Urbina-Blanco, C. A.; Poater, A.; Lebl, T.; Manzini, S.; Slawin, A. M. Z.; Cavallo, L.; Nolan, S. P. The Activation Mechanism of Ru-Indenylidene Complexes in Olefin Metathesis. J. Am. Chem. Soc. 2013, 135, 7073-7079.
Recent literature accounts have challenged the notion that all olefin metathesis reactions catalyzed by Ru-NHC complexes are identical, beginning with dissociation of a ligand trans to the NHC. However, these accounts have focused on the structurally unique Hoveyda-type complexes, which are distinguished by a weak intramolecular ether chelate and lower steric bulk in the 16 electron species. This post addresses a recent account by Urbina-Blanco, et al. that elucidates an associative (interchange) mechanism for a popular phosphine containing phenyl indenylidene catalyst (1).
The authors point out that understanding mechanisms for organometallic catalysts leads to the design of next generation complexes. The importance of the activity is highlighted not only by the inclusion of Yves Chauvin as a co-recipient of the 2005 Nobel Prize for his work toward understanding metathesis, but by many other contributions in the field of organometallic catalyzed organic reactions that have led to new, general synthetic methodology.
Since the advent of sIMesRu(Cl)2(PR3)(CHPh), 2, Grubbs’ second generation catalyst, the mechanism of reactivity for all similar catalysts has been assumed to be more or less the same. The general scheme goes: 1) phosphine dissociation to provide a reactive 14 e– intermediate 2) olefin binding 3) metallacyle formation 4) metallacyle decomposition and 5) release of the new olefin product from the metal coordination sphere. In the recent JACS account, the authors study the phenyl indenylidene catalyst motif alongside classical Grubbs’ catalysts (1st and 2nd generation for both). According to experimental and DFT studies, some catalysts prefer the expected mechanism while one, sIMesRu(Cl)2(PCy3)(PhInd), 1, proceeds through an interchange mechanism wherein the phosphine is displaced by an incoming substrate (think SN2). This is illuminated by negative entropy terms derived from Arrhenius plots for phosphine exchange and irreversible olefin metathesis reaction with alkyl vinyl ethers.
It is well known that steric and electronic influences on the overall olefin metathesis reaction profile are complicated. Hypothetically, each activation barrier could respond differently to added bulk in the NHC. The position of that bulk on the NHC (i.e. ortho or para on the aromatic ring) could effect metallacyle formation/decomposition but not olefin coordination. The contributions from sterics and electronics can be synergistic for one part of the mechanism but work against each other in the next step. Thus, it is no surprise that the authors find that situation in their latest studies. As it turns out, a key contributing factor in the difference between 1 and 2 is the ability of the benzylidene fragment to rotate and form an agostic bond to the Ru center, stabilizing the 14 e- intermediate. Complex 1 is devoid of this stabilization due to hindered rotation, which facilitates the interchange mechanism. Adding a new NHC (sIPr) adds sufficient bulk to prevent olefin coordination to the 16 e– species necessary for the interchange mechanism and forces a change to the classical phosphine dissociation mechanism.
The ramifications of these insights are not abundantly clear, but they are certainly thought provoking! It immediately leads me to wonder about the mechanisms of other common catalysts (like the dimethyl vinylidenes). Does an agostic interaction stabilize the corresponding 14 e– complex? Does this catalyst proceed through an interchange mechanism as well? Perhaps the Nolan and Cavallo groups will provide more insight into the ubiquity of this mechanism in a follow up full paper.