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.

From whose bourns no traveller returns, puzzles the will: UV/Vis, 19F NMR, and fluorescence spectroscopic studies (see graphic) provide no evidence supporting the boomerang mechanism in Grubbs–Hoveyda complexes, which is the return of the isopropoxy styrene as a benzylidene ether ligand following its release during catalyst initiation.

Conversion-time data were recorded for various ring-closing metathesis (RCM) reactions that lead to five- or six-membered cyclic olefins by using different precatalysts of the Hoveyda type. Slowly activated precatalysts were found to produce more RCM product than rapidly activated complexes, but this comes at the price of slower product formation. A kinetic model for the analysis of the conversion-time data was derived, which is based on the conversion of the precatalyst (Pcat) into the active species (Acat), with the rate constant k(act), followed by two parallel reactions: 1) the catalytic reaction, which utilizes Acat to convert reactants into products, with the rate k(cat), and 2) the conversion of Acat into the inactive species (Dcat), with the rate k(dec). The calculations employ two experimental parameters: the concentration of the substrate (c(S)) at a given time and the rate of substrate conversion (-dc(S)/dt). This provides a direct measure of the concentration of Acat and enables the calculation of the pseudo-first-order rate constants k(act), k(cat), and k(dec) and of k(S) (for the RCM conversion of the respective substrate by Acat). Most of the RCM reactions studied with different precatalysts are characterized by fast k(cat) rates and by the k(dec) value being greater than the k(act) value, which leads to quasistationarity for Acat. The active species formed during the activation step was shown to be the same, regardless of the nature of different Pcats. The decomposition of Acat occurs along two parallel pathways, a unimolecular (or pseudo-first-order) reaction and a bimolecular reaction involving two ruthenium complexes. Electron-deficient precatalysts display higher rates of catalyst deactivation than their electron-rich relatives. Slowly initiating Pcats act as a reservoir, by generating small stationary concentrations of Acat. Based on this, it can be understood why the use of different precatalysts results in different substrate conversions in olefin metathesis reactions.

The diazaborole Me3Sn–B{N(Dipp)CH}2 (1; B{N(Dipp)CH}2=N,N′-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaborolyl) was prepared by the reaction of Me3SnCl with one equivalent of Li[B{N(Dipp)CH}2]. Single crystals of 1 were obtained from hexane (triclinic space group P1̅). The diazaborole 1 was mono-deprotonated at the heterocycle upon treatment with Li[Me] to give product 2. In contrast to Li[B{N(Dipp)CH}2] which reacted with P4 to give the tetraphosphenediide Li2[{HC(Dipp)N}2B–P(1)P(2)P(3)P(4)–B{N(Dipp)CH}2] (3; δP=364.5, –29.4; 1JP(2),P(3)=–509.8 Hz, 1JP(1),P(2)=–434.3 Hz, 2JP(1),P(3)=–3.7 Hz, 3JP(1),P(4)=178.9 Hz) and the triphosphenide Li[{HC(Dipp)N}2B–PPP–B{N(Dipp)CH}2] (δP=665.1, 175.4; 1JP,P=500 Hz), the stannyl derivative 1 did not activate white phosphorus. The reaction of 1 with GaCl3 yielded either Me2ClSn–B{N(Dipp)CH}2 (4) or MeCl2Sn–B{N(Dipp)CH}2 (5) depending on the molar ratio of the reactants. The monochlorinated diazaborole Me2ClSn–B{N(Dipp)CH}2 was also obtained by the reaction of 1 with AsCl3.