Mechanistic Studies on 14-Electron Ruthenacyclobutanes:  Degenerate Exchange with Free Ethylene

Abstract

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Calpha-Cbeta agostic interactions with the Ru center. Intramolecular exchange of Calpha and Cbeta is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Calpha and Cbeta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2C=CH2] (4.8(3) x 10-4 M-1 s-1). Activation parameters for this process are DeltaH++ = 13.2(5) kcal mol-1 and DeltaS++ = -15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.

Tailored Ru‐NHC Heterogeneous Catalysts for Alkene Metathesis

“…The deviation from the expected linearity is a clear indication of a change in catalyst structure occurring during initiation of the initial metallocarbene. Recent studies have shown that the olefin can approach either cis or trans with respect to the NHC ligand, [19][20][21] which could explain the observed deviation until a steady state is reached. We have therefore used to characterize the catalysts (Scheme 2 a, L 1 = PCy 3 vs. NHC): 1) the E/Z ratio at low conversions and 2) the extrapolated E/Z ratio of products at 3 and L 2 = NHC, respectively.…”

Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst

“…In the specific case of alkene metathesis, it has been proposed that carbenes and metallacyclobutanes are key reaction intermediates (16). Although they have been unambiguously prepared and observed for homogeneous catalysts (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29), observing them as heterogeneous catalysts has proved to be a formidable challenge (30)(31)(32). Note that because the surface can play such a crucial role, one should never assume that the intermediates are identical in heterogeneous and homogeneous systems (31,32).…”

Olefin Metathesis with Ru-Based Catalysts Exchanging the Typical N-Heterocyclic Carbenes by a Phosphine–Phosphonium Ylide

“…Although most of the existing literature supports the trans coordination mode (demonstrated for the Grubbs II system B [42,43]), both pathways may exist depending on the type of ligand and nature of the olefin. Indeed, since 2000, several studies have tried to reveal the preference for the trans mode and/or find alternative ways to promote the cis mode [44,45]: the issue remains an open debate in NHC-ruthenium catalysis [46,47]. In summary, a trans binding preferential can be generally postulated, but the special nature of the present phosphine-phosphonium ylide ligand might provide an example where the olefin can be coordinated in the cis mode: Correa and Cavallo [48] indeed showed that the trans/cis selectivity was based on a delicate balance between steric and electronic effects, also depending on the solvent.…”