Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity

Abstract

The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solid-state NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl2(H2IMes)­(PCy3)­(CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl2(PCy3)2(CHR) (GI, GIm). The compiled rate constants for PCy3 dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The 31P chemical shift (CS) tensors for the PCy3 ligand exhibited the expected changes (i.e., those characteristic of an increased Ru–P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the 13C CS tensors. Whereas calculations on truncated models predict significant differences in 13C CS tensor values for GII compared with GIIm, the experimental values are equivalent, implying a compensating effect that weakens the RuC interaction in the benzylidene complex. Published X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane by steric interactions with the benzylidene phenyl group, an effect absent in GIIm and GIm. During PCy3 loss from the [Ru]CHPh systems, established processes of alkylidene rotation transform Ph–Cl repulsion into Ph–PCy3 repulsion. Displacing the PCy3 ligand below the plane does not relieve this conflict, instead incurring steric interactions with the H2IMes ligand. Enhanced PCy3 lability in the benzylidene complexes, relative to their methylidene analogues, is hence proposed to originate in the steric pressure exerted by the Ph substituent.

Bis(Cyclic Alkyl Amino Carbene) Ruthenium Complexes: A Versatile, Highly Efficient Tool for Olefin Metathesis

“…[25] The product was isolated as ared solid in 70 %yield. Surprisingly, the signal from the phosphorus atom was not present in 31 PNMR spectra, which suggests substitution of both phosphines by CAACs. The 1 HNMR spectra of product was complicated owing to the presence of rotational isomers, [26] and could not provide reliable information about its structure.…”

“…Therefore,t he most plausible explanation for the very broad range of activities covered by catalysts 15 is that the degree of steric repulsion between two CAACs determines the initiation rate. [31] Next, ethenolysis of methyl oleate 7 was performed. Also in this case, 12 g was used for the comparative reasons.…”

Alkyne gem‐Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character

“…Complementary to the information gained from the 1 H PHIP NMR experiments about carbene formation and stability,s olid-state 13 CNMR allows the electronic character of these organometallic intermediates to be assessed. [25][26][27] While isotropic chemical shifts (d iso = (d 11 + d 22 + d 33 )/3) are obtained by solution NMR, the three principal components of the chemical shift tensor (d 11 ! d 22 !…”

“…Forinstance,the rather low-lying p*(M=C) orbital of aFischer carbene entails massive deshielding,especially of d 11 ,w hile nucleophilic Schrock alkylidenes experience as ignificant contribution of their high energy p(M = C) orbital to d 11 .B ased on such patterns,m embers of these families can apriori be distinguished on spectroscopic grounds. [25][26][27] This powerful analytical tool was applied to complexes 14-18 (Figure 3), which correspond to the stationary points A-D of the reaction coordinate of gem-hydrogenation;t he computed data were calibrated with the experimental values available for 14, 17 and 18.Ithas previously been noted that the C-atoms of 1-(prop-1-yn-1-yl)cyclohexan-1-ol (d iso = 84/ 80 ppm) experience large but unequal deshielding upon formation of complex 14 (d iso = 159/130 ppm; Figure 4b). [29,30] This fact was nicely reproduced by DFT calculations,w hich indicate that this effect originates from the low lying LUMO essentially composed of av acant metal d-orbital interacting with the p(CÀC) bonding orbital (see the Supporting Information (SI) for more details).…”

ChemInform Abstract: Ethenolysis: A Green Catalytic Tool to Cleave Carbon—Carbon Double Bonds

“…Fogg and co-workersr ecently reported that, in second-generation Grubbsc atalysts, the steric pressureo ft he benzylidene ligand,c ompared to the resting-state ruthenium methylidene, facilitated the loss of the PCy 3 ligand thuse nhancing the initiation and propagations teps in olefin metathesis. [39] Metal hydrides appear to be important decomposition products,a st hey contribute to isomerization reaction. [13,23,24,40,41] However,t he groups of Wagener and Fogg found that commonly proposed ruthenium hydride species arisingf rom decomposition of Ru=CH 2 species or catalyst decomposition did not efficientlyp romote isomerization reactions.…”