Lewis Acidities of Indol-3-ylmethylium Ions and Intrinsic Barriers of Their Reactions with Phosphines and Pyridines

Eur. J. Org. Chem., 2016, 23, 4050-4058


Rate and equilibrium constants for the reactions of indol-3-ylmethylium ions with triarylphosphines and pyridines were measured spectrophotometrically. The resulting Lewis acid-base adducts were characterized by NMR and X-ray crystallography. All rate constants, except those for the reactions of sterically hindered bis(indol-3-yl)methylium ion 1j with phosphines, agreed within a factor of 4 with those calculated by the correlation log k20 °C = sN(N + E) from previously reported electrophilicity parameters E for indol-3-ylmethylium ions and N and sN parameters for phosphines and pyridines. Lewis acidities (LA) of indol-3-ylmethylium ions were calculated from the correlation log K20 °C = LA + LB using the benzhydrylium-based Lewis basicities (LB) of phosphines and pyridines and the equilibrium constants for the reactions of these Lewis bases with indol-3-ylmethylium ions. The agreement of LA values derived from equilibrium constants of their reactions with different types of Lewis bases shows that indol-3-ylmethylium ions have a steric demand similar to the p- and m-substituted benzhydrylium ions. For some reactions of indol-3-ylmethylium ions with phosphines and pyridines, rate and equilibrium constants could be determined, which allowed us to calculate the Marcus intrinsic barriers and compare them with those for analogous reactions with the dicarbonyl(η5-cyclopentadienyl)-(η2-ethene)iron cation.

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