Computational Investigations of Enantioselection in Carbon–Carbon Bond Forming Reactions of Ruthenium Guanidinobenzimidazole Second Coordination Sphere Hydrogen Bond Donor Catalysts

Abstract

The NH 2 group of 2-guanidinobenzimidazole (GBI) can be replaced by (R C R C )-NHCH(CH 2 ) 4 CHNMe 2 and elaborated to the enantiopure chelate salts (− . These catalyze highly enantioselective additions of 1,3-dicarbonyl compounds to nitroalkenes. The mechanism and basis for enantioselection are probed by DFT calculations. First, the parent GBI complex [(η 5 -C 5 H 5 )Ru(CO)-(GBI)] + PF 6 − (1 + PF 6 − ) is examined. This species has only ruthenium-centered chirality and must be used with a trialkylamine, as it lacks the internal base of 2 + PF 6 − . The dicarbonyl compound initially hydrogen bonds to the NH triad of the GBI ligand, but the transition states leading to each product enantiomer are essentially equal in energy. In contrast, after similar bonding of the dicarbonyl compound to (− , a proton is transferred to the :NMe 2 moiety, giving an enolate and a HNMe 2 + group. The latter mediates the introduction of trans-β-nitrostyrene such that one enolate π face attacks the C si C re Ph face to give an addition product with an R configuration, in agreement with experiment. Thus, the configurations of the catalyst carbon stereocenters control the product stereochemistry. Interactions in competing transition states are analyzed.

Recent Developments in Enantioselective Transition Metal Catalysis Featuring Attractive Noncovalent Interactions between Ligand and Substrate

“…Following calculations, it was proposed that the complex and the malonate readily associate via NH···O hydrogen bonds, while the tertiary amine acts as a Brønsted base. 71 Once protonated, this can hydrogen bond with the nitroalkene ( Figure 14 c). These diastereomeric complexes could catalyze the addition of malonate esters to nitroalkenes with high yields and enantioselectivities while the free GBI ligand alone exhibited modest reactivity, highlighting the importance of the metal center for preorganization ( Figure 14 d).…”

Rhodium(III) Werner Complexes with 1,2‐Diphenylethylenediamine Ligands: Syntheses, Structures, and Applications as Chiral Hydrogen Bond Donor Catalysts and Agents for Enantiomer Purity Determinations

“…Furthermore, the transition states associated with all of the enantioselective reactions catalyzed by the cobalt and rhodium complexes in Figure 1 and Scheme 1 remain “black boxes”, [6,28] although certain rate data suggest that in some cases substrate access to a C 3 face is essential [11a] . As can be appreciated from I and II in Figure 1, as many as 4–5 NH groups may be simultaneously engaged.…”

An Air‐ and Water‐Stable Hydrogen‐Bond‐Donor Catalyst for the Enantioselective Generation of Quaternary Carbon Stereocenters by Additions of Substituted Cyanoacetate Esters to Acetylenic Esters

“…Some headway has been made in computationally determining the mechanisms of enantioselective addition reactions catalyzed by metal containing hydrogen bond donors . However, these have involved ruthenium catalysts where the NH groups are remote from the metal and only three can simultaneously participate in the transition state.…”