FORMATION AND REARRANGEMENT OF (SILOXYMETHYL)ZIRCONOCENE CHLORIDE COMPLEXES

The zirconocene eta2-formaldehyde dimer 6 reacts with trialkylchlorosilanes R3SiCl (R = ethyl, methyl, isopropyl) to yield the (eta2-(trialkylsiloxy)methyl)zirconocene chloride complexes Cp2-Zr(Cl)CH2OSiR, (7a-c). These complexes exhibit internal O-coordination; they belong to a metallaoxirane structural type. Thermolysis of 7b,c at 110-degrees-C leads to equilibration with the isomers 8b,c and 9b,c. Complexes 8 are the acyclic CP2Zr(Cl)CH2OSiR3 isomers of 7, whereas dyotropic rearrangement has resulted in the formation of the CP2Zr(Cl)OCH2SiR3 products 9. The kinetics of the thermally induced rearrangement of 7b (R = CH3) is described by a 8b half arrow right over half arrow left 7b --> 9b kinetic scheme. At 110-degrees-C the K(8b half arrow right over half arrow left 7b) equilibrium constant is 0.35. The Gibbs activation energy of the 7b --> 8b isomerization at 110-degrees-C is DELTAG(double dagger)isom(383 K) = 30.9 +/- 0.5 kcal mol-1. The activation barrier of the 7b --> 9b transformation is in the expected range of a concerted dyotropic rearrangement process at DELTAG(double dagger)dyo(383 K) = 31.5 +/- 0.5 kcal mol-1. Treatment of 6 with triphenylchlorosilane has resulted in the direct formation of the acyclic addition product CP2Zr(Cl)CH2OSiPh3 (8d). Our study underlines the important participation of metallaoxirane type structures in dyotropic rearrangement systems.