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

Abstract

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 Ntosylimines and 1,4-additions to enones was investigated. For this purpose, cationic dimeric Rh(I) complex [(Rh(1)) 2 Cl]SbF 6 and cationic monomeric Rh(I) complex [RhOH 2 (2)]SbF 6 were synthesized from oxazolidinone-substituted 3-phenylnorbornadiene ligands 1 and 2, which differ in the substitution pattern at oxazolidinone C-5′ (CMe 2 vs CH 2 ) 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 [RhOH 2 (2)]SbF 6 gave better yields as compared to dimeric complexes [RhCl(1)] 2 and [(Rh(1)) 2 Cl]SbF 6 . Regarding enantioselectivities, dimeric Rh species [RhCl(1)] 2 and [(Rh(1)) 2 Cl]SbF 6 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.

Efficient and Spatially Controlled Functionalization of SBA‐15 and Initial Results in Asymmetric Rh‐Catalyzed 1,2‐Additions under Confinement

“…In order to test the influence of the linker between diene ligand and mesoporous materials, three different chiral norbornadienes 2 a-c with terminal alkyne suitable for click immobilization were envisioned. As outlined in Scheme 4, the synthesis of norbornadienes 2 commenced with the treatment of known oxazolidinone-substituted 3-phenylnorbornadiene ligands 1 a, b [38] with ethynyl magnesium bromide. Depending on the substitution pattern at C 5 of the oxazolidinone auxiliary (CMe 2 vs CH 2 ), either the propargylic ketone 2 a was obtained by exocyclic cleavage from 1 a or the tethered amide 2 b was isolated from 1 b. Saponification of 2 b to alcohol 4 and subsequent Williamson etherification [39] with propargylic bromide yielded the respective propargylic ether 2 c in 89 % yield (Scheme 4, path B).…”

Interplay of Polarity and Confinement in Asymmetric Catalysis with Chiral Rh Diene Complexes in Microemulsions

“…During our previous studies we noticed that the catalytic performance depended strongly on the polarity and steric and electronic effects of the diene ligand [18] . Moreover, monomeric neutral and cationic Rh‐diene complexes differed significantly from their dimeric counterparts regarding enantiocontrol in microemulsions vs. conventional solvents [17] . Therefore, we surmised that the type of counterion bound to the Rh complex and thus the charge density at the Rh centre might control the outcome of the catalytic reaction.…”

“…This observation is in good agreement with our previous results for the similar complex [Rh(L1)OH 2 ]SbF 6 (Scheme S1, Supporting Information). [17] A comparison of crystallographic data of the monomeric complexes [Rh(L2)X] (X = OTf, OTs, OAc, PO 2 F 2 ) and [Rh(L2)OH 2 ] SbF 6 is given in Table 1.…”

“…[12] Recently, we investigated the effect of nanoscale confinement comprising a highly dynamic amphiphilic surfactant film by means of the Rh-catalyzed asymmetric 1,2-addition of triphenylboroxine to N-tosylimines utilizing chiral Rh-diene complexes in microemulsions. [17,18] Rh-diene complexes were chosen as benchmark catalysts because they are very versatile and catalyse a large variety of reactions. [19,20] Furthermore they are rather slow in conventional solvents such as dioxane.…”

Hierarchical Silica Inverse Opals as a Catalyst Support for Asymmetric Molecular Heterogeneous Catalysis with Chiral Rh‐diene Complexes

“…Then, hierarchical macro-mesoporous SiO 2 À IOs were used for the first-time as a catalyst support for chiral Rh-diene complexes, and their catalytic activity and selectivity with respect to the porous material was studied. As a catalytic model reaction, the Rh-diene-catalyzed 1,2-addition of phenylboroxine to Ntosylimines was chosen, because the corresponding homogeneous catalysis has been previously studied both experimentally and theoretically [41][42][43][57][58][59][60] and thus could serve as a benchmark for our novel solid catalyst.…”