Thiirane opening

Figure 1 Bond lengths (A) for the transition state of thiirane opening by MeS in the presence of HCO2H.

The isolation of thiirane oxides prepared by oxidation of the corresponding thiiranes has been somewhat problematic, and the isolation of thiirane dioxides from the direct oxidation of thiiranes or thiirane oxides has yet to be reported. Traditional methods of sulfide oxidation typically give low yields of impure materials when applied to thiiranes. Opening of the strained three-membered ring is the primary cause of these difficulties. 

Photolysis of 4-aminothiazolium salts in aqueous solution may cause ring opening, probably via a thiirane. to give a-cyano-jS-aminovinyl-disulfide or a-acylacetamidine derivatives (22). 

The facility with which oxiranes may be prepared and the ease with which they undergo ring opening with nucleophiles or electrophiles makes them useful synthons. Aziridines and especially thiiranes have been less widely exploited in this respect. 

A dramatic decrease in the magnitude of the magnetic susceptibility anisotropy is observed on going from thiirane to the open-chain analog, dimethyl sulfide, and has been attributed to non-local or ring-current effects (70JCP(52)5291). The decrease also is observed to a somewhat lesser extent in oxirane relative to dimethyl ether. 

Thiirane 1-oxide undergoes acid-catalyzed ring opening by ethanethiol to give ethyl 2-ethylthioethyl disulfide. Treatment of thiirane 1,1-dioxide with thiolate anions, sodium sulfide or thiourea gives /3-mercaptosulfinic acid derivatives (75S55). Thiiranium ions are attacked at carbon by most sulfur nucleophiles (79ACR282), but see Section 5.06.3.4.3 for exceptions. 

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

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. 

The nucleophilic attack of strong bases (e.g. hydroxide ion, alkoxide ions and carbanions) on either the a-carbon111 or the sulfur atom of the sulfone group99,113 of the thiirane dioxides is the initial key step that is responsible for the subsequent ring opening and further reaction. The formation of a three-membered a-sulfonyl carbanion is not observed in these cases (equation 11). 

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. 

Similarly, the stereospecific formation of cis-2-butene from cis-2,3-dimethylthiirane dioxide19 may be rationalized in terms of a stereospecific ring opening to give the threo-sulfinate 120 which, in turn, decomposes stereospecifically to yield the cis-alkene, hydroxide ion and sulfur dioxide73. The parent thiirane dioxide fragments analogously to ethylene, hydroxide ion and sulfur dioxide (equation 49). 

The mechanistic interpretation of the acid-catalyzed ring opening reaction of thiirane oxides125 is based on the push-pull mechanism162 with a transition state in which the bonded hydrogen atom plays a major role (see equation 59). 

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