Isomerization of an η3-allyl to an (E)-1-propenyl in (C5Me5)(C5Me4CH2)TiC3H5:: Mechanism, kinetics, and thermodynamics

The isomerization of Cp*FvTi(eta(3)-C3H5) (1) to Cp*FvTi((E)-CH=CHMe) (5) proceeds through four intermediates, Cp*Ti-2(eta 2-allene) (2), two geometrical isomers of Cp*FvTiCMe=CH2 (3a,b), and Cp*Ti-2(eta(2)-propyne) (4) (Cp*, C5Me5; Fv, C5Me4CH2). The process was monitored at four temperatures between 10 and 40 degrees C starting from 1 or the mixture 3a,b. The time-dependent concentration profiles of all compounds were simulated with a mathematical model of 10 rate constants. The model is based on the assumption that the isomerization proceeds through reversible first-order reaction steps in the sequence 1 reversible arrow 2 reversible arrow 3a reversible arrow 3b reversible arrow 4 reversible arrow 5. Equilibrium constants yield Gibbs free energy differences between the complexes ranging from 0 to -8 kJ/mol. The activation enthalpies for the forward reactions lie between 75 and 102 kJ/mol and between 74 and 111 kJ/mol for the reverse reactions. The entropy of activation is negative for the forward and reverse reactions between 1 and 2 and between 4 and 5 and positive for transitions involving the sterically congested isomers 3a,b, except for the rotation of the isopropenyl group in 3a.