On the Mechanism of the Initiation Reaction in Grubbs-Hoveyda Complexes

Grubbs-Hoveyda-type complexes with variable 4-R (complexes 1: 4-R = NEt2, OiPr, H, F, NO2) and 5-R substituents (complexes 2: S-R = NEt2, OiPr, Me, F, NO2) at the 2-isopropoxy benzylidene ether ligand and with variable 4-R substituents (complexes 3: 4-R = H, NO2) at the 2-methoxy benzylidene ether ligand were synthesized and the respective Ru(II/III) redox potentials (ranging from Delta E = +0.46 to +1.04 V), and UV-vis spectra recorded. The initiation kinetics of complexes 1-3 with the olefins diethyl diallyl malonate (DEDAM), butyl vinyl ether (BuVE), 1-hexene, styrene, and 3,3-dimethylbut-1-ene were investigated using UV-vis spectroscopy. Electron-withdrawing groups at the benzylidene ether ligands were found to increase the initiation rates, while electron-donating groups lead to slower precatalyst activation; accordingly with DEDAM, the complex 1(NO2) initiates almost 100 times faster than 1(NEt2). The 4-R substituents (para to the benzylidene carbon) were found to have a stronger influence on physical and kinetic properties of complexes 1 and 2 than that of S-R groups para to the ether oxygen. The DEDAM-induced initiation reactions of complexes 1 and 2 are classified as two-step reactions with an element of reversibility. The hyperbolic fit of the k(obs) vs [DEDAM] plots is interpreted according to a dissociative mechanism (D). Kinetic studies employing BuVE showed that the initiation reactions simultaneously follow two different mechanistic pathways, since the k(obs) vs [olefin] plots are best fitted to k(obs) = k(D).k(4)/k(-D).[olefin]/(1 + k(4)/k(-D).[olefin]) + k(1).[olefin]. The k(1).[olefin] term dominates the initiation behavior of the sterically less demanding complexes 3 and was shown to correspond to an interchange mechanism with associative mode of activation (I-a), leading to very fast precatalyst activation at high olefin concentrations. Equilibrium and rate constants for the reactions of complexes 1-3 with the bulky PCy3 were determined. In general, sterically demanding olefins (DEDAM, styrene) and Grubbs-Hoveyda type complexes 1 and 2 preferentially initiate according to the dissociative pathway; for the less bulky olefins (BuVE, 1-hexene) and complexes 1 and 2 both D and I-a are important. Activation parameters for BuVE reactions and complexes 1(NEt2), 1(H), and 1(NO2) were determined, and Delta S-double dagger was found to be negative (Delta S-double dagger. = 113 to 167 J.K-1.mol(-1)) providing additional support for the la catalyst activation.