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 PCl3 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 FeCl2 or with [Fe(η6-C9H12)(η5-C5H5)][PF6] respectively. The structure of diphosphaferrocene 4 was determined. Two conformations are present in the cell:  a C2 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 C2h 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 × 105). The catalytic activity of 6 in coupling reactions between
phenylboronic acid and 3-bromothiophene, 2-bromoanisole, and bromobenzene was also
investigated.