Computational and Experimental Insights into Asymmetric Rh‐Catalyzed Hydrocarboxylation with CO₂

Abstract

The asymmetric Rh-catalyzed hydrocarboxylation of α,β-unsaturated carbonyl compounds was originally developed by Mikami and co-workers but gives only moderate enantiomeric excesses. In order to understand the factors controlling the enantioselectivity and to propose novel ligands for this reaction, we have used computational and experimental methods to study the Rh-catalyzed hydrocarboxylation with different bidentate ligands. The analysis of the CÀ CO 2 bond formation transition states with DFT methods shows a preference for outer-sphere CO 2 insertion, where CO 2 can undergo a backside or frontside reaction with the nucleophile. The two ligands that prefer a frontside reaction, StackPhos and t Bu-BOX, display an intriguing stacking interaction between CO 2 and an N-heterocyclic ring of the ligand (imidazole or oxazoline). Our experimental results support the computationally predicted low enantiomeric excesses and highlight the difficulty in developing a highly selective version of this reaction.

Steric and dispersion effect of chiral diphosphine ligand on copper-catalyzed enantioselective incorporation of CO2with styrene: a computational study

“…Afterward, the Mikami group 10 reported the catalytic asymmetric hydrocarboxylation of the prochiral α,β-unsaturated esters with CO 2 by using the cationic ( S )-SEGPHOS-Rh complex as the catalyst and AgSbF 6 as the cocatalyst, which led to the corresponding carboxylation product with moderate enantioselectivity (66% ee value). Of note, the Hopmann group 11 carried out the computational and experimental approaches to understand the factors that control the enantioselectivity in such reaction systems. Their DFT calculations showed that CO 2 insertion into the substrate preferably undergoes an outer -sphere C–CO 2 bond formation transition state, where the Rh centre is η 6 chelated with the phenyl ring of the substrate, and is far away from the nucleophile CO 2 simultaneously.…”

Understanding the Influence of Lewis Acids on CO2 Hydrogenation: The Critical Effect Is on Formate Rotation

“…For insertion of CO 2 into metal–R bonds (R = hydride or alkyl/aryl), generally two mechanisms can be considered (Scheme A): i ) An inner sphere path , involving an interaction between CO 2 and the metal at the transition state for H–CO 2 bond formation (TS inn ). ,− Note that a precoordination of CO 2 is not required ,,,− Subsequently, the resulting η 1 -σ-intermediate rearranges (TS2) , to allow for formation of a κ 1 -O-(or κ 2 -O,O-)­carboxylate species. It is important to note that although TS inn and TS2 both are cyclic, they are entirely different transition states, whose optimized geometries will show distinct features.…”