Thiirene 1,1-dioxides

Both cis- and rrans-l-arylsulfonyl-2-arylsulfenyl propenes (56) underwent a Smiles rearrangement under electron impact at 20 and 70 eV and formed a diarylsulfide ion [M — 104]+ (equation 27a)39 through a process where a bond between the R C H group and the sulfide sulfur is formed and a rearomatization occurs by a loss of the neutral thiirene dioxide or a simultaneous expulsion of SOz and propyne. The ion m/z 148 was also obtained from all of the sulfonyl-sulfides, 56 (equation 27b) and here the loss of R2 seemed to be related to the bond strength39. In addition to the above compounds 56 exhibited some simple cleavages before and after sulfone-sulfinate rearrangements. 

Nucleophilic attack on carbon in thiirane and thiirene dioxides. . ... 

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). 

The structural features and the spectroscopic characteristics of the thiirene dioxide system (22) are of special theoretical interest since, on the basis of analogy with cyclopropenone (23), it is a possible nonbenzenoid aromatic system with all the physical and chemical implications involved. Aromatic and/or conjugative effects, if any, require transmission through the d-orbitals of the sulfur atom. 

TABLE 4. Selected IR stretching frequencies of sulfone and sulfoxide groups in thiirane and thiirene dioxides and oxides... 

Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

Since a similarity between the rates of decomposition of thiirene dioxide complexes and those of thiirane dioxides was found, it was suggested103 that upon coordination the carbon-carbon bond order of thiirene dioxides decreases and the ligand becomes thiirane dioxide-like. The role of the metal is thus to saturate the carbon-carbon double bond so that the reactivity of the coordinated thiirene dioxide approaches that of the thermally less stable thiirane dioxide. 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

It is highly probable that the lesser stability of thiirene dioxides compared with that of the thiirene oxides simply reflects the more facile extrusion of sulfur dioxide relative to that of sulfur monoxide. In fact, the same effect is probably operative in the case of the cis- and trans-diphenylthiirane oxides (16g,h)110 compared with cis- and trans-diphenylthiirane dioxides (17d,e)99 the former were found to be more stable toward thermal decomposition than the latter. 

The issue of the acidity of a-hydrogens in thiirene oxides and dioxides is dealt with only in the dioxide series, since neither the parent, nor any mono-substituted thiirene oxide, is known to date. Thus the study of the reaction of 2-methylthiirene dioxide (19c) with aqueous sodium hydroxide revealed that the hydroxide ion is presumably diverted from attack at the sulfony 1 group (which is the usual pattern for hydroxide ion attack on thiirene dioxides) by the pronounced acidity of the vinyl proton of this compound113 (see equation 14). 

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). 

An illustrative example of the Michael reaction is that of the thiirene dioxide 19b with either hydroxylamine or hydrazine to give desoxybenzoin oxime (87) and desoxybenzoin azine (88), respectively, in good yields6 (see equation 29). The results were interpreted in terms of an initial nucleophilic addition to the a, j8-unsaturated sulfone system, followed by loss of sulfur dioxide and tautomerization. Interestingly, the treatment of the corresponding thiirene oxide (18a) with hydroxylamine also afforded 86 (as well as the dioxime of benzoin), albeit in a lower yield, but apparently via the same mechanistic pathway6. 

Although the nucleophilic addition of secondary amines to thiirene dioxides can be interpreted as following the same mechanistic pathway, the reaction was found to be second order in amine119 (which is typical for the addition of amines to olefins in appropriate solvents13 2 133), and the addition is syn. As a result, mechanisms with a cyclic-concerted addition across the carbon-carbon bond, or a stepwise addition involving two molecules of amine per one molecule of thiirene dioxide, have been proposed. 

Thus, like a, /1-unsaturated ketones and sulfones, both thiirene dioxides and thiirene oxides are preferentially attacked by the less basic nucleophiles on the vinylic carbon atom2. This would lead to formally 1,4 Michael-type adducts and/or other products resulting from further transformations following the initial formation of the a-sulfonyl and a-sulfoxy carbanions. 

Route d is a hydrogenation of thiirene dioxides. Since the preparation of thiirene dioxides is rather laborious, and many of them are prepared from the corresponding thiirane oxides6, this method has practically no preparative value, and the only example reported is the reduction of 18a to cis- 17d in a very low yield (8%)21. 

Cyanide and benzenesulfinate ions react with thiirene dioxides in an analogous manner (equation 54). 

The softer, less basic potassium bromide and iodide did not react with the thiirene dioxide 19b. The latter was also inert towards potassium thiocyanate, selenocyanate or nitrile. It did react, however, with potassium thiophenoxide in DMF at room temperature to yield, most probably, the vinyl sulfmate 138 isolated as the corresponding sulfone39 (equation 56). 

Although thiirene dioxides do not react with typical tertiary amines like triethylamine, they do react with the amidine 1,5-diazobicyclo-[4.3.0]-non-5-ene (DBN) to give a 1 1 adduct betaine119158 141, analogously to the reaction of thiirene dioxides with soft nucleophiles (equation 57). 

As formal a, /i-unsaturated sulfones and sulfoxides, respectively, both thiirene dioxides (19) and thiirene oxides (18) should be capable, in principle, of undergoing cycloaddition reactions with either electron-rich olefins or serving as electrophilic dipolarophiles in 2 + 3 cycloadditions. The ultimate products in such cycloadditions are expected to be a consequence of rearrangements of the initially formed cycloadducts, and/or loss of sulfur dioxide (or sulfur monoxide) following the cycloaddition step, depending on the particular reaction conditions. The relative ease of the cycloaddition should provide some indication concerning the extent of the aromaticity in these systems2. 

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