Thiophene 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... 

Whereas the initial hydrogenation both breaks a K bond, as well as destroys any aromatic stabilization, the second hydrogenation only breaks the k bond. Is this difference large as in benzene (see discussion at right), or is it much smaller or neglible Is thiophene aromatic ... 

The planar form of phosphole is a first-order saddle point on the potential energy surface, 16—24 kcal/ mol above the minimum (at different levels of the theory). ° (The calculated barriers are the highest at the HF level, which underestimates aromatic stabilization of the planar saddle point, while the MP2 results are at the low end.) It has been demonstrated by calculation of the NMR properties, structural parameters, ° and geometric aromaticity indices as the Bird index ° and the BDSHRT, ° as well as the stabilization energies (with planarized phosphorus in the reference structures) ° and NIGS values ° that the planar form of phosphole has an even larger aromaticity than pyrrole or thiophene. 

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]. 

The fact remains that selenophene and its derivatives have aromatic character. This statement is, however, not easy to substantiate with quantitative data mainly because there are too many ambiguities for parent five-membered ring heterocycles. Some aromaticity indices for five-membered rings with one heteroatom, among them selenophene, were summarized and estimated values for thiophene and selenophene were similar [26], The data obtained from photoelectron ionization energies show that the aromatic stabilization in thiophene is slightly larger than for selenophene [27],... 

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. 

The NICS of each ring, as a criterion of aromaticity, has been used to explain the stability order of benzo[/)]thio-phene and its isomer. The results indicate that the benzene ring is aromatic in all the systems. The five-membered ring of benzo[. ]thiophene is also aromatic, whereas in benzo[r]thiophene it is nonaromatic. This could be an explanation of the stability of the former molecule. The MOS and the condensed Fukui functions derived from the electronic-structure calculations explain the expected electrophilic substitution of these compounds. The theoretical structure, ionization energies, order of aromaticity, stability, and reactivity are in good agreement with the experimental results <2003T6415>. 

Dihydrobenzo[3]thiophene-2-one is considerably more acidic (pA a = 8.82) than the corresponding benzofuran derivative this reflects the greater aromatic stabilization of the conjugate base of the former <2006JOC8203>. 

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]. 

Because of the high aromatic stabilization of the thiophene nucleus and the tendency of sulfur to poison the catalyst, the hydrogenation of thiophenes is extremely difficult. In general, harsh conditions and high catalyst concentrations are required53. Hydrogenation occurs with excellent cis diastereoselectivity. as shown in the reduction of the biotin precursor 154. 

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