Palladium-Catalyzed Decarbonylative Nucleophilic Halogenation of Acyl Fluorides and Chlorides: Synthesis of Aryl Halides via Reductive Elimination of the C-X (X = I, Br, and Cl) Bond and Mechanistic Implications

Aryl halides are widely recognized as crucial and versatile feedstocks for organic synthesis. However, in palladium-catalyzed reactions, while oxidative addition of carbon-halogen bonds is thermodynamically favorable, the reverse reaction-reductive elimination with the formation of carbon-halogen bonds- poses a significant challenge. As part of conducting a series of decarbonylative transformations of acyl halides, we developed a decarbonylative nucleophilic halogenation of acyl fluorides and chlorides through Pd-mediated reductive elimination of the C-X bond. These reactions enable the synthesis of aryl iodides, bromides, and chlorides using alkali metal halides. Regarding the reaction mechanism, the Xantphos ligand emerges as a crucial factor in promoting reductive elimination, leading to the formation of a stable Pd(0) intermediate and an oxidative adduct trans-(Xantphos)Pd(ArCO)X. Two proposed mechanisms involve Xantphos-promoted outer-sphere nucleophilic substitution and direct transhalogenation between acyl halides and alkali metal halides. In the latter mechanism, acyl fluorides or acyl chlorides react with alkali metal halides to form the corresponding acyl iodides or acyl bromides in situ and under mild conditions through decarbonylation, yielding the desired aryl halides via unimolecular fragment coupling. Importantly, it is evident that controlling the rate of acyl halide formation through the appropriate combination of substrates and alkali metal halides is crucial for the success of this reaction. Indeed, we found that the gradual formation of acyl iodide is pivotal in managing the undesired generation of I-2, a known catalyst poison. This observation enables us to fine-tune reaction conditions, thereby improving the selectivity of the desired transformation. As a result, we achieve enhanced yields of the final products and establish more sustainable and robust catalytic processes. This advancement not only boosts the applicability and reliability of our synthetic methodology but also underscores the potential for broader adoption in organic synthesis.