The Role of Bidentate Fluorenylphosphines in Palladium-Catalyzed Cross-Coupling Reactions

Abstract

Seven new bidentate phosphine ligands, in which two 9-(dicyclohexylphosphino)fluoren-9′-yl units are bridged with n-alkanediyl-(C 1 -C 5 ) or o/m-xylenediyl linkers were synthesized and characterized as the respective air-stable phosphonium salts. Pd complexes of the new bidentate ligands proved to be highly active catalysts for Buchwald-Hartwig amination and Suzuki and Sonogashira coupling using aryl bromides and chlorides as substrates. A study comparing the catalytic activity of the Pd complexes of the bidentate phosphine ligands with those of closely related monodentate fluorenyl phosphines gave insights into the influence of the various bridging units on the catalytic transformations. In Suzuki and amination reactions the diphosphine 2b with the shortest linker, a -CH 2 -unit, turned out to be by far the best ligand. In the Sonogashira coupling monodentate phosphines rendered the most active Pd catalysts, while 2b fails.

Sterically demanding trialkylphosphines for palladium-catalyzed cross coupling reactions—alternatives to PtBu3

“…158 To add even more variability in the ligand design two fluorenes can be linked via reaction of the respective anion with bifunctional carbon centered electrophiles (a,o-dibromides) to generate potential bidentate ligands, whose steric and electronic properties, as well as the bite angle can be adjusted via variable bridging units. 159 …”

“…Potential bidentate ligands of the cataCXium s F type can be obtained using related procedures starting from respective difluorenylalkanes or -xylenes (Scheme 28). 159 The reactivity of the fluorenyl unit enables the synthesis of water-soluble 9-fluorenyldialkylphosphines with sulfonato groups. 162 The sulfonation of trialkylphosphines with concentrated sulfuric acid appears to be critical with a view to the oxidation-prone phosphorus.…”

Sulfonated N‐Heterocyclic Carbenes for Pd‐Catalyzed Sonogashira and Suzuki–Miyaura Coupling in Aqueous Solvents

“…[19] However, while chelating phosphines appear to be a very good choice for Buchwald-Hartwig amination and Suzuki reactions, they tend to be less ideal for Sonogashira reactions. [20] Water seems to be able to relieve the inhibition of the catalyst. It is likely that this highly polar solvent preferentially associates with the heterocyclic substrates (via hydrogen bonding) [21] rather than coordinating to the catalytically active Pd center.…”

Cross‐Coupling Reactions to sp Carbon Atoms

“…In contrast to their laboratory counterparts, suitable industrial In particular, such low-coordinate species are readily obtained with bulky monodentate ligands [52]. In the case of bulky phosphines, the species formed on oxidative addition of haloarenes to [Pd(0)(PR 3 ) 2 ] is generally accepted to be of the Pd(II)L-type, and further increasing the phosphine bulkiness (e.g., P(o-Tol) 3 ) is believed to shift the equilibrium between 16 and 17 toward dimeric 17 (Scheme 9.8) [53].…”