Synthesizing Highly Fluorinated Oligophenyls via Negishi Coupling of Fluoroarylzinc Pivalates

Previously established general synthetic methods for the synthesis of highly fluorinated biphenyls using Suzuki–Miyaura protocols require the use of organoboron compounds, which are not very stable under reactions conditions and thus need to be used in large excess. Herein, we report an improved general strategy for the synthesis of highly fluorinated biphenyls, terphenyls, and phenyl-substituted terphenyls using organozinc pivalates. The influence of several parameters was investigated: (1) in a series of monodentate phosphine ligands, X-Phos showed the best performance; (2) a higher yield was obtained for substrates bearing less steric hindrance or lower amount of fluorine substitution; (3) as iodinated substrates decomposed during the reaction, brominated electrophiles were found to be superior. The presented protocol is scalable, versatile, and works with commonly used and commercially available phosphine ligands (X-Phos) and palladium sources (Pd2dba3). Also, it does not require excess nucleophile for terphenyl synthesis and only a slight excess is needed for the preparation of phenyl substituted terphenyls.

Reductive Coupling of (Fluoro)pyridines by Linear 3d-Metal(I) Silylamides of Cr-Co: A Tale of C-C Bond Formation, C-F Bond Cleavage and a Pyridyl Radical Anion

Herein, we disclose the facile reduction of pyridine (and its derivatives) by linear 3d-metal(I) silylamides (M=Cr-Co). This reaction resulted in intermolecular C-C coupling to give dinuclear metal(II) complexes bearing a bridging 4,4'-dihydrobipyridyl ligand. For iron, we demonstrated that the C-C coupling is reversible in solution, either directly or by reaction with substrates, via a presumed monomeric metal(II) complex bearing a pyridyl radical anion. In the course of this investigation, we also observed that the dinuclear metal(II) complex incorporating iron facilitated the isomerisation of 1,4-cyclohexadiene to 1,3-cyclohexadiene as well as equimolar amounts of benzene and cyclohexene. Furthermore, we synthesised and structurally characterised a non-3d-metal-bound pyridyl radical anion. The reactions of the silylamides with perfluoropyridine led to C-F bond cleavage with the formation of metal(II) fluoride complexes of manganese, iron and cobalt along with the homocoupling or reductive degradation of the substrate. In the case of cobalt, the use of lesser fluorinated pyridines led to C-F bond cleavage but no homocoupling. Overall, in this paper we provide insights into the multifaceted behaviour of simple (fluoro)pyridines in the presence of moderately to highly reducing metal complexes.

Synthesis of Polyflourinated Biphenyls; Pushing the Boundaries of Suzuki-Miyaura Cross Coupling with Electron-Poor Substrates

Polyfluorinated biphenyls are interesting and promising substrates for many different applications. Unfortunately, all current methods for the syntheses of these compounds only work for a hand full of molecules or only in very special cases. Thus, many of these compounds are still inaccessible to date. Here we report a general strategy for the synthesis of a wide range of highly fluorinated biphenyls. In our studies we investigated crucial parameters, such as different phosphine ligands and the influence of various nucleophiles and electrophiles with different degrees of fluorination. These results extend the scope of the already very versatile Suzuki–Miyaura reaction toward the synthesis of very electron-poor products, making these more readily accessible. The presented methodology is scalable and versatile without the need for elaborate phosphine ligands or Pd-precatalysts.