Synthesis of internal alkynes using new palladium <i>N</i>-heterocyclic carbene catalytic system

Abstract

Alkyne is a recurring functional group in a wide range of natural products and other bioactive compounds. It is as well a versatile intermediate in synthesis. A direct method for incorporating a triple bond in molecules is termed Sonogashira reaction, and it typically employs palladium and copper catalysts. However, the use of copper catalysts mediates homocoupling of terminal alkynes. In order to address the homocoupling problem, copper-free reactions are considered. In this study, a new palladium-N-heterocyclic carbene-pyridine complex was synthesized and tested as catalysts in the coupling reaction of aryl iodides with terminal alkynes. The catalyst was synthesized by alkylation of imidazole to yield N-methylimidazole (H2), which was further alkylated to yield dialkyl imidazolium salt (H4). The dialkyl imidazolium salt was further reacted with palladium bromide (PdBr₂) in 3-methylpyridine using potassium carbonate (K₂CO₃) as a base to yield the Palladium N-Heterocyclic carbene pyridine complex (H10). The products obtained were characterized using ¹HNMR and FT-IR spectroscopic techniques which supported their proposed structures. The elemental compositions were established on the basis of C, H, N elemental analysis. Using H10 as catalyst, a mild protocol for the Sonogashira coupling reaction was developed. Using 1.0 mol% catalyst loading, acetonitrile-water (1:1) as a solvent system, KOH as a base, and room temperature were found as a suitable condition for the synthesis of internal alkynes from the reaction of aryl halides with aryl alkynes. The new catalytic system was compatible with various functional groups and worked effectively with unactivated, activated and deactivated aryl iodides.