The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

Abstract

The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…Notably, anti complexes disclosed an unexpectedly high propensity to the ring closure of the most hindered diolefins 9 and 11, rivaling with the benchmark catalysts GIItol and HGIItol. This is in contrast with results reported for analogous systems incorporating symmetrical N-substituents [26,27], where instead syn isomers were found better performing, and suggests that the impact of backbone configuration of unsymmetrical N-substituted NHCs on catalyst properties is not easily predictable or interpretable. The catalytic behavior of catalysts 11-GII and 11-HGII was also investigated in the CM of allylbenzene (3) and cis-1,4-diacetoxy-2-butene (4), depicted in Scheme 1.…”

“…The analogous phosphine-free catalysts 7-HGII and 8-HGII were easily synthesized with standard procedures in good yields (60%-88%) [27]. All obtained phosphine-free catalysts were able to convert 1 to 2 in a few minutes at 60 °C, with performances comparable to HGIItol, as emerged from data reported in Table 1.…”

“…Phosphine-containing Ru catalysts bearing N-o-isopropylphenyl groups in place of N-o-tolyl (8-synGII and 8-antiGII, Figure 6) can be prepared in good yields (60%-65%) with the same procedures adopted for 7-synGII and 7-antiGII [27]. As expected by the increased catalyst bulkiness, 8-synGII and 8-antiGII exhibited higher activity in the RCM of substrates 1 and 9 [41][42][43][44][45] with respect to the corresponding o-tolyl N-substituted catalysts (Tables 1 and 2), whereas the RCM of the more encumbered 11 appeared slowed down ( Table 3).…”

“…The role of the NHC backbone configuration in the RCM of challenging hindered substrates, investigated by theoretical studies on monophosphine o-tolyl N-substituted catalysts, revealed that the rate determining step occurs at the very beginning of the RCM, during the first CM of the substrate, and the corresponding transition state of the syn complex, that induces a syn orientation of the o-tolyl groups, presents lower energy than the anti complex mainly due to steric reasons [27].…”

“…Modifications of the NHC ligand include the nature of the ring backbone (saturated or unsaturated), substitution at the nitrogen atoms and at the carbon atoms of the backbone, ring-size variation, introduction of heteroatoms in the skeleton, and introduction of chirality [11][12][13][14][15]. Among these, modifications of the steric and electronic properties of substituents on the backbone and/or the nitrogen atoms have had a significant impact on catalyst activity, stability and selectivity in several metathesis applications [16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”