Expanding the Family of Hoveyda–Grubbs Catalysts Containing Unsymmetrical NHC Ligands

Abstract

A series of Hoveyda–Grubbs second-generation catalysts containing N-alkyl/N′-aryl N-heterocyclic carbene (NHC) ligands were synthesized and investigated in representative olefin metathesis reactions. Steric perturbations of unsymmetrical NHCs were achieved through modulation of the hindrance of alkyl (neopentyl, neophyl, cyclohexyl) and aryl (2-isopropylphenyl, mesityl) substituents at the nitrogen atoms in combination with different backbone configurations ( syn and anti). The NHC substitution patterns strongly influence the stability and reactivity of the corresponding complexes. In general, complexes bearing an anti NHC backbone are more stable and more active than their corresponding syn isomers. In both the series, the presence of bulky, highly branched N-alkyl groups tends to give reduced catalytic differences between syn and anti isomers, whereas the nature of the N′-aryl substituent (2-isopropylphenyl or mesityl) gives rise to different activity and/or selectivity. Of note, an N′-mesityl catalyst with anti backbone was found to be highly competent in the ethenolysis of ethyl oleate, achieving up to 90% selectivity for the formation of terminal olefins.

Helicenes as Chirality‐Inducing Groups in Transition‐Metal Catalysis: The First Helically Chiral Olefin Metathesis Catalyst

“…[9] On the downside, opportunitiesf or steric and electronic modulation through backbones ubstituentsa re restricted for thesel igand systems. [36,37] As as ubstituent for the second nitrogen we chose an N-mesityl group, because we expectedt his substituent to be sufficiently bulky to stabilizet he catalytically active 14-elec-tron species arising from dissociation of the temporary ligand, but not too sterically demanding to inhibita ny catalytic activity.For the synthesis of the required imidazolium salt, Fürstner's method [38] was used. One of the earliest examples for this strategy is the first chiral Ru-metathesis catalyst 5a.…”

“…Our synthesis of such ac atalyst (M)-6 starts from enantiopure 2-amino[6]helicene ((M)-1). [37] In summary,w eh ave devised as ynthesis of the first olefin metathesis catalyst with ah elicallyc hiral NHC-ligand. Unsymmetrical NHCs can be used to modulate steric congestion around the metal centera nd influence conformational preferencesa rising from ar otation around the Ru-NHC bond.…”

“…In addition, few other helicene substituted imidazolium salts have been synthesized through different routes, but not ligated to transition metals. [36,37] As as ubstituent for the second nitrogen we chose an N-mesityl group, because we expectedt his substituent to be sufficiently bulky to stabilizet he catalytically active 14-elec-tron species arising from dissociation of the temporary ligand, but not too sterically demanding to inhibita ny catalytic activity.For the synthesis of the required imidazolium salt, Fürstner's method [38] was used. Our synthesis of such ac atalyst (M)-6 starts from enantiopure 2-amino[6]helicene ((M)-1).…”

“…"Percent buried volume" was introduced by Nolan, Cavallo, and co-workersa saparameter to quantify the bulkiness of NHC ligands. [37] In summary,w eh ave devised as ynthesis of the first olefin metathesis catalyst with ah elicallyc hiral NHC-ligand. [47,48] For instance, for complex 5c a%V bur value of 33.7 was reported, [48,49] and for ad erivativeo fi ts unsaturated IMes-analogue a% V bur value of 31.9 was calculated.…”

“…Our synthesis of such ac atalyst (M)-6 starts from enantiopure 2-amino [6]helicene ((M)-1). [36,37] As as ubstituent for the second nitrogen we chose an N-mesityl group, because we expectedt his substituent to be sufficiently bulky to stabilizet he catalytically active 14-elec-tron species arising from dissociation of the temporary ligand, but not too sterically demanding to inhibita ny catalytic activity.For the synthesis of the required imidazolium salt, Fürstner's method [38] was used. Unsymmetrical NHCs can be used to modulate steric congestion around the metal centera nd influence conformational preferencesa rising from ar otation around the Ru-NHC bond.…”