Given the wide applicability of transition-metal catalysis, methods to control these processes are becoming increasingly important. In cases in which the substrate or product inhibit the activity of the catalyst, catalytic control is particularly useful. Pyridine-containing substrates, for example, possess a lone electron pair capable of interacting with and deactivating the metal catalyst. A possible solution to circumvent this issue is to use a catalytically inert building block that interacts with the pyridine substrate, thus preventing catalyst deactivation.
Rafael Gramage-Doria, CNRS-Université de Rennes 1, Rennes Cedex, France, and colleagues have developed a method to control palladium-catalyzed Suzuki and Heck reactions of pyridine substrates. Catalytically inert zinc-porphryin and -salphen complexes interact non-covalently with halopyridines, causing their properties to change but not those of the palladium catalyst. These non-covalent interactions were studied by 1H NMR and UV/Vis spectroscopy and X-ray diffraction.
Ortho-substituted halopyridines did not interact with either Zn complex, whereas both meta- and para-substituted halopyridines did, with higher binding constants for the zinc-salphen complex. In Suzuki cross-couplings, conversions were higher for meta- and para-substitution in the presence of zinc-porphyrin. In Heck cross-couplings, conversions were higher when zinc-salphen was used. As expected, conversions for ortho-substituted substrates were not affected. This method could also be useful for other transition-metal catalyzed reactions involving catalyst poisoning by N-containing substrates.
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