Isomerization of an Enantiomerically Pure Phosphorus-Bridged [1]Ferrocenophane

The enantiomerically pure phospha[1]­ferrocenophane 4-C1 was prepared
through a salt-metathesis reaction between tBuPCl2 and a chiral dilithioferrocene derivative (Li2fc3‑Pen), which was equipped with two 3-pentyl
groups in α positions with respect to lithium on the Cp rings
(Sp,Sp isomer; C2 symmetry). The chiral 4-C1 isomerizes in reaction
mixtures to give the Cs-symmetrical phospha[1]­ferrocenophane 4-Cs. This
thermal isomerization involves haptotropic η5 to
η1 shifts of Cp rings and is catalyzed by the chelating
ligand 1,1′-(tBuClP)2fc3‑Pen, which is a byproduct of the salt-metathesis reaction. Both phospha[1]­ferrocenophanes, 4-C1 and 4-Cs, were isolated and characterized as pure
compounds; the molecular structure of 4-Cs was determined
by a single-crystal X-ray analysis. According to DFT calculations,
the equilibrium constant K° for 4-C1 ⇌ 4-Cs is 4.43 (ΔESCF =
−3.74 kcal/mol; ΔH° = −3.81
kcal/mol; ΔG° = −3.68 kcal/mol).
As deduced from calculated molecular geometries, the thermodynamic
difference between both isomers is mainly caused by a steric repulsion
between the tBu group on phosphorus and one of the
3-pentyl groups on a Cp ring in the isomer 4-C1.