Thiirene dioxides synthesis

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

The situation is even more problematic in the unsaturated series the elusive thiirenes cannot serve as starting materials for the synthesis of thiirene oxides 18 via direct oxidation, and the laborious synthetic method used to prepare the most commonly known and studied aryl-substituted thiirene oxides 18 does not make the latter attractive as starting materials for preparing the corresponding thiirene dioxides . Fortunately there are much better and versatile methods available for the synthesis of the sulfones 19 (equation 35). 

Most thiirene dioxides (and oxides) have been prepared through a modified Ramberg-Backlund reaction as the last crucial cyclization step, as illustrated in equation 40 for the benzylic series . Synthesis of thiirene dioxides requires two major modifications of the originally employed reaction first, the inorganic base has to be replaced by the less basic and less nucleophilic triethylamine - and second, the aqueous media has to be substituted by an aprotic organic solvent (e.g. methylene chloride). Under these mild reaction conditions the isolation of aryl-substituted thiirene dioxides (and oxides) is feasible . In fact, this is the most convenient way for the preparation of the aryl-disubstituted three-membered ring sulfones and sulfoxides. ... 

The use of thiirene dioxide 292 as a dipolarophile in the synthesis of thiazine 1,1-dioxides was already mentioned in CHEC(1984) <1975CL1153>. The same compound also reacts with nitrile ylide 293 to afford 294 <1984JOC1300> (Equation 97). 

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

Oxathiole 3,3-dioxides may be obtained in a similar manner. When dibromide (112) is treated with triethylamine, the expected thiirene 1,1-dioxide is not formed but rather a nucleophilic displacement via the enolate oxygen atom occurs, as depicted in Scheme 26. Removal of the bromine and benzoyl moieties yields an efficient synthesis of 5-phenyl-l,3-oxathiole 3,3-dioxide (74JOC2722). This work shows that the previously reported synthesis of 2,3-dibenzoylthiirene dioxide from (112) is incorrect. 

There are three main synthetic methods for synthesis of thiiranes C-S bond-forming reactions, C-G bond-forming reactions, and two bond-forming reactions. These methods are described in detail in CHEC(1984) and CHEC-11(1996), and the references until 1995 are cited therein <1984CHEC(7)131, 1996CHEC-11(1)174>. Described here are methods published after 1996 for synthesis of thiirane, thiirane-Toxide, thirane-1,1-dioxide, and thiirene-1-oxide. 

A unique approach to the synthesis of fused-ring thiiranes and thiirenes involves the utilization of cycloaddition methodology. The use of diphenylthiirene oxide and the corresponding dioxide as dieneophiles in [2 + 2], [4 + 2], and 1,3-dipolar cycloadditions is well established. Often the initial... 

A-2-Unsaturated three-membered systems are unknown as stable molecules because they would have a four-electron 7t-system, and thus be antiaromatic. 1//-Azirines occur as reactive intermediates and there is evidence for the existence of 2-thiirene in a low-temperature matrix. Azirines, by contrast, are well-known stable compounds. Thiirene A.A-dioxides are also stable molecules, probably best likened to cyclopropenones. The chemistry of saturated three-membered heterocycles is, however, very extensive - epoxides (oxiranes), and to a lesser extent, aziridines are important intermediates in general synthesis. 

Formation.—Experimental details are now available for the synthesis of diphenylthiiren 1,1-dioxide (32) from a,a -dibromodibenzyl sulphone by treatment with triethylamine. It is also formed by base-catalysed dehydrohalogenation of a,a-dichlorobenzyl benzyl sulphone, whilst similar treatment of a,a -dibromobenzyl sulphoxide (18) afforded diphenylthiiren 1-oxide. Alkyl-substituted thiiren 1,1-dioxides (33), (34), and (35), which could not be synthesized by this method, were obtained by dehydro-bromination of the appropriate bromothiiran 1,1-dioxides (36), (37), and (38), catalysed by triethylamine or l,5-diazabicyclo[4,3,0]non-5-ene. The bromothiiran 1,1-dioxides were synthesized from the appropriate... 

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