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

Grubbs-Hoveyda-type complexes with variable 4-R (complexes 1: 4-R = NEt(2), OiPr, H, F, NO(2)) and 5-R substituents (complexes 2: 5-R = NEt(2), OiPr, Me, F, NO(2)) at the 2-isopropoxy benzylidene ether ligand and with variable 4-R substituents (complexes 3: 4-R = H, NO(2)) at the 2-methoxy benzylidene ether ligand were synthesized and the respective Ru(II/III) redox potentials (ranging from ΔE = +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(NO(2)) initiates almost 100 times faster than 1(NEt(2)). 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 5-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(I)·olefin. The k(I)·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 PCy(3) 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(NEt(2)), 1(H), and 1(NO(2)) were determined, and ΔS(‡) was found to be negative (ΔS(‡) = -113 to -167 J·K(-1)·mol(-1)) providing additional support for the I(a) catalyst activation.