Tri(hetero)substituted Carbonium Ions. III. Ring-size Effects on the Dual Reactivities of 2-Dimethylamino-1,3-dithiolanylium Ion and Its Ring Analogs as Ambident Electrophiles

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Abstract
The following ring analogs of 2-dimethylamino-1,3-dithiolanylium perchlorate (Ia) were prepared: 2-dimethylamino-1,3-dithianylium perchlorate (IV), 2-dimethylamino-1,3-dithiepanylium perchlorate (V) and an open-chain analog, bis(methylthio)dimethylaminocarbonium perchlorate (VI). In order to investigate the structural effect on the reactivities of the tri(hetero)carbonium ions at two reactive sites, i. e., the centered carbon (sp2) (a) and the S-methylene carbon (sp3) atoms (b), the reactions of these carbonium ions with hydroxide ion (mode A) and with N,N-dimethyldithiocarbamate ion (mode B), respectively, were studied at 30°C. The rates of mode A of these carbonium ions, relative to that of Ia, are 1 : 15.0 (IV) : 32.5 (V) : 1.06×104 (VI), and of mode B are 1 : 6.5 : 23.5 : ≈2.0×104. In their electronic spectra a definite trend that as the ring-size increases there is a strong shift of absorption maxima toward the lower energies was observed. The fact suggests that the stability of the carbonium ion decreases with an increase in ring-size. On the basis of the schematic energy profiles for the two modes as well as the structural and kinetical aspects, it has been assumed that one of common factors contributing to the ringsize effect on both reactivities would be the stability of the carbonium ion.