Thienothiophenes aromaticity

In some sulfur extrusions from 2,7-di-/ Tt-butylthiepins a byproduct containing two sulfur atoms is obtained, along with the aromatic compound. The byproduct has been characterized as a thienothiophene by single crystal X-ray analysis.92... 

The aromatic 7r-electron system of thienothiophenes and related compounds containing two condensed rings comprises electrons from three carbon-carbon double bonds and unshared electron pairs from two heteroatoms, and thus is similar to that of naphthalene. 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

The presence of benzo[6]thiophene in commercial naphthalene, its possible contamination with isomeric thienothiophenes 1 and 2, and their ability to poison aromatic hydrogenation catalysts led Maxted and Walker to develop detoxification by a preliminary short hydrogenation, in which thienothiophenes 1 and 2, and benzo[6]-thiophene are adsorbed on the catalyst. This is followed by their hydrogenation products that can easUy be oxidized with hydrogen peroxide or permolybdic acid to nontoxic sulfones subsequent hydrogenation of the aromatic hydrocarbons is then performed as usual. 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Semiempirical methods of calculation with consideration of all valence electrons have been used only recently but already have given results on the reactivities of some aromatic and heteroaromatic com-pounds. " Thus, to analyze the reactivities of thiophene and the isomeric thienothiophenes 1-3 to electrophilic substitution, the semiempirical SCF LCAO MO method CNDO/2 was used, taking into account all valence electrons.The 3s, 3p, and 3d orbitals have been taken into account for the sulfur atom. Tlie reactivities were estimated from the difference between bond energies of the initial and the protonated molecule (in a complex). ... 

In 1948 Maxted and Walker studied the detoxification of catalyst poisons in the hydrogenation of aromatic hydrocarbons and found that the isomeric thienothiophenes 1 and 2 could be converted into the sul-fones of fully hydrogenated thienothiophenes 1 and 2, which do not poison the catalysts. This conversion is performed by brief preliminary hydrogenation and subsequent oxidation by hydrogen peroxide or per-molybdic acid. However, no data on the isolation or foe properties of these disulfones are available. It has been reported that direct oxidation of thienothiophenes 1 and 2 does not produce sulfones. 

The three isomeric classical thienothiophenes (3), (7) and (8) are known and do exhibit varying degrees of aromatic stabilization. 

Polymers having a central core consisting of [2,3-b]-thienothiophene were prepared having a M > 6000 daltons and M > 9000 daltons and used as semiconductors or charge transport materials in electronic devices. By varying the aromatic or ahphatic content of this material, a X ax between 380 and 462 nm was obtained. 

The least stable of the four isomeric thienothiophenes is thieno[3,4-c] thiophene, which has defied all efforts towards its isolation. Its tetraphenyl derivative (28) has been synthesized and its PE spectrum, obtained by Schweig and coworkers34, is interpreted as verifying that all of these nonclassical condensed thiophenes are aromatic like thiophene itself. The HOMO of 28 is of au symmetry and appears at the very low IE of 6.19 eV. When referenced against its counterpart in naphthalene, it is shown to be missing stabilization from the n orbitals, which is the explanation for its high reactivity. 

It seems that the thienothiophene 6.5 has not been tested as such in azo coupling with aromatic diazonium salts. 

Poly(thienothiophenes), low band-gap conjugated polymers with a polythiophenelike chain where an aromatic thienothiophene moiety is fused to each thiophene ring, were studied using Raman spectroscopy and photoinduced IR adsorbtion (2002JPC(B)3583). 

Recently Schleyer et al. studied the aromaticity of bicyclics, furofuran, thienothiophene, benzofuran. 

Chart 4. Aromaticity Indices HOMA said NICS of Furofuran, Thienothiophenes, Benzofuran, and Benzothiophene... 

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