Synthesis of monodentate ferrocenylphosphines and their application to the palladium-catalyzed Suzuki reaction of aryl chlorides

Article abstract of DOI:10.1021/jo0265479

Racemic and enantiopure (_pS)-1-bromo-2-methylferrocene 6 were synthesized in 4 steps from 2-(4,4-dimethyloxazolinyl)ferrocene and (S)-2-(4-methylethyloxazolinyl)ferrocene, respectively (46 and 81% overall yield). Bromolithium exchange and addition of ClPR₂ gave the corresponding racemic or enantiopure 2-methylferrocenyl phosphine ligands 2-MeFcPR₂ 11 (R = Ph), 12 (R = Cy), and 13 (R = tBu) in 28-93% yield. Use of PCl₃ gave the C₃-symmetric phosphine (2-MeFc)₃P 5 from (_pS)-6 (72% yield) but racemic 6 did not lead to the formation of triferrocenyl phosphines. Combination of 5 and Pd₂(dba)₃ gave an active catalyst for the Suzuki reaction of aryl chlorides, for example, 4-chlorotoluene and phenylboronic acid reacted at only 60 °C in dioxane (86% yield). Other examples are reported together with the use of 12 in this same protocol. From the X-ray crystal structure of 5 the cone angle was determined as 211°. With this, and the electronic character of 11, 12, and other phosphines (derived from vco of trans-[(R₃P)₂Rh(CO)Cl]), an analysis is made of the steric and electronic influences on ligand activity in the Suzuki reaction.