Sulfur compounds heteroatoms, five-membered rings

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. 

The neighboring group effect becomes particularly apparent within the SaS series relative to the oxidation of a simple sulfide. The generally lower oxidation potentials for the selenium- and phosphorus-containing compounds not only prove participation of these heteroatoms but also reflect their lower ionization potentials compared with sulfur. As discussed before, the best orbital overlap for formation of the 2(r/l(r three-electron bond is obtained for the five-membered ring (n = 3), evidenced by the lowest oxidation potentials and the most blue-shifted absorptions. (Remember that absolute optical transition energies may... 

Sulfur containing heterocycles are also common Compounds m which sulfur is the heteroatom m three four five and six membered rings as well as larger rings are all well known Two interesting heterocyclic compounds that contain sulfur-sulfur bonds are hpoic acid and lenthiomne... 

Only a few examples of ring syntheses by transformation of another ring have been published, as exemplified by the preparation of the sultines 168 by ring enlargement of five-membeted thiolane 1-oxides 166 (cf Section 8.10.9.2.3) and the 3,6-dihydro-l,2-dithiins 202 by catalytic transformation of vinylthiiranes 201 (cf Section 8.10.9.4.2). Because possibilities to synthesize six-membered rings with oxygen and/or sulfur as heteroatoms in 1,2-positions are rather limited, these reactions have been covered already in Section 8.10.9 together with alternative syntheses from alicyclic compounds. 

Seven-membered rings with four and more heteroatoms attract attention because of their biological activity. An important class of compounds is that of the seven-membered cyclic polysulfides with five sulfur atoms, which are well known as lenthionine and varacin. 

The reaction has been the subject of reviews,316,45 from which it will be seen that the reaction can be applied to synthesis of five- and six-membered oxygen-heterocycles, sulfur-heterocycles, bicyclic compounds, and compounds containing an additional heteroatom in the ring. 

As in five-membered heterocycles, ring strain in six-membered heterocycles is of little or no importance. Pyran and thiine (thiopyran), with an oxygen or sulfur atom, respectively, and pyridine, with a nitrogen atom, are the parent compounds of six-membered neutral heterocycles with one heteroatom and the maximum number of noncumulative double bonds. In contrast to pyran and thiine, pyridine exists as a cyclic conjugated system. However, by (formal) abstraction of a hydride ion, both pyran and thiine can be converted into the corresponding cyclic conjugated cations, i.e. the pyrylium and the thiinium ions (thiopyrylium ion). 

Furan and thiophene rings are electron-rich, five-membered aromatic systems. The donation of a pair of electrons by the heteroatom into the 7t-system is essential to the aromaticity of these compounds. Due to the highly electronegative oxygen heteroatom, the aromaticity of hiran is comparable to pyrrole. The aromaticity of benzofuran is comparable to indole for similar reasons. On the other hand, the aromaticity of thiophene resembles benzene due to the decreased electronegativity of the sulfur heteroatom. For similar reasons, the aromaticity of benzothiophene is comparable to naphthalene. ... 

Chapter 21 introduces aromatic hydrocarbons and their unique chemistry. In another class of aromatic compounds, heteroatoms replace one or more of the ring carbons. These compounds are collectively known as heterocycles or heterocyclic aromatic compounds, and they comprise a class of compounds so large that an entire course is easily built around their chemistry. Heterocycles are seen in several places in this book, including a brief introduction to their nomenclature in Chapter 5 (Section 5.6) and in Chapter 8 (Section 8.9). The most common heterocycles include five- and six-membered monocyclic derivatives that contain nitrogen, oxygen, or sulfur. Several important bicyclic derivatives contain nitrogen. The use of heterocycles in medicine and industry is extensive. This chapter will expand the aromatic chemistry from Chapter 21 and introduce the world of heterocyclic chemistry. 

---