Substituent effects in reductions of heteroaromatic cations

A set of 11 each of 2,4,6-triphenylpyrylium, -thiopyrylium and -N-methylpyridinium tetrafluoroborates carrying a range of substituents in the phenyl rings were prepared. First and second wave reduction potentials were deternimed. For the thiopyrylium series there are linear correlations between scaled potentials (Edegrees/0.05915) and summed Hammett constants for substituents in the pendant phenyl groups (sigma = 2.29 and 3.38 for first and second waves respectively). For the pyrylium series, a good linear relationship (sigma = 2.79) is obtained for all substituent patterns for the first wave reduction potentials, but for the second wave there are separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2- and 6-phenyls. For the pyridinium series, the first wave potentials show separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls, but a single linear relationship for the second wave potentials. These are related to particular structural features in the cations, radicals and anions in each series. Rates and products were determined for reductions of the pyrylium and thiopyrylium cations by sodium cyanoborohydride and of all cations by sodium borohydride in acetonitrile solution. Reactions are first order in reducing agent and cation. Primary kinetic isotope effects were determined for borohydride reduction of the least reactive of each of the series of cations. Plots of logarithms of second-order rate constants against summed Hammett constants for substituents in the pendant phenyl groups are linear for all combinations of reagent and cation with 0.91 < sigma < 1.50 across all substituent patterns. For parent pyrylium and thiopyryliums, kBH(4)/kCNBH(3) = 8.4 x 10(4) and 1.5 x 10(4), respectively, and for reductions by borohydride the reactivities of the pyrylium, thiopyrylium and pyridinium, series decrease in the order 1.4 x 10(5):8.8 x 10(3):1. Constant selectivities are not observed. Comparison of the correlations for electrochemical reduction and for hydride addition leads to the conclusion that charge neutralization in the hydride addition transition states runs ahead of bonding changes at the originating B-H bond. Copyright (C) 2002 John Wiley Sons, Ltd.