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

Abstract

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.

Solvent effects on Grubbs’ pre-catalyst initiation rates

“…The interchange mechanism supported by our computational work 17 and also by Plenio's experimental data, 18 is earlier with respect to the Ru⋯O extension, and includes the approach of an alkene molecule towards the Ru centre (EVE in the case of initiation) and some sharing of alkene electron density with the Ru centre. The three types of transition states have different shapes, volumes and polarities so it is not surprising that there is no simple relationship between dielectric constant or E T (30) for either pre-catalyst, and that multi-parameter approaches are required.…”

Optimized reaction conditions for the cross-metathesis of methyl oleate and oleylamine with ethyl acrylate

“…In this case also, a longer reaction time did not improve the conversion and again polymerization of 5 occurred. A possible explanation for the poor performance of 2 might be either the increased steric demand of the 2-oxo-3-butyl substituent compared with the isopropyl group present in 1 or j 3 coordination of the carbene ligand or even a combination of both effects [14]. Finally, 0.1 mol% 3 was used as the catalyst and after a reaction time of 16 h 58% conversion of 4 was observed (cf.…”

Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes

“…Conversion of S7 to S8 using triisopropylsilyl triflate and 2,6-lutidine under analogous conditions to those for the conversion of S3/S4 to S5/S6 provided additional S8 (1.69 g, 71% yield). 1 H NMR (500 MHz, CDCl 3 ) δ 7.64-7.60 (m, 1H), 7.25-7.20 (m, 1H), 7.13 (td, J 1 = 7.5 Hz, J 2 = 1.0 Hz, 1H), 6.92 (dd, J 1 = 8.0 Hz, J 2 = 1.0 Hz, 1H), 6.61 (dd, J 1 = 13.8 Hz, J 2 = 6.1 Hz, 1H), 4.87 (s, 2H), 4.66 (dd, J 1 = 13.8 Hz, J 2 = 1.7 Hz, 1H), 4.39 (dd, J 1 = 6.1 Hz, J 2 = 1.7 Hz, 1H), 1.24-1.15 (m, 3H), 1.10 (d, J = 6.9 Hz, 18H); 13 ((2-cyclopropoxybenzyl)oxy)triisopropylsilane (S9): 8 To an oven-dried 100 mL round-bottom flask containing a Teflon-coated magnetic stir bar under an Ar atmosphere, were added triisopropyl((2-(vinyloxy)benzyl)oxy)silane (S8) (2.10 g, 6.85 mmol), DCM (50 mL), and chloroiodomethane (3.0 mL, 41.1 mmol). The solution was cooled to 0 °C in an ice bath, and diethylzinc solution (1.0 M in hexane) was added dropwise over 30 min.…”