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

Abstract

Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2F2) were synthesized and characterized by X‐ray crystallography and theoretical calculations. These complexes were used in Rh‐catalyzed 1,2‐additions of phenylboroxine to N‐tosylimine in microemulsions stabilized either exclusively by n‐octyl‐β‐D‐glucopyranoside (C8G1) or a C8G1‐film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 % (90 %ee) for the surfactant film doped with 5 wt% of AOT as compared to 52 % (58 %ee) for neat C8G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies.

Determination of accessibility and spatial distribution of chiral Rh diene complexes immobilized on SBA-15 via phosphine-based solid-state NMR probe molecules

“…More likely, the presence of the rigid oxazolidinone unit in [RhCl( L1 )] and [RhCl( L1 )PPh 3 ] and the possible coordination of the oxazolidinone carbonyl to the Rh as compared to the more flexible immobilized Rh catalyst should lead to a different conformational preference despite the same absolute configuration and substitution pattern of the ligand (plus spacer). 25 The exchange of the oxazolidinone by the linker was necessary for immobilization. While the TONs calculated from the Rh loadings determined by ICP-OES were similar, the corrected TON that was calculated from the amount of accessible Rh was twice as high for the catalyst SBA-15-IN-1 with a lower concentration.…”

“…1,2-and 1,4-adducts, cycloaddition and rearrangement products. [17][18][19] In addition, asymmetric Rh diene complexes immobilized on various supports [20][21][22][23] and Rh diene complexes under liquid 24,25 and solid confinement 26,27 have been reported. In silica pores adsorbed hydridocarbonyltris(triphenylphosphine)rhodium (HRh(CO) (PPh 3 ) 3 ) in PPh 3 and solvent showed modified reaction rates and regioselectivities in hydroformylation of propene.…”

Tethering chiral Rh diene complexes inside mesoporous solids: experimental and theoretical study of substituent, pore and linker effects on asymmetric catalysis

“…1–6 Several approaches have been undertaken to obtain confined reaction spaces with pore sizes of 2–50 nm suitable for catalysis, such as artificial metalloenzymes, 7–9 supramolecular cages 10 or mesoporous solids 11,12 among others. 13–21…”

“…[1][2][3][4][5][6] Several approaches have been undertaken to obtain confined reaction spaces with pore sizes of 2-50 nm suitable for catalysis, such as artificial metalloenzymes, [7][8][9] supramolecular cages 10 or mesoporous solids 11,12 among others. [13][14][15][16][17][18][19][20][21] The immobilization of chiral soluble catalysts in mesoporous solids is very attractive for asymmetric heterogeneous catalysis. [22][23][24][25] For example, the confined spaces in such mesoporous solids can stabilize transition states and intermediates, e.g.…”