Sulfur as heteroatom

Sulfur as Heteroatom. Thiols and sulfides are protonated on sulfur in superacid media and give mono- and dialkylsulfonium ions, respectively.136 Thio-carboxylic acids, 5-alkyl esters, thioesters, dithioesters, and thiocarbonates in similar media also form stable protonated ions541,647 such as cations 348-353. 

Minkwitz and co-workers have recently obtained the X-ray crystal structure of protonated thiocarbonic acid, protonated (9,5-dimethyl dithiocarbonate, and protonated dimethyl trithiocarbonate. Protonated thiocarbonic acid 354 has C—S bond lengths of 1.683-1.708 A.662 The small differences imply a nonequal distribution of the positive charge over the S atoms. All hydrogen atoms have trans positions and they are 

Sulfur-stabilized heteroaromatic species such as 356 and 357 are also known.566 

A series of thioketones have also been protonated to form carbosulfonium ions 358541,542 and studied by 13C NMR spectroscopy.665 It was found that deshielding of the thiocarbonyl carbon is much less pronounced than that of ketones. This shows that there is significant mercaptocarbenium 358b contribution to the overall protonated thioketone structure. However, this is not the case in the case of protonated ketones. 

Olah and coworkers have used ab initio methods (MP2/6-311+G level) to calculate the structures of diprotonated superelectrophilic3-5 formaldehyde and thioformaldehyde dications.666 In both cases, the heteroatom-diprotonated structure 

S-methylated and O-methylated dimethyl sulfoxides 147 and 148 have been very extensively studied180.  

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Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

With sulfur as heteroatom (a) Zamaev lA, Shklover VE, Ovchinnikov YE, Struchkov YT, Astankov AV, Nedel kin VI, Sergeyev VA (1989) Acta Cryst, Sect C 45 1531 ... 

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. 

Table 4.66. l3C Chemical Shifts of Heterocycles with sp2 Ring Carbons (<5C in ppm). Part 1 Oxygen and Sulfur as Heteroatoms [416-424]. 

Oxygen has been used as an example of an exocylic substituent in heterocycles (7)—(12) but other groups such as imine, thiocarbonyl, and substituted methylene are common. The majority of 6-6 heterocycles are associated with nitrogen, oxygen, and sulfur as heteroatoms other heteroatoms such as selenium and phosphorus are less common. 

Mass spectrometry has been widely used for the elucidation of the structures of macrocycles with both oxygen and sulfur as heteroatoms. However, in most cases only molecular compositions are reported and detailed fragmentation patterns are scarce. The mass spectra of macrocyclic polyethers have been evaluated and fragmentation patterns compared <76JCS(P2)206>. Within the series of compounds studied, molecular ion peaks were always observed. Only small differences in the mass spectra of compounds with different aromatic ring substitutions were observed. A protonated... 

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