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.
The most promising types of iodoazolium compounds exhibiting high catalytic activity toward halogen abstraction and carbonyl activation are highlighted.
Sulfonium and selenonium salts, represented by S-aryl dibenzothiophenium and Se-aryl dibenzoselenophenium
triflates, were found to exhibit remarkable catalytic activity in
the model Groebke–Blackburn–Bienaymé reaction.
Kinetic analysis and density functional theory (DFT) calculations
indicated that their catalytic effect is induced by the ligation of
the reaction substrates to the σ-holes on the S or Se atom of
the cations. The experimental data indicated that although 10-fold
excess of the chloride totally inhibits the catalytic activity of
the sulfonium salts, the selenonium salt remains catalytically active,
which can be explained by the experimentally found lower binding constant
of the selenonium derivative to chloride in comparison with the sulfonium
analogue. Both types of salts exhibit lower catalytic activity in
the model reaction than dibenziodolium species.
Based upon the experimentally obtained kinetic data on iodonium salt catalyzed nucleophilic addition of isocyanide to imine leading to imidazopyridine species, a reliable model for DFT calculations has been suggested....
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