Role of Substrate Substituents in Alkene Metathesis Mediated by a Ru Alkylidene Catalyst

Abstract

Quantum mechanical calculations on the mechanism of olefin metathesis with a variety of substituents mediated by a Ru alkylidene catalyst reveal multistep processes along the general reactants → adduct → coordination complex → metallacycle → decoordination complex → products pathway for two consecutive turnovers. Net energy barriers in solution do not exceed 12 kcal mol −1 during the [Ru]The complex series of steps is initially driven by the evolution of the Ru(catalyst)•••C(olefin) contact. Dissection of bonding interactions using the tools provided by the natural bond orbitals and by the quantum theory of atoms in molecules methods indicate that each contact in the Ru(catalystcyclic reactive center undergoes the following series of transformations in different orders: no interaction → long range → σ → → π. Every single contact in this reactive center gains/loses an entire σ bond during the •••TS → metallacycle → TS••• interval. The lowest point in the potential energy surface is usually the metallacycle. For the first turnover, cycloreversion and final elimination of the products exhibit late transition states leading to higher relative energy barriers. Conversely, for the second turnover, it is the metallacycle to decoordination complex transformation step which leads to the highest barriers, constituting the rate-determining step for the entire process. Each step of the reaction is best described as a highly asynchronous process. Electron-withdrawing groups exhibit the largest overall barriers by virtue of destabilizing the emerging π bond in the final R 1 R 2 CCR 1 R 2 olefin during the second turnover.

The Janus face of high trans-effect carbenes in olefin metathesis: gateway to both productivity and decomposition

“…78 All catalysts studied exhibited a higher barrier to styrene selfmetathesis than ethylene metathesis, presumably resulting from both the steric bulk 85 and the electron-withdrawing properties of the phenyl substituent. 86 For the H 2 IMes catalyst nG, the difference is 6.6 kcal mol À1 , vs. 2.6 or 3.1 kcal mol À1 for nG-C1 Ph or nG-C2 Me , respectively. This reinforces the more facile reaction of nG with ethylene discussed above.…”

Analysis of Conformational Preferences in Caffeine

“…In this scenario, honoring Frank Weinhold on occasion of his 80th birthday, we resort to NBO and other analysis methods to resolve this structural conundrum. NBO, developed over the last few decades in the group of Frank Weinhold [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ], is a widely used and highly successful strategy in the analysis of a large body of chemical problems [ 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. Specifically, for the conformational problem in caffeine, for each methyl rotation, we study hyperconjugation, electron delocalization involving the methyl groups, NBO donor–acceptor interactions, and the effect of all these factors on geometrical variables.…”

Formation and evolution of C–C, C–O, CO and C–N bonds in chemical reactions of prebiotic interest

“…A variety of reactions has been successfully studied in the past by our group using this methodology. [30][31][32][33][34][35][36][37] 2 Computational methods Reaction paths starting with HCN and H 2 O and linking the set of molecular precursors of large biomolecules listed above, i. e., carbon dioxide, formaldehyde, formic acid, formaldimine, glycolaldehyde, glycine, glycolonitrile, and oxazole derivatives, were determined both in gas phase and in aqueous medium, as modelled by the Polarizable Continuum Model (PCM) approach 38,39 via intrinsic reaction coordinate (IRC) calculations using second order Moller-Plesset perturbation theory at the MP2/6-311++G(d, p) level. The latter is chosen so that the ambiguity and non systematic problems in the treatment of electron correlation and dispersion proper of Density Functional Theory (DFT) methods are eliminated.…”

“…A variety of reactions has been successfully studied in the past by our group using this methodology. 30–37…”