Thiirenium cations

Nesterenko and Staninets used quantum-chemical methods to examine the reaction of the phenylsulfenium ion (PhS ) with alkynes XCCY (X, Y = H, Me, Ph, CN) <2004TEC77>. Reaction was found to occur in two steps in general, forming first an acyclic cation 33 that could close to a thiirenium cation 34 or to a four-membered ring cation 35 (Scheme 4). The thiirenium ion 34 could rearrange to cation 35. All steps were described as reversible. 

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. 

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... 

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). 

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). 

A different source of arenesulphenyl cations has been reported by Montevecchi and coworkers . They showed that the BFs-promoted reaction of 4 -nitrobenzenesulphena-nilide (35) with aryl-substituted alkynes in poorly nucleophilic solvents such as chlorobenzene generally led to bissulphides (36) and sulphimides (37) in addition to diphenyl disulphide and 4-nitroaniline. In acetonitrile as solvent, products of capture of the thiirenium ion by the solvent were also observed (equation 22). The thiirenium ion from phenylacetylene reacted even with the poorly nucleophilic solvent chlorobenzene to ( )-PhSCH=CPhC6H4Cl. With alkyl-substituted alkynes in chlorobenzene as solvent, a small amount of ( )-2-fluorovinyl sulphide PhSC(R )=CF(R ) was also detected. The yield of this sulphide could be increased when the reaction was performed in the presence of tetrabutylammonium tetrafluoroborate. Terminal alkynes gave the corresponding 2-fluorovinyl sulphides in 35-55% and internal alkynes in 65-87% yield. The procedure was unsuccessful for di-tcrt-butylacetylene and gave low yields for arylacetylenes. In acetic acid as solvent the thiirenium ion was captured as ( )-PhSC(R )=CR (OAc). 

Molecular Orbital (M.O.) studies of reaction intermediates have been reported for the rearrangement of l-propenethiol, for the addition of sulphenyl halides to ethylene (sulphurane rather than thiiranium salt as intermediate), and for similar additions to alkynes (thiirenium ions or /ff-alkylthiovinyl cations, depending upon the substituents). Electronic effects of substituents on energy levels of orbitals of... 

---