STRUCTURE-REACTIVITY CORRELATIONS FOR THE FORMATION OF ZIRCONOCENE ETA-2-IMINE COMPLEXES FROM AMINES

The formation of eta2-imine-zirconocene complexes (zirconaziridines) (CP2Zr(NR1CR2R3) by elimination of R4H from CP2Zr(R4)(NR1CHR2R3) has been investigated with regard to the variation in R1, R2, R3, and R4, in particular by making use of Hammett type structure/rate correlations (R1, R2, R4 = p-XC6H4, X = Me2N, MeO, H, Cl, CO2Me; R1, rho = 3.2; R2, rho = 0.5; R4, rho = -1.6). The elimination is first order in the zirconocene complex, has a deuterium isotope effect for the hydrogen eliminated of 8.2 at 20-degrees-C, and kinetic studies on Cp2Zr(Me)(NPhCHMe2) give the activation parameters DELTAH(double dagger) = 100 kJ mol-1 and DELTAS(double dagger) = -19 J K-1 mol-1 for the elimination of methane. A cyclometalation involving deprotonation alpha to nitrogen by (R4)- best fits the data. The relationship between the rate of the reaction and the structure of the amine shows a marked dependency on both electronic and steric effects ranging between no reaction after 48 h at 110-degrees-C for piperidine to below room temperature for silylamines and benzylanilines. For the first time.eta2-imine complexes have been formed even from simple amines such as dibutylamine and trapped with an alkyne to form secondary allylic amines on workup. In the absence of a trap eta2-(PhN=CMe2)ZrCP2 rearranges via a rapidly reversible hydride shift to afford a eta3-azaallyl-zirconocene hydride.