| dc.description.abstract | Metal-catalyzed cross-coupling reactions of aryl halides or pseudohalides with nucleophiles have been ubiquitous and powerful tools to construct new C–C bonds in synthetic organic chemistry. Among the nucleophiles, arylboronic acids and their esters (arylboronates) are the most popular reagents due to their versatile reactivity and reasonable stability. In 2016, our group reported a highly efficient aromatic C–H borylation catalyzed by (POCOP)Ir complexes. Here we continued the development of the (pincer)Ir-catalyzed system by surveying a series of iridium and rhodium complexes of pincer ligands. We have disclosed that only iridium complexes supported by pincer ligands with a central aryl donor are capable of catalyzing aromatic C–H borylation. The newly tested (PCP)Ir complex has shown improved chemoselectivity (C–H borylation vs. olefin hydroboration, and sp² C–H vs. sp³ C–H borylation) compared to the previously reported (POCOP)Ir complexes. In terms of regioselectivity, C–H borylation of PhF showed certain variations when different precatalysts were examined. On the other hand, C–H borylation of PhCF₃ generally followed the state-of-the-art C–H borylation catalyzed by the iridium complexes supported by neutral bidentate ligands.
In addition to arylboron reagents, group 14 main group reagents have also shown their utility in cross-coupling chemistry. Germanium, positioned between silicon and tin on the periodic table, however, receives much less attention. There are only a few examples of cross-coupling reactions using organogermanes as nucleophiles in the existing literature. Here we have developed a robust and efficient ligandless Pd-catalyzed germylation protocol to prepare arylgermanes with tert-butoxy substituents from aryl bromides. The germylation reagent, sodium tri-tert-butoxygermanate, is readily prepared from commercially available GeCl₂·C₄H₈O₂. This highly efficient catalytic system has a wide substrate scope with excellent isolated yields and only produces NaBr as a byproduct.
The newly prepared arylgermanes have been studied in the subsequent Pd-catalyzed cross-coupling reaction. We undertook a detailed screening of reaction conditions to identify the optimal condition. Using fluoride as a base is required for successful catalysis and adding water significantly increases the yield of the desired cross-coupling product. The possible roles of fluoride and water have been discussed. The reaction scope and preliminary chemoselective cross-coupling experiments have shown that the newly prepared arylgermanes indeed have useful and unique reactivity compared to the existing reagents. | |
