Thiabenzenes, oxidation

Epoxides are believed to be intermediates in the conversion of the enol ethers of 1,3-diketones (ketoaldehydes are less satisfactory) into 2,4-substituted furans by means of the trimethylsulfonium ylids. No epoxides could be isolated, however, nor was it necessary to use acid to effect cy-clization. Methoxydimethylsulfonium ylids were less efficient and tended to produce thiabenzene oxides instead, so Scheme 8 remains speculative.57 The use of thioenols instead of 1,3-diones is advantageous.233... 

Methyl-3,5-diphenyl-l//-thiopyran 1-oxide forms red air-stable complexes with carbonyls of chromium, molybdenum and tungsten. (78CB1709). X-Ray diffraction measurements on these compounds confirm the non-planarity of the thiabenzene oxide nucleus and reinforce the evidence for high inversion barriers at sulfur. These complexes are found in isomeric forms with either the sulfur-oxygen bond axial to the half-chair conformation adopted by the ring, or the S- methyl group axial the complexes cannot be interconverted. 

Thia... s. a. Thio..., S-Heterocyclics Thiaareninm salts s. Thiopyrylium salts Thiabenzene oxides... 

Thiabenzene Oxide Complexes.—Reactions of l-methyl-3,S-diphenylthiabenzene-1-oxide (L) with (MeCN)aM(CO)3 (M = Cr, Mo, or W) have afforded complexes of the type LM(CO)s crystal-structure analyses have established that the cyclic ligand is non-planar with only the five carbon atoms bonded to the metal atom. ... 

Evidence has been provided that photolysis of 2/f-thiopyran 1,1-dioxides and their benzo-derivatives does involve sulphene intermediates. Details of the synthesis of a thiabenzene oxide have appeared. ... 

Although not a sulphone, the (formally) sexivalent sulphur compound (41) is included here a new synthesis for thiabenzene oxides of this type has been reported. ... 

Thiabenzenes should be considered as sulfonium betaines. They react readily with acids to give mixtures of 2H- and 4/f-thiinium salts, behave as dienes with dienophiles (Equation 9), and can be oxidized to sulfoxides. The sulfimide (450) is an aza analogue of a thiabenzene and it is oxidized by KMn04 to the corresponding sulfoximide. 

The variable valency of sulfur allows the formation of S-oxides (11) which unlike thiabenzene 1-oxides are non-aromatic and exist in half-boat conformations (78CC197). On the other hand, acid treatment of 6-hydroxy-6H- 1,2-oxazines (12) yields cations (13) which are analogous to pyrylium salts (74ZOR1513). 

Thiabenzene I-oxide. The reaction of 1,3-disubstituted 2-propynones such as l,3-diphenyl-2-propyn-I-onc (1) with dimethyloxosulfonium methylide in DMSO at 16.5° gives l-mcthyl-3,5-disubstituted thiabenzene I-oxides in the case of (I) the product is l-methyl-3,5-diphenylthiabenzene l-oxide (3), obtained in 76% yield. If the reaction is carried out in THF-DMSO at - 8°, the intermediate (2) can be isolilted it probably arises by Michael addition of the reagent to (I). 

Thiabenzene 1-oxides (e.g., 102) have also figured in the above discussion. Earlier papers313-318 did not rule out the possibility of aromatic conjugation in these compounds, but NMR data are suggestive of ylide character, and bonding of the ylide type has recently been firmly proposed by Hortmann and Harris.313 314 Somewhat related systems are the anions derived from cyclic sulfones such as 103 and 104. Pagani et aZ.319-321 have... 

A novel approach to thiabenzenes has been developed in the course of attempts to synthesize 1-alkylthiabenzenes. Reduction of the known > thiabenzene 1-oxide (10) with trichlorosilane gave a mixture of the thiopyrans (11) and (12), from which the 2/7-isomer (11) was isolated and methylated with MeI-AgBF4. Deprotonation of the resulting sulphonium salt (13) with t-butyl-lithium in pentane-DMSO gave an orange solution of the thiabenzene (14), which regenerated the salt (13) when treated with fluoroboric acid. The thiabenzene decomposed upon attempted isolation. 

A notable feature of the n.m.r. spectrum of the thiabenzene (14), prepared in situ in [ HglDMSO, is the high-field resonance of 2-H, 4-H, and 6-H. Addition of CDgCOOD to the thiabenzene in [ He]DMSO caused rapid H-D exchange at positions 2, 4, and 6, giving the deuteriated salt (15). Addition of a limited amount of deuterium oxide to a solution... 

Full details have appeared of the synthesis of thiabenzene 1-oxides from the reaction of acetylenic ketones with dimethyloxosulphonium methylide (Scheme 1), following a preliminary account. By working at low temperatures with THF-DMSO solutions, intermediate allylides (22)—(24) could... 

Table Chemical shifts (8) in the H n.m.r. spectra of the thiabenzene 1-oxides (16)—(20). Solutions in CDCI3 TMS as internal reference...

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