Thiirane dioxides reactions

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

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). 

The reaction of thiirane dioxides with reagents that are weak nucleophiles but strong... 

The elimination of sulfur dioxide from thiirane dioxides leading to the corresponding alkenes is not the only result of base-induced reactions other products are also formed. This fact raises the question of the mechanistic pathway of this reaction. In general, the thiirane dioxide is treated with a large excess of the base in an appropriate solvent for several hours at room temperature or below. Bases commonly used are 2n NaOH (in water), NaOCH3 (in methanol), t-BuO-K + (in f-BuOH) and BuLi (in tetrahydrofuran) or KOH-CCU (in t-BuOH)16-19"112 113. 

Similarly, the reaction of the parent thiirane dioxide, the 2-chloro- and 2,3-cis-dimethylthiirane dioxides with either Grignard or alkyl lithium reagents, has been studied extensively. The fair-to-good yields of the sulfinates (62) obtained (48-82%), accompanied by ethylene (or the corresponding alkenes for substituted thiirane dioxide), have been interpreted in terms of initial nucleophilic attack of the base on the sulfur atom as depicted in equation 17116. 

Actually, thiirane dioxides (17) have so far never been isolated in these reactions cis- and trans-olefins were the main products, and all attempts to obtain the three-membered ring system and prevent the loss of SO 2 failed. Hence, the method can be used only for the in situ formation of intermediates. 

In a typical procedure61144 the sulfonyl chloride in ether is added to an etheral solution of the diazoalkane and triethylamine. Filtration and evaporation gives the relatively pure thiirane dioxide. Further purification can be easily achieved by recrystallizations preferentially below room temperature in order to avoid fragmentation of the product into sulfur dioxide and the olefin. In general, when the temperature of the above reaction is lowered, the yields are improved without a drastic decrease in reactivity144. Many thiirane dioxides have been successfully synthesized through this method and a detailed list of them can be found elsewhere2. 

The use of excess diazoalkane in its reaction with sulfur dioxide will necessarily lead to symmetrically substituted thiirane dioxides. When monoalkyl or monoaryl diazoalkanes are used, mixtures of cis- and trans-isomers are formed18-19-99. 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

A nucleophilic attack of the hydroxide (or the alkoxide) ions on the sulfur atom of the thiirane dioxide ring to give sulfonic acids or similar intermediates, which then decompose to alkenes and bisulfite ion, has been suggested for these reactions " - . 

Treatment of 19b with phenylmagnesium bromide gives diphenylacetylene (66) and the salt of benzenesulfmic acid Lithium aluminium hydride reacts with 19b similarly. These ring-opening reactions are similar to the reactions of organometallic reagents with the analogous thiirane dioxides (equation 17 above). 

The Ramberg-Backlund Reaction. a-Halosulfones undergo a related rearrangement known as the Ramberg-Backlund reaction.91 The carbanion formed by deprotonation gives an unstable thiirane dioxide that decomposes with elimination of sulfur dioxide. This elimination step is considered to be a concerted cycloelimination. 

Alkylidene sulfenes (75), generally prepared by the dehydrohalogenation of alkylsulfonyl chlorides, add readily to electron-rich multiple bonds. For example, with enamines, the thietane dioxide (e.g., 76) is formed diazoalkanes yield thiirane dioxides (episulfones) and imines (Schiff bases) afford 1,2-thiazetidine 1,1-dioxides. There are available numerous reviews of sulfenes, including cycloaddition reactions.102... 

Cheletropic reactions, in which a single atom is added or extruded, comprise a special case of cycloaddition reactions. Figure 14.4 displays correlation diagrams for two typical cheletropic reactions, the loss of SO2 from a thiirane dioxide (Figure 14.4a) and the loss of CO from a norbornadienone (Figure 14.4b). The addition of a carbene to an olefin is another example which is discussed below (Figure 14.9a). 

The transformation of an a-halo sulfone into an alkene takes place in three steps (Scheme 7). Step 1 involves reversible formation of the a -anion. Under the basic reaction conditions, there is a rapid equilibration of the a-halo sulfone with its a- and a -anions. Only the a -anion leads to step 2 which is intramolecular displacement of a halide ion to give a thiirane 1,1 -dioxide, generally as a mixture of cis and trans isomers. This is the rate-limiting step. Finally, in step 3, the thiirane dioxides lose SO2 to give the stereoisomeric alkenes. This transformation can take place via two distinct pathways one is thermal, while the other involves base catalysis. Both pathways are stereospecific trans- or c/5-thiirane 1,1-dioxides give only ( )- or (Z)-alkenes, respectively. Consequently, the stereochemical outcome of the overall reaction is determined at the ring-forming step 2. 

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