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N-Vinyl and N-Aryl Hydroxypyridinium Ions: Charge-Activated Catalysts with Electron-Withdrawing Groups

Cite this: J. Org. Chem. 2020, 85, 9, 6017–6026
Publication Date (Web):April 8, 2020
Copyright © 2020 American Chemical Society

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    Charge-enhanced Brønsted acid organocatalysts with electron-withdrawing substituents were synthesized, and their relative acidities were characterized by computations, 1:1 binding equilibrium constants (K1:1) with a UV–vis active sensor, 31P NMR shifts upon coordination with triethylphosphine oxide, and in one case by infrared spectroscopy. Pseudo-first-order rate constants were determined for the Friedel–Crafts alkylations of N-methylindole with trans-β-nitrostyrene and 2,2,2-trifluoroacetophenone and the Diels–Alder reaction of cyclopentadiene with methyl vinyl ketone. These results along with kinetic isotope effect determinations revealed that the rate-determining step in the Friedel–Crafts transformations can shift from carbon–carbon bond formation to proton transfer to the catalyst’s conjugate base. This leads to an inverted parabolic reaction rate profile and slower reactions with more acidic catalysts in some cases. Electron-withdrawing groups placed on the N-vinyl and N-aryl substituents of hydroxypyridinium ion salts lead to enhanced acidities, more acidic catalysts than trifluoroacetic acid, and a linear correlation between the logarithms of the Diels–Alder rate constants and measured K1:1 values.

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    • Kinetic, UV–vis titration, and Ph3PO 31P chemical shift data; plots of ln kDiels–Alder versus ln K1:1 and ΔGacid°; NMR spectra; and computed structures and energies, and the complete citation to ref (23) (PDF)

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