Abstract
During a Ru-catalyzed reaction of an olefin with an alkylidene moiety that leads to a metallacycle intermediate, the cis insertion of the olefin can occur from two different directions, namely side and bottom with respect to the phosphine or N-heterocyclic ligand (NHC), depending on the first or second generation Grubbs catalyst. Here, DFT calculations unravel to which extent the bottom coordination of olefins with respect is favored over the side coordination through screening a wide range of catalysts, including first and second generation Grubbs catalysts as well as the subsequent Hoveyda derivatives. The equilibrium between bottom and side coordination is influenced by sterics, electronics, and polarity of the solvent. The side attack is favored for sterically less demanding NHC and/or alkylidene ligands. Moreover the generation of a 14-electron species is also discussed, with either pyridine or phosphine ligands to dissociate.
Highlights
Organic synthesis is based on reactions that drive the formation of carbon–carbon bonds [1]
Olefin metathesis represents a metal-catalyzed redistribution of carbon–carbon double bonds [2,3,4,5,6] and provides a route to unsaturated molecules that are often challenging or impossible to prepare by any other means
The area of ruthenium-catalyzed olefin metathesis reactions is an outstanding field for the synthesis of C–C double bonds [7,8,9]
Summary
Organic synthesis is based on reactions that drive the formation of carbon–carbon bonds [1]. The inclusion of a polar solvent and the absence of strong steric effects, i.e., with less bulky ligands (less than SIMes) and/or substrates [50,51], favored the sidebound structures over the bottom-bound ones as suggested by Goddard and Grubbs, respectively [16,17,18] This is a possible explanation of why there is experimental evidence for some structures with side confirmation. System 7 has been thoroughly studied because of its simplicity, playing especially with the NHC ligand, replacing one or both mesityl groups by CH3, CF3, t-Bu, H, F, and combinations between them to observe the effects of an asymmetric NHC ligand This system 7 was taken to discuss about the evolution of the precatalysts, PRE-II, PRE-I and PRE-0 (see Scheme 1). Here we consider that the qualitative comparison between the set of studied catalysts and even the quantitative trends (side/bottom) should not be affected by this omission
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