Regioselective Synthesis of Chromones via Cyclocarbonylative Sonogashira Coupling Catalyzed by Highly Active Bridged-Bis(N-Heterocyclic Carbene)Palladium(II) Complexes

The
one-pot regioselective and catalytic synthesis of bioactive
chromones and flavones was achieved via phosphine-free cyclocarbonylative
Sonogashira coupling reactions of 2-iodophenols with aryl alkynes,
alkyl alkynes, and dialkynes. The reactions are catalyzed by new dibromidobis­(NHC)­palladium­(II)
complexes. The new bridged N,N′-substituted
benzimidazolium salts (L1, L2, and L3) and their palladium complexes C1, C2, and C3 were designed, prepared, and fully characterized
using different physical and spectroscopic techniques. The molecular
structures of complexes C1 and C3 were determined
by single-crystal X-ray diffraction analysis. They showed a distorted
square planar geometry, where the Pd­(II) ion is bonded to the carbon
atoms of two cis NHC carbene ligands and two cis bromido anions. These
complexes displayed a high catalytic activity in cyclocarbonylative
Sonogashira coupling reactions with low catalyst loadings. The regioselectivity
of these reactions was controlled by using diethylamine as the base
and DMF as the solvent.