Furan aromatic stabilization

Cyclic compounds that contain at least one atom other than carbon within their ring are called heterocyclic compounds, and those that possess aromatic stability are called het erocyclic aromatic compounds Some representative heterocyclic aromatic compounds are pyridine pyrrole furan and thiophene The structures and the lUPAC numbering system used m naming their derivatives are shown In their stability and chemical behav lor all these compounds resemble benzene more than they resemble alkenes... 

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

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... 

The most extensive research on furanic polyamides is recent (33) and deals essentially with furanic-aromatic structures, although an important effort was also devoted to all-fiiranic compositions. The reaction of the diacid 11a with various aromatic diamines leads to high-molecular weight polymers with good thermal stability and ciystallinity. Structure 23, obtained with p-phenylenediamine, exhibited features resembling closely those of polyaramides ... 

The electrochemical properties of 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[l,2-A4,5- ]difuran 72 have been investigated by cyclic voltammetry in the search for new materials with enhanced conducting properties <2000JOC2577>. Upon single-electron reduction the central quinonoid ring gains the aromatic stability of benzene, whereas the fused heterocyclic part loses the aromaticity of the furan nucleus. 

Other elements can also participate in the formation of aromatic species. Furan, pyrrole, and thiophene are all aromatic molecules. This is due to the fact that if the heteroatom is sp2 hybridized, then a doubly occupied p orbital interacts with the carbon 2p orbitals to give an MO array which contains six it electrons and is aromatic. Note that in the development of the MO diagram for these systems the identity of the heteroatom is not important. It is only important in determining the magnitude of the aromatic stabilization. 

Pyrrole is believed to be more aromatic than furan, with an aromatic stabilization energy estimated to be 100-130 kj mole-1, thus only with such extremely powerful dienophiles as tetrakis(trifluoromethyl)Dewar thiophene were the IEDA adducts isolated. Therefore, several attempts to achieve an IEDAR with pyrroles using high pressure have been made [9-14]. 

Another theoretical criterion applied to estimation of aromaticity of homo- and heteroaromatic ring system is aromatic stabilization energy (ASE). Based on this approach, the aromatic sequence of five-membered ring systems (ASE in kcal mol-1) is pyrrole (20.6) > thiophene (18.6) > selenophene (16.7) > phosphole (3.2) [29], According to geometric criterion HOMA, based on the harmonic oscillator model [30-33], thiophene is more aromatic than pyrrole and the decreasing order of aromaticity is thiophene (0.891) > pyrrole (0.879) > selenophene (0.877) > furan (0.298) > phosphole (0.236) [29],... 

A recent study of proton transfer from rhenium Fisher-type carbine complexes (13) shows that the reactions lead to the formation of an aromatic product (14), following the same rules as reactions that lead to the formation of products stabilized by simple resonance. The conjugate bases of these carbine complexes represent aromatic heterocycles, i.e., substituted furan, selenophene, and thiophene derivatives, respectively. The aromatic stabilization of these heterocycles is known to follow the order furan < selenophene < thiophene (Scheme 1) [43],... 

Regarding the closed-ring isomers, the difference in behavior between those diarylethenes with furan, thiophene, or thiazole rings and those with pyrrole, indole, or phenyl rings agrees well with the theoretical prediction that the thermal stability depends on the aromatic stabilization energy of the aryl group.1201... 

The differing amounts of aromatic stabilization for benzene, pyrrole, furan, and thiophene demonstrate that aromatic stabilization occurs in varying degrees, depending on the structure of the compound. Some compounds have a large aromatic stabilization that dramatically affects their stabilities and chemical reactions. Others may have only a small stabilization and have stabilities and reactions that are more comparable to a normal alkene. 

B3LYP/6-31G calculations on the tautomers of succinic anhydride 105 show that the enols 106 and 107 are disfavored by 24.1 and 41.1 kcalmoF1, respectively. This is in spite of the aromatic stabilization in the furan 107 and in line with the general instability of anhydride enols . For 2-hydroxythiophene 110, similar calculations show the following relative stability 108 (0.0kcalmol-1), 109 (4.23 kcalmoF1), and 110 (15.72kcal mol J) . 

The merits of NIGS, obtained by an upgraded computational protocol, as a measure of the aromaticity, nonaromaticity, or antiaromaticity of, inter alia, planar five-membered heterocycles (including furan) have been discussed. The subtypes NICS(0),izz and NICS(l)zz appear to perform most reliably in providing a linear relationship with aromatic stabilization energies (ASEs) <20060L863>. 

Photochromic reactions of cw-l,2-diarylethenes are the extension of photochemical electrocyclization of cw-stilbene, which yields dihydrophenanthrene. When the aromatic stabilization energy (aromaticity) of at least one aryl group is low (such as furan, thiophene, benzothiophene) and the nonhydrogen substituents are located on the ring-forming carbon atoms, the thermally irreversible photochromism is observed. When the aromaticity of both aryl groups are high (such as phenyl, indolyl, or pyrryl), the diarylethene is thermally reversible [31]. 

The aromatic character of benzo[6]-fused heterocycles was evaluated through uniformity of their ring bond order (Table 19). It is very interesting to mention that little aromatic stability for benzene is lost by [6] fusing with furan, pyrrole. 

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. 

By using an aromatic aldehyde carrying an electron-releasing group the intermediate cation can be stabilized. This is the basis of the widely-used Ehrlich colour reaction for pyrroles, indoles and furans which have a free reactive nuclear position (Scheme 21). 

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