower in energy for substitution at the 2 position over the 3 position. 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The resonance energy of 0.93 v.e. for furane, C4H4O, arises from the same structures as for pyrrole. 

Furan, Pyrrole, and Thiophene.-—The carbon-heteroatom distances found in furan, pyrrole, and thiophene correspond to 5 = = 5%, 12 = = 6%, and 17 = = 10% double-bond character, respectively. Resonance of the normal structure I with structures of the types II and III (X = O, NH, S) is assumed to be responsible for this double-bond character, while excited structures characteristic... 

The polar character of the group R will also have a bearing on the formation and stabilization of the furan ring. In accordance with the theory of resonance, the classic structure... 

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. 

Other studies show the presence of a diversity of compounds other than limonoids as defenses in the tissues of Meliaceae. Woody tissues of Trichilia trifolia afforded three novel dolabellanes with flexible Curing structures. These substances were very active antifeedants in the Sitophilus bioassay (Ramirez et al., 2000). T. martiana seeds yielded large amounts of 2-((Z,Z)-6,9-heptadecadienyl)furan. T. hirta and T. americana bark have yielded novel steroids by insect bioassay-guided isolation and application of a nanoprobe nuclear magnetic resonance (NMR) technique for structure elucidation (Chaurest etal.,1996). Compounds isolated included hydroxyandrosta-l,4-diene-3,16-dione (Fig. 1.4) and derivatives. However, studies by Wheeler et al. (2001) suggest that other unidentified compounds may also be... 

---