Experimental and Theoretical Study on the Role of Monomeric vs Dimeric Rhodium Oxazolidinone Norbornadiene Complexes in Catalytic Asymmetric 1,2- and 1,4-Additions

The influence of nuclearity and charge of chiral Rh diene complexes on the activity and enantioselectivity in catalytic asymmetric 1,2-additions of organoboron reagents to N-tosylimines and 1,4-additions to enones was investigated. For this purpose, cationic dimeric Rh­(I) complex [(Rh­(1))2Cl]­SbF6 and cationic monomeric Rh­(I) complex [RhOH2(2)]­SbF6 were synthesized from oxazolidinone-substituted 3-phenylnorbornadiene ligands 1 and 2, which differ in the substitution pattern at oxazolidinone C-5′ (CMe2 vs CH2) and compared with the corresponding neutral dimeric and monomeric Rh­(I) complexes [RhCl­(1)]2 and [RhCl­(2)]. Structural, electronic, and mechanistic insights were gained by X-ray crystallography, cyclic voltammetry (CV), X-ray absorption spectroscopy (XAS), and DFT calculations. CV revealed an increased stability of cationic vs neutral Rh complexes toward oxidation. Comparison of solid-state and solution XAS (extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES)) data showed that the monomeric Rh complex [RhCl­(2)] maintained its electronic state and coordination sphere in solution, whereas the dimeric Rh complex [RhCl­(1)]2 exchanges bridging chloro ligands by dioxane molecules in solution. In both 1,2- and 1,4-addition reactions, monomeric Rh complexes [RhCl­(2)] and [RhOH2(2)]­SbF6 gave better yields as compared to dimeric complexes [RhCl­(1)]2 and [(Rh­(1))2Cl]­SbF6. Regarding enantioselectivities, dimeric Rh species [RhCl­(1)]2 and [(Rh­(1))2Cl]­SbF6 performed better than monomeric Rh species in the 1,2-addition, while the opposite was true for the 1,4-addition. Neutral Rh complexes performed better than cationic complexes. Microemulsions improved the yields of 1,2-additions due to a most probable enrichment of Rh complexes in the amphiphilic film and provided a strong influence of the complex nuclearity and charge on the stereocontrol. A strong nonlinear-like effect (NLLE) was observed in 1,2-additions, when diastereomeric mixtures of ligands 1 and epi-1 were employed. The pronounced substrate dependency of the 1,4-addition could be rationalized by DFT calculations.