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 π-nucleophiles 8­(a–j) (silylated enol ethers and ketene acetals) were
studied kinetically using photometric monitoring at 20 °C. The
resulting second-order rate constants were found to follow the correlation
log k(20 °C) = sN(N + E), in which nucleophiles
are characterized by the two solvent-dependent parameters N and sN, and electrophiles
are characterized by one parameter, E. From the previously
reported N and sN 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 sN was analyzed and demonstrated
to be due to a mistake of the value reported in the literature.