Octaethyldiphosphaferrocene: An Efficient Ligand in the Palladium-Catalyzed Suzuki Cross-Coupling Reaction

Syntheses of octaethyldiphosphaferrocene 4 and tetraethylphosphaferrocene 5 are presented. Tetraethylzirconacyclopentadiene 1 reacts with PCl 3 in dichloromethane to yield the 1-chlorotetraethylphosphole 2, which upon reduction by lithium in excess, yields the teraethylphospholide anion 3 . Anion 3 was subsequently converted into 4 or 5 by treatment with FeCl 2 or with [Fe(η 6 -C 9 H 12 )(η 5 -C 5 H 5 )][PF 6 ] respectively. The structure of diphosphaferrocene 4 was determined. Two conformations are present in the cell: a C 2 conformation (α = 51.1°) in which the P atom of each ring is located above the α-carbon atom of the other ring, and a C 2 h conformation (α = 180°) in which P atoms point in opposite directions. Reaction of ligand 4 with [Pd(dba) 2 ] yields the bis(octaethyldiphosphaferrocene)palladium(0) complex 6 . An X-ray crystallographic study reveals that the overall geometry around palladium is nearly tetrahedral and that both ligands, whose geometry is not significantly perturbed, adopt a bridging mode involving a side-on coordination of the lone pairs. Complex 6 behaves as an efficient catalyst for the coupling reaction between phenylboronic acid and 4-bromoacetophenone in refluxing toluene. A conversion of 98% was obtained with 1 × 10 - 4 % of catalyst (TON = 9.80 × 10 5 ). The catalytic activity of 6 in coupling reactions between phenylboronic acid and 3-bromothiophene, 2-bromoanisole, and bromobenzene was also investigated.