Chalcogenametallacyclohexadienes by thermally induced migratory ring enlargement of furyl- and thienylzirconocene complexes

Article abstract of DOI:10.1021/om00040a040

Zirconocene dichloride reacts with 2 molar equiv of (2-furyl)lithium to give bis(2-furyl)zirconocene (1a). The (σ-furyl)zirconocene complexes Cp₂ZrR(2-furyl) (R = CH₃, Ph, SiMe₃) were similarly prepared by treatment of the respective Cp₂Zr(R)Cl complexes with (2-furyl)lithium. Cp₂Zr(SiMe₃)(2-thienyl) was obtained from the reaction of Cp₂Zr(SiMe₃)Cl with (2-thienyl)lithium. The Cp₂ZrR(2-furyl) complexes with R = 2-furyl, methyl, or phenyl all undergo an intramolecular high-temperature (≥180°C) σ,σ-exchange reaction to give the oxazirconacyclohexadienes Cp₂ZrOCH=CHCH=C(R) (2a-c). The 1a → 2a rearrangement follows first-order kinetics in the temperature range 140-180°C and is characterized by the activation parameters ΔH? = 30.9 ± 2.0 kcal mol^-1 and ΔS? = -9 ± 5 cal mol^-1 K^-1. The complexes Cp₂Zr(SiMe₃)(2-furyl) (1d) and Cp₂Zr(SiMe₃)(2-thienyl) (1e) undergo the analogous dyotropic rearrangements much faster. The 1d → 2d isomerization is fast at 0°C and already takes place during the formation of 1d from Cp₂Zr(SiMC₃)Cl and (2-furyl)lithium. The activation parameters of the 1e → 2e ring enlargement reaction of the (2-thienyl)metallocene system are ΔH? = 20.4 ± 2.0 kcal mol^-1 and ΔS? = -12 ± 5 cal mol^-1 K^-1. The oxa- and thiazirconacyclohexadienes Cp₂Zr-X-CH=CHCH=C(SiMe₃) (X = O, 2d; X = S, 2e) were characterized by X-ray diffraction. Complex 2d crystallizes in space group P1 with cell parameters a = 9.783 (1) A?, b = 11.514 (1) A?, c = 15.806 (1) A?, α = 96.27 (1)°, β = 101.64 (1)°, γ = 98.89 (1)°, Z = 4, R = 0.037, and R_w = 0.053. Complex 2e crystallizes in space group Cc with cell parameters a = 13.988 (4) A?, b = 30.774 (2) A?, c = 9.788 (4) A?, β = 122.93 (1)°, Z = 8, R = 0.034, and R_w = 0.035. Both chalcogenazirconacyclohexadienes are monomeric in the solid state and exhibit nonplanar metallacyclic conformations with the metal-chalcogen vectors rotated significantly relative to the planes of the endocyclic conjugated diene moieties.