Flexibility of N-Heterocyclic Carbene Ligands in Ruthenium Complexes Relevant to Olefin Metathesis and Their Impact in the First Coordination Sphere of the Metal

Abstract

We present a detailed static and dynamics characterization of 11 N-heterocyclic carbene (NHC) ligands in Ru complexes of the general formula (NHC)Cl(2)Ru horizontal lineCH(2). Analysis of the dynamic trajectories indicates that the nature of the N substituent can result in extremely different flexibilities of the Ru complexes. In almost all the cases the N substituent trans to the Ru-ylidene bond is severely folded so that it protects the vacant coordination position at the Ru center. Limited flexibility is instead associated with the N substituent on the side of the Ru-ylidene bond. NHCs with a single ortho substituent, either a simple Me or a bulkier i-Pr group, have a preferential folding that bends the unsubstituted side of the ring toward the halide-Ru-halide plane. Analysis of the dynamics trajectories in terms of buried volume indicates that the real bulkiness of these systems can be somewhat modulated, and this flexibility is a key feature that allows NHCs to modulate their encumbrance around the metal in order to make room for bulky substrates. Analysis of the buried volume in terms of steric maps showed that NHCs with mesityl or 2,6-diisopropylphenyl N substituents have quite different reactive pockets: rather flat with constant pressure on the halide-Ru-halide plane in the former and vault-shaped with higher pressure on the sides in the latter. Regarding the NHCs with an ortho tolyl or i-Pr group on the N substituent, the steric maps quantify the higher impact of the unsubstituted side of the ligand in the first coordination sphere of the metal and evidence the overall C(s)- and C(2)-symmetric reactive pockets of the corresponding complexes. We believe that a detailed characterization of the differently shaped reactive pockets is a further conceptual tool that can be used to rationalize the experimentally different performances of catalysts bearing these ligands or to devise new applications.

Breaking the regioselectivity rule for acrylate insertion in the Mizoroki–Heck reaction

“…Steric maps (29,30) based on calculated ground state geometries of 1-DMSO and 2b-DMSO show that all phosphine sulfonato ligands exert some steric pressure in the two bottom quadrants (Fig. 6 C and F) of the first coordination sphere around the palladium atom.…”

The Measure of All Rings—N‐Heterocyclic Carbenes

“…by Cavallo and co-workers takes into account the impact of the flexibility of NHC ligands and, arguably, represents the starting point for a dynamic model that more precisely describes the actual metal environment during catalysis. [60] The effect of backbone (un)saturation or annulation upon the steric demand is quite small (Table 7, entries 3 and 5, 7 and 8; Figure 14), but variation of the substituents on the 4-and 5-positions allows the fine tuning of the steric properties (Table 7, entries 7 and 9). In some selected cases, backbone substitution can exhibit a rather strong effect upon the buried volume (Table 7, entries 3 and 10).…”

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…Significantly, for the first time the AROCM of norbornenes with allyltrimethylsylane as the cross-partner was investigated (e.g., AROCM of 36, Scheme 10). The fixed, non-rotable N-aryl unit in the NHC ligand of catalysts 25-GII and 26-HGII led to higher enantioselectivities for both E/Z stereoisomers of the formed olefin with respect to Grubbs catalyst 18a-antiGII, in which the possible partial N-aryl rotation gives rise to a more flexible reaction pocket, resulting in a lower enantioselectivity [59]. One year later the same group reported ruthenium catalysts 25-GII and 26-HGII coordinated with a new type of chiral NHC ligand presenting an intramolecular linkage between the N-aryl and the backbone which creates a rigid chiral environment around the metal [58].…”