Thiiranes oxidation

Acid-catalyzed ring opening of thiirane oxides.423... 

Reactions of thiirane oxides with metal salts.424... 

Thiirane oxides (3 x = 1) were rather rare and not well characterized until about 20 years ago20. Since 1965 synthetic methods for their preparation have been consistently and systematically explored2. They are rather thermodynamically stable compounds— compared to their closely-related thiirane dioxides—provided they have an anticonfiguration with respect to the substituents and the sulfinyl oxygen. Also they are more resistant than the corresponding sulfones toward ring opening by either nucleophiles or electrophiles. 

A unique characteristic feature of the cyclic three-membered ring sulfones and sulfoxides is the dramatic increase in the length of the carbon-carbon single bonds and the carbon-carbon double bonds in the series of thiirane-thiirane oxide-thiirane dioxide (20a -> 16a -> 17a), and thiirene-thiirene oxide-thiirene dioxide (21 -> 18a -> 19b). 

Interestingly, the oxygen-17 chemical shifts for the thiirane oxide (16a) and thiirane dioxide (17a) were found to be 71 and 111 ppm (downfield from natural-abundance 170 in HzO), respectively. The oxygen-17 shift reveals that this oxygen is the most highly shielded oxygen atom so far reported80,70. 

It has been generally assumed that thermal decomposition of thiirane oxides proceeds stereospecifically to the corresponding olefins by elimination of sulfur monoxide, possibly through a concerted nonlinear chelatropic reaction96 with retention of configuration of the liberated olefin. 

Based on extensive studies associated with the Ramberg-Backlund rearrangement15 and its mechanism2 16"19,111,112, including the treatment of thiirane oxides with bases, the following conclusions emerge ... 

In contrast to thiirane oxides, the electrophilic oxidation of thiirene oxides to thiirene dioxides is feasible, probably because both the starting material and the end product can survive the reaction conditions (equation 21). 

To what extent the above suggests that the sulfoxide sulfur of thiirene oxides is more nucleophilic than that of thiirane oxides remains an open question. 

There are several reactions in which the sulfoxy oxygen exhibits its nucleophilicity, the most noticeable being the thermal rearrangement of thiirane oxides (in the presence of a suitable disposed /J-hydrogen) to allylic sulfenic acids2,63,105 (see equation 9 in Section III.C.l). 

Theoretical considerations (previously discussed in Section III.B.3) predict the oxygen moiety in the sulfoxide function of thiirene oxides to be relatively nonreactive12, that is, less nucleophilic than the sulfoxy oxygen of either thiirane oxides or ordinary acyclic sulfoxides. 

A systematic study632 in which substituted thiiranes were oxidized to the corresponding thiirane oxides determined the geometrical position of the oxygen atom by complete NMR and microwave analysis. 

Mono- and cis-di-substituted thiirane oxides can theoretically exist in the syn- (s) and anti- (a) configurations shown below ... 

To date, several well-established methods are available for the convenient preparation of thiirane oxides, the two main ones being the controlled oxidation of thiiranes63a and the reaction of sulfenes with diazoalkanes636. 

All the asymmetric thiirane oxides which have been obtained through this procedure are mixtures of the two possible cis- and trans- syn- and anti-) configurations, but the antiisomer predominates. 

Any attempt to separate the two components by the usual chromatographic methods failed owing to the instability of the thiirane oxides, which easily lose sulfur monoxide to give the corresponding olefins152. 

The above reaction is a convincing example of an intermolecular hydrogen abstraction leading essentially to the same result as obtained in the pyrolysis of alkyl-substituted thiirane oxides through an intramolecular /(-elimination of hydrogen. 

The above explains the key roles of (a) the nucleophilicity of the nucleophile (b) the substituent(s) (c) the polarity of the reaction medium and (d) the the bulkiness of the nucleophile, in determining the regio- and stereo-specificity of the reaction. The reaction of alkyl chloromethyl ethers with thiirane oxides to give sulfenic esters128 appears to be mechanistically analogous. 

Whereas acyclic sulfoxides form complexes with various metal salts, thiirane oxides react with copper(II) chloride or bromide163 in benzene at room temperature to give the thiolsulfonate 146a. In alcoholic solution below 0 °C the major products are sulfinates (149). Similar results are obtained in the reaction of thiirane oxides with ethanesulfinyl chloride163 as summarized in equation 60. 

Thus the reaction of the three geometrical isomers of 2,4-hexadiene with thiirane oxide afforded the three related 3-thiolene 5-oxides 154 depicted in equation 62104... 

Interestingly, preliminary calculations (3-21G basis set) estimate the AH of the triplet SO (and ethylene) generation from the parent thiirane oxide (16a) to be about 18kcalmol-1 166. 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

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