Synthesis and Catalytic Evaluation of Phosphanylferrocene Ligands with Cationic Guanidinium Pendants and Varied Phosphane Substituents
Abstrakt
This contribution expands the still narrow class of functional ferrocene phosphanes with polar cationic groups, focusing on the synthesis and catalytic use of a series of phosphanylferrocene ligands bearing positively charged guanidinium tags, [R(2)PfcCH(2)NHC(NH2)(2)]Cl (3a-d), where fc = ferrocene-1,1 '-diyl, R = isopropyl (a), cyclohexyl (b), phenyl (c), and 2-furyl (d). To probe the influence of phosphane substituents, these compounds were studied as supporting ligands in Pd-catalyzed Suzuki-Miyaura cross-coupling of acyl chlorides with arylboronic acids, in analogous coupling of aryl bromides with arylboronic acids, and in Rh-catalyzed hydroformylation of 1-hexene using trans-[RhCl(CO){R(2)PfcCH(2)NHC(NH2)(2)-kappa P}(2)]Cl-2 complexes (4a-d) as pre-catalysts.
The outcome of the cross-coupling reactions strongly depended on the starting materials, and no ligand generated a universally applicable catalyst when combined with Pd(OAc)(2). In the hydroformylation reactions, the catalyst based on 4d led to lower conversions than all others, which performed rather similarly.
Overall, the phenyl-substituted phosphane 3c emerged as a good compromise, giving rise to reasonably efficient and stable catalysts in most cases (except for Suzuki-Miyaura biaryl cross-couplings, wherein electron-rich alkylphosphanes performed better than 3c).