Tri(hetero)substituted Carbonium Ions. VII. Effect of the Ring System on the Ambident Reactivity of Cyclic Dithiocarbamidium Ions

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Abstract
In order to study the effect of structure on the ambident reactivity of cyclic dithiocarbamidium ions, two endo-imimum type cations, 2-methylthio-3-methyl-4,5-dihydrothiazolium ion (5) and 2,3,5,6-tetrahydrothiazolo-[2,3-b]-thiazolium ion (6) were prepared. Their electrophilic reactivities were studied and compared with those of the exo-iminium type cations, 2-dimethylamino-1,3-dithiolan-2-ylium ion (1) and its ring analogues. 3-Di-methylamino-2,4-benzodithiepan-3-ylium ion (7) was prepared and its ambident reactivity was studied. Examination of the products obtained from their reactions with various nucleophiles revealed that the three cation have two reactive sites, i.e., the central (sp2) carbon (site a) and the S-methylene (or S-methyl) (sp3) carbon atoms (site b). Significant differences in the electrophilic behavior of the endo- and exo-iminium type cations were observed. They were discussed in terms of the structures as estimated from the spectra. Kinetic experiments showed that the reactivities at site a of these cations with hydroxide ion, relative to that of 1, are 22.5 (5): 16.6 (6): 27.2 (7), and at site b with N,N-dimethyldithiocarbamate ion are 3.3 : 282 : >104. The reactivity order suggests that the reactivities at the two sites, especially at site b, are very sensitive to a change in the ring system of the cations.

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