Click Immobilization of Organometallic Pincer Catalysts for C-C Coupling Reactions

Model systems for the simple, high-yielding, efficient click immobilization of homogeneous pincer catalysts are demonstrated. We present the synthesis of azide- and isocyanate-functionalized high pore volume, low surface area silica materials. Both para-ethynyl-functionalized palladium and platinum pincer organometallics (pincer = [C6H3(CH2NMe2)(2)-2,6](-)) were coupled with azido-functionalized silica material, yielding the corresponding tethered complexes in high yields with no complex degradation. Analogous para-alcohol-functionalized palladium and platinum pincer complexes were coupled with isocyanate-functionalized silica material to yield immobilized organometallics in high yields. The acquired immobilized organometallic pincer complexes were characterized using various techniques (solid-state IR and NMR, elemental content analysis, surface area measurements). Palladium-containing materials were applied as Lewis acid catalysts in the double Michael addition reaction between ethyl cyanoacetate and methyl vinyl ketone. When measured on a per palladium site basis, the novel materials were found to be as active as the homogeneous forms of the catalyst. This led to the discovery that one of the support materials, with no palladium organometallic units, however, with linker triazole groups present, was also an active catalyst, albeit with slightly diminished activity compared to the palladium-containing material. The palladium-containing materials were also applied in allylic stannylation catalysis and were found to be active catalysts, approximately three times slower than the homogeneous catalysts when measured on a per palladium site basis. The support material was not found to be an active catalyst in this reaction. The immobilized palladium catalysts have been applied sequentially for the two-step conversion of an allyl chloride to an allylic alkylation product.