Thiirenium ion

Irradiation of a 2-methvlthio-4-hydroxythiazolium hydroxide inner salt (12) leads to exchange of the carbon atoms located in the 2 and 4 positions, probably via a thiirenium ion (Scheme 13) (5). 

With 2-methyl- and 2,4-dimethylthiazole, the methyl thiirenium ion (m/e 72) is obtained, which can easily lose a hydrogen radical to give the ml ell ion (confirmed by the metastable peak). This latter can rearrange by ring expansion to give the thietenyl cation whose structure was confirmed in certain spectra by the presence of a metastable peak corresponding to the decomposition of the m/e 71 ion to give the thioformyl cation m/e 45, probably by elimination of acetylene. 

As in the case of thiazole and the alkylthiazoles, cleavage of the thiazole ring takes place at the 1,2 and 3,4 bonds, confirmed by a metastable peak. The other important peaks result from fragmentation of thiirenium ion, in the case of 4- and 5-phenylthiazole and of the phenyl ring. These latter are generally present in the spectra of all comptmnds with benzene-ring substituents, they occur at m/e 77, 76, 75, 51, 50, 39 (124). The ion m/e 45 (HCS" ) is always present. 

Thiirenium ions formed as intermediates in the solvolysis of tran5-/3-thiovinyl sulfonates react with methanol to give product with retention of configuration (Scheme 75) (79MI50600). 

The formation of 2,3-di-f-butyl-l-methylthiirenium chloride from fran5-3-chloro-4-methylthio-2,2,5,5-tetramethyl-3-hexene is quantitative (by NMR) in liquid sulfur dioxide (Scheme 128) (82JOC590). Similar thiirenium ions are intermediates in the reactions of -thiovinyl derivatives (79MI50600). 

Thiiranium and thiirenium ions as reaction intermediates and building blocks in organic synthesis 97G177. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

The addition of organodichalcogenides to alkynes does not necessarily require the aid of transition metal catalysts. Indeed, the reaction proceeds via different mechanisms under various conditions (radical, In, CsOH,183 SnCl4,184 and phase-transfer catalysts185). Treatment of a mixture of (PhS)2 and terminal alkynes with GaCl3 affords (E)-products (E Z=>20 1).186 The reaction is assumed to involve a thiirenium ion as the intermediate (Scheme 38). 

The endoperoxide transfers a sulfur atom to alkenes forming thiiranes (episulfides) the process is made more efficient in the presence of the tetraphenylporphyrinato cobalt(ll) complex <1996CC177>. With cyclic alkynes such as cyclooctyne, an interesting thiirenium ion intermediate could be observed <2002JA8316>. 

The same team studied also Art-butyl-substituted thiiranium and thiirenium ions in reactions with disulfides, which led to the formation of thietanium and thietium ions, respectively. However, this paper only reported upon the kinetics of this... 

If the reaction was carried out in liquid S02 at —60 °C the thiirenium ion (59) was formed, which in CDC13 at 20 °C yielded (56). This represents the first seven-membered trans-cycloalkene, which is isolable at room temperature. In refluxing chloroform (56) isomerized to the corresponding cis-cycloalkene the activation parameters of this trans-cis-isomerization were determined 86). 

G. H. Schmid Thiiranium and Thiirenium Ions, in Topics in Sulfur Chemistry, Vol. 3, 101, Thieme-Verlag, Stuttgart 1977... 

Both kinetic data and the nature of the products concur to exclude that linear vinyl cations are formed in any case in the addition reaction. There is little doubt that a bridged species is involved as intermediate and its structure may well be that of a thiirenium ion (53) more or less... 

It has been concluded that the cationic intermediate involved in the reaction of esters 147 has a bridged structure indicated as that of a thiirenium ion (53). 

It is apparent that, whereas for an open vinyl cation the effect of a substituents is very large in the order H < Aik < Ph and the effect of / substituents is very small, in the case of thiirenium ions, the effect of substituents bound to the carbon atoms of the three-membered ring of the thiirenium ion 53 follows the order H < < Ph < Aik which is, by the way, that observed for cyclopropenium ions (Breslow et al., 1962). 

Ion cyclotron resonance (i.c.r.) spectroscopy has allowed Buttrill (1970) to observe the ion-molecule reaction between hydrogen sulphide and acetylene (acetylene-d2). The products are HCS+(DCS+) and C2HsS+ (C2D2HS+). For the latter ion, the structure consistent with the labelling experiments is that of the thiirenium ion ... 

The effect of internal factors in the stability of bridged cations has been discussed in the case of thiirenium ions generated by addition of... 

The stereochemistry of the reactions of vinyl cations with nucleophiles is predictably different depending on their geometry, (a) High stereospecificity is expected from bridged ions (trans addition to acetylenes and retention of configuration in substitution reactions of vinyl derivatives) and is experimentally observed in the case of thiirenium ions, (b) From free linear cations with two j3 substituents of equal size, complete racemization is expected and is fully verified in the substitution products from l,2-dianisyl-2-phenylvinylbromide (section II,C,2). 

Reactivity of Substituents at Sulfur - Thiiranium Ions, Thiirenium Ions, and Episulfuranes... 

Modena and co-workers examined the relevance of the symmetry of the lowest unoccupied molecular orbital (LUMO) of the electrophile in substitutions at vinyl carbon of thiirene <1995JA2297>. The computational levels included 3-21G //3-21G, 6-31G //3-21G, and 6-311G //3-21G. In attack at the vinyl carbon of a thiirenium ion, for example, the thiirenium ion is the electrophile and the attacking nucleophile is a neutral species with a lone pair, or an anion. It was found that in cases where the first vacant a- and n-levels differ in energy by more than 0.01 hartree, there is a good correspondence between the symmetry of the lowest unoccupied orbital and the stereochemical... 

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