Structure and Reactivity of Indolylmethylium Ions: Scope and Limitations in Synthetic Applications

Eight substituted aryl(indol-3-yl)methylium tetrafluoroborates 3(a-h)-BF4 and three bis(indol-3-yl)methylium tetrafluoroborates 3(i-k)-BF4 have been synthesized and characterized by NMR spectroscopy and X-ray crystallography. Their reactions with pi-nucleophiles 8 (a-j) (silylated enol ethers and ketene acetals) were studied kinetically using photometric monitoring at 20 degrees C. The resulting second-order rate constants were found to follow the correlation log k(20 degrees C) = s(N)(N + E), in which nucleophiles are characterized by the two solvent-dependent parameters N and s(N), and electrophiles are characterized by one parameter, E. From the previously reported N and s(N) parameters of the employed nucleophiles and the measured rate constants, the electrophilicities of the indol-3-ylmethylium ions 3(a-k) were derived and used to predict potential nucleophilic reaction partners. A discrepancy between published rate constants for the reactions of morpholine and piperidine with the (2-methylindol-3-yl)phenylmethylium ion 3h and those calculated from E, N, and s(N) was analyzed and demonstrated to be due to a mistake of the value reported in the literature.