Transition metal catalyzed cross-coupling reactions are cornerstones of organometallic chemistry. Among them, the heterobimetallic Pd(0)/Cu(I) Sonogashira catalyst system for C-C coupling reactions between aryl or alkenyl halides and terminal alkynes has become the most important method to prepare arylalkynes and conjugated enynes, which are precursors for natural products, pharmaceuticals, and organic materials. However, the mechanism of the Sonogashira reaction is still not well understood. Three of the basic questions are: (a) Is the catalysis propagated by molecular species, or metallic Cu or Pd nanoparticles? (b) What role does a coordinated monodentate or chelating phosphine ligand play? (c) Do the Pd(0) and Cu(I) components have to come into direct contact with one another to perform the coupling reaction? These questions will be answered in the presentation. By immobilizing the components on silica via phosphine linkers, the different factors could be disentangled and deeper insight gained by a combination of NMR spectroscopy, coordination chemistry, and catalysis. Recently, we pioneered solid-state NMR of immobilized linkers and catalysts in the presence of solvents (HRMAS).1,2 This method provides spectra with unprecedented signal resolution and allows distance measurements via 2D HRMAS NMR. HRMAS proved that surface-bound Pd complexes can migrate over tethered, but uncomplexed phosphines.1,2 Multinuclear HRMAS has also been employed to study the immobilized Pd(0) and Cu(I) components of immobilized Sonogashira catalyst systems with respect to their mobilities, their interactions with the substrates, potential intermediates, and resting states. A variety of different mono- and bidentate phosphines has been applied as linkers. The influences of different solvents, surface coverages, oxide support materials, and temperatures on the metal complexes will be explained in the presentation. In parallel, Sonogashira catalysis has been performed with these batches under different experimental settings, and a correlation between the HRMAS results and the activity and lifetimes of the catalysts has been established. Using this multifaceted approach immobilized Sonogashira catalysts with unprecedented activities and lifetimes have been generated.

Immobilized Sonogashira catalyst systems

253rd National Meeting (2017)

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