Palladium complexes of the N-fused heterocycle derived abnormal N-heterocyclic carbenes for the much-preferred Cu-free and the amine-free Sonogashira coupling in air

Alex John, Sudipta Modak, Mahesh Madasu, Madanakrishna Katari, Prasenjit Ghosh

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Abstract

Abnormal N-heterocyclic carbenes of the N-fused heterocycles were employed in stabilizing a series of PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed complexes of the type (abnormal-NHC) PdI2(pyridine). Specifically, imidazo[1,2-a]pyridine based abnormal complexes, trans-{(1-ethyl-2-(4-R-phenyl)-imidazol-3-ylidene[1,2-a]pyridine)} PdI2(pyridine) [R = F (1b), Cl (2b), Br (3b) and Me (4b)] were synthesized from the reaction of corresponding imidazo[1,2-a]pyridinium iodide salts (1-4)a with PdCl2 in pyridine in the presence of K 2CO3 as a base. The (1-4)b complexes carried out the Sonogashira couplings of the aryl bromide and iodide substrates with terminal alkynes in a mixed aqueous medium in air under the Cu-free and amine-free conditions. The density functional theory studies revealed that in the (1-4)b complexes, the Ccarbene-Pd σ-bonding interaction was significantly stronger than the trans Pd-Npyridine σ-bonding interaction.

Original languageEnglish (US)
Pages (from-to)20-29
Number of pages10
JournalPolyhedron
Volume64
DOIs
StatePublished - 2013

Bibliographical note

Funding Information:
We thank Department of Science and Technology (Grant No: SR/S1/IC-15/2011 ), New Delhi, for financial support of this research. We are grateful to the National Single Crystal X-ray Diffraction Facility and Sophisticated Analytical Instrument Facility at IIT Bombay, India, for the crystallographic and other characterization data. Computational facilities from the IIT Bombay Computer Center are gratefully acknowledged. S.M. thanks CSIR, New Delhi and M.K. thanks BRNS, Mumbai for research fellowship.

Keywords

  • Abnormal N-heterocyclic carbene (NHC)
  • Amine-free
  • Cu-free
  • DFT studies
  • Palladium
  • Sonogashira reaction

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