(Ferrocenylthienyl)phosphines: Synthesis, electrochemistry and their use in Suzuki-Miyaura C,C coupling

Abstract

(Ferrocenylthienyl)phosphines of type 2-PR2-5-Fc-cC4H2S (R = Ph (3a), tBu (3b)) and 2-PR2-3,4-Fc2-cC4HS (R = Ph (4a), tBu (4b); Fc = Fe(η5-C5H4)(η5-C5H5)) and their selenium derivatives were synthesized by consecutive synthetic methodologies. The molecular structure of 3a and 4a,b in the solid state show inter- and intramolecular (T-shaped) π–π interactions for 3a and 4a, resulting in dimeric and 1D polymeric structures.Phosphines 3a,b and 4a,b were applied in the Pd-catalyzed Suzuki-Miyaura C,C cross coupling reaction of aryl halides and boronic acids yielding the respective biaryls. Compound 3b shows a higher activity in C,C cross coupling reactions as compared to 3a and previously published thiophene-based phosphines and aminophosphines. Moreover sterically hindered and deactivated aryl bromides could be coupled at Pd loadings as low as 0.05 mol-% and at temperatures between 25 °C and 50 °C. However, the application of diferrocenylthienylphosphines 4a,b in Suzuki-Miyaura reactions resulted in decreased yields of the corresponding biaryls.In the context of electrochemical investigations on these compounds, a strong dependency of the individual electrode reactions on the phosphorus substituents were found. During UV–Vis/NIR spectroelectrochemical studies, only a very limited iron-iron electronic interaction in 4a-Se+ could be detected. Thus, 4a-Se+ corresponds to a very weak coupled class II system according to the classification of Robin and Day.