Influence of syn and anti Configurations of NHC Backbone on Ru-Catalyzed Olefin Metathesis

Abstract

The synthesis and characterization of two ruthenium-based olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with syn (12) and anti (13) methyl groups on the backbone and o-tolyl N-substituents are reported. The catalytic performance of both 12 and 13 has been evaluated in ring-closing metathesis, ring-opening metathesis polymerization, and cross-metathesis reactions. The results have been compared to those obtained in the presence of the Grubbs second-generation catalyst 3a, lacking substituents on the NHC backbone. Of particular interest, compound 12 outperformed both 3a and 13 in the ring-closing metathesis of hindered olefins, demonstrating to be the most active monophosphine catalyst in this class of metathesis reactions up to now. The influence of syn and anti stereochemical relationship of methyl groups on the NHC backbone of 12 and 13 in the formation of sterically encumbered olefins has been discussed. The X-ray structure of novel ruthenium complex 15, generated during an attempt to prepare pyridine-based complex 14 and identified as a decomposition product derived from C−H activation of N-tolyl substituents, is also reported.

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…No significant difference between the two isomers of 7 with dissimilar backbones was appreciated in CM of 3 and 4, being both catalysts less active than GII, with slightly higher E selectivity. The behavior of the two catalysts was also flattened in the ROMP of 6, where both catalysts exhibit high activities as well as E/Z ratios very similar to GII and GIItol [26].…”

“…7-antiGII showed a lower activity than 7-synGII in the RCM of all three malonate derivatives, with an enhanced gap for hindered substrates where only half of yield with respect to 7-synGII was reached in the ring-closure of 11. In 2009, the effect of NHC backbone configurations in ruthenium catalysts bearing aromatic N-tolyl groups was explored by Grisi and coworkers (7-synGII and 7-antiGII, Figure 6) [26]. The synthesis of 7-synGII and 7-antiGII proceeded in good yield (55%-60%), although it required several steps, including the preparation of meso and chiral 1,2-diamines [38] to achieve the corresponding NHC ligand precursors.…”

“…Surprisingly, two isomeric compounds, corresponding to different conformations of the N-tolyl substituents (10a-synGII and 10b-synGII, Figure 8), were obtained. Since complex 7-synGII as well as GIItol exist as a mixture of rotational isomers both in the solid state and in solution [18,26,48], the obtainment of separate, stable rotational isomers was strictly related to the steric pressure exerted by the bulky phenyl groups on the backbone. Phosphine-containing complexes 10a-synGII and 10b-synGII were easily converted in the corresponding phosphine-free catalysts 10a-synHGII (95% yield) and 10b-synHGII (84% yield) by treatment with 2-isopropoxystyrene.…”

“…In the challenging RCM of malonate derivative 11, complex 10a-synGII gave the better result obtained with a phosphine-containing catalysts up to now (92% conversion within 30 min) (Table 4), proving to be more efficient than complexes 7-synGII and GIItol (Table 3). In comparison to the catalytic beaviour of complexes 7-synGII and GIItol, existing as a mixture of inseparable syn and anti NHC conformational isomers [18,26,48], the catalytic behavior of 10a-synGII, possessing frozen syn N-tolyl groups, furnished the unequivocal evidence for the importance of correctly disposed N-aryl groups to successfully accomplish RCM reactions. As for phosphine-free complex 10a-synHGII, although the differences in activity with respect to complexes 10b-synHGII and HGIItol were less pronounced (see Tables 3 and 4), it turned out to be the most efficient, allowing the ring-closure of the difficult substrate 11 also at a low catalyst loading (0.5 mol %).…”

“…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].…”