Diversity Synthesis via C−H Bond Functionalization: Concept-Guided Development of New C-Arylation Methods for Imidazoles

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Abstract
Herein, we have formulated the concept of systematic derivatization of a structural motif via C-H bond functionalization. This concept may not only serve as a blueprint for new strategies in diversity synthesis but also provide systematic guidance for the identification of unsolved and important synthetic challenges. To illustrate this point, 2-phenylimidazole was selected as the core motif for this study, a choice inspired by numerous azole-based synthetics, including pharmaceuticals (compound SB 202190), and also fluorescent and chemiluminescent probes. We were able to show that systematic and comprehensive arylation of the 2-phenylimidazole core was feasible, and in the context of this study new arylation methods were developed. The direct 4-arylation of free 2-phenylimidazole was achieved with iodoarenes as the aryl donors in the presence of palladium catalyst (Pd/Ph(3)P) and magnesium oxide as the base. A complete switch from C-4 to C-2' arylation was accomplished using a ruthenium catalyst [CpRu(Ph(3)P)(2)Cl] and Cs(2)CO(3). The corresponding transformations for (N,2)-diphenylimidazole (C-5 and C-2' arylation) were accomplished via the palladium-based method [Pd(OAc)(2)/Ph(3)P/Cs(2)CO(3)] and a rhodium-catalyzed procedure [Rh(acac)(CO)(2)/Cs(2)CO(3)], respectively. All of the arylation methods described herein demonstrated broad synthetic scope, high efficiency, and exclusive selectivity. Furthermore, these new methods proved to be orthogonal to one another and applicable to sequential arylation schemes. With these methods in hand, arrays of arylated imidazoles may now be accessed in a direct manner from 2-phenylimidazole. This strategy stands in sharp contrast to a traditional approach, wherein a distinct and multistep synthesis would be required for each analogue.