From thiiren-1,1 -dioxides

The isolation of the -isomer 139 was in fact unexpected, since all tetrasubstituted olefins previously obtained from thiirene dioxide have been assigned the cis-configuration with respect to the two phenyl substituents based on the principle of least motion during the ring opening to olefins ". It might well be, therefore, that the -isomer is obtained through the isomerization of the initially formed Z-isomer. 

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. 

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. 

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). 

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. 

Thiirene dioxides readily react with an entire spectrum of enamines to provide novel acyclic and cyclic systems172. These products result mostly from carbon-carbon or carbon-sulfur bond cleavage in the intermediate fused thiirane dioxide 167 (equation 67). 

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

Based on experimental results and complementary calculations, an out-of-plane n-delocalization is suggested for thiirene dioxides . As far as the thiirene oxide is concerned, theoretical calculations predict possible spiroconjugative-type interaction between the n c=c orbital of the ring and the Jt-orbitals of the SO (which leads to aromatic stabilization and a n charge transfer backward from the SO to the C=C). There exists, however, a rather strong destabilization effect, due to the jt so(dxz)-orbital. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

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