Diaryliodoniums as Hybrid Hydrogen- and Halogen-Bond-Donating Organocatalysts for the Groebke–Blackburn–Bienaymé Reaction

Dibenziodolium and
diphenyliodonium triflates display high catalytic
activity for the multicomponent reaction that leads to a series of
imidazopyridines. Density functional theory (DFT) calculations indicate
that both the salts can play the role of hybrid hydrogen- and halogen-bond-donating
organocatalysts, which electrophilically activate the carbonyl and
imine groups during the reaction process. The ortho-H atoms in the vicinal position to the I atom play a dual role:
forming additional noncovalent bonds with the ligated substrate and
increasing the maximum electrostatic potential on the σ-hole
at the iodine atom owing to the effects of polarization. Dibenziodolium
triflate exhibits higher catalytic activity, and the results obtained
from 1H nuclear magnetic resonance (NMR) titrations, in
conjunction with those from DFT calculations, indicate that this could
be explained in terms of the additional energy required for the rotation
of the phenyl ring in the diphenyliodonium cation during ligation
of the substrate.