CHALCOGENAMETALLACYCLOHEXADIENES BY THERMALLY INDUCED MIGRATORY RING ENLARGEMENT OF FURYLZIRCONOCENE AND THIENYLZIRCONOCENE COMPLEXES

Zirconocene dichloride reacts with 2 molar equiv of (2-furyl)lithium to give bis(2-furyl)zirconocene (1a). The (sigma-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 (greater-than-or-equal-to 180-degrees-C) sigma,sigma-exchange reaction to give the oxazirconacyclohexadienes Cp2 activated ZrOCH = CHCH = C(R) (2a-c). The 1a --> 2a rearrangement follows first-order kinetics in the temperature range 140-180-degrees-C and is characterized by the activation parameters DELTA-H double-ended-dagger = 30.9 +/- 2.0 kcal mol-1 and DELTA-S double-ended-dagger = -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-degrees-C and already takes place during the formation of 1d from Cp2Zr(SiMe3)Cl and (2-furyl)lithium. The activation parameters of the 1e --> 2e ring enlargement reaction of the (2-thienyl)metallocene system are DELTA-H double-ended-dagger = 20.4 +/- 2.0 kcal mol-1 and DELTA-S double-ended-dagger = -12 +/- 5 cal mol-1 K-1. The oxa- and thiazirconacyclohexadienes Cp2 activated Zr - 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) angstrom, b = 11.514 (1) angstrom, c = 15.806 (1) angstrom, alpha = 96.27 (1)-degrees, beta = 101.64 (1)-degrees, gamma = 98.89 (1)-degrees, Z = 4, R = 0.037, and R(w) = 0.053. Complex 2e crystallizes in space group Cc with cell parameters a = 13.988 (4) angstrom, b = 30.774 (2) angstrom, c = 9.788 (4) angstrom, beta = 122.93 (1)-degrees, 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.