Asymmetric Catalysis in Liquid Confinement: Probing the Performance of Novel Chiral Rhodium–Diene Complexes in Microemulsions and Conventional Solvents

Abstract

The role of liquid confinement on the asymmetric Rh catalysis was studied using the 1,2-addition of phenylboroxine (2)t oN-tosylimine 1 in the presence of [RhCl(C 2 H 4 ) 2 ] 2 and chiral dienel igandsa sb enchmark reaction. To get access to Rh complexes of differentpolarity,enantiomerically pure C 2 -symmetric p-substituted 3,6-diphenylbicyclo[3.3.0]octadienes 4 and diastereomerically enriched unsymmetric norbornadienes 5 and 6 carrying either the Evans or the SuperQuat auxiliary were synthesized.Amicroemulsion containing the equal amounts of H 2 O/KOH and toluene/reactants was formulated using the hydrophilic sugar surfactant n-octyl b-d-glucopyranoside( C 8 G 1 )t om ediate the miscibility between the nonpolarr eactants and KOH, neededt oa ctivate the Rh-diene complex. Prominent features of this organized reaction medium are its temperature insensitivity as well as the presence of water and toluenerich compartments with ad omain size of 55 confirmed by small-angle X-ray scattering (SAXS). Althoughb icyclooctadiene ligands 4a,b,e performed equally well under homogeneous and microemulsion conditions, ligands 4c,d gave a different chemoselectivity.F or norbornadienes 5, 6,h owever, microemulsions markedly improved conversion and enantioselectivity as well as reaction rate, as was confirmed by kinetic studies using ligand 5b.Scheme1.EnvisionedRh-catalyzed asymmetric 1,2-additions in the presence of novel chiral bicyclo[3.3.0]octadiene and norbornadiene basedl igands 4-6 (ME:microemulsion). Scheme2.Synthesis of novel chiral bicyclo[3.3.0.]octadiene ligands 4.For details see the SupportingInformation.

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

“…The corresponding homogeneously Rh-catalyzed 1,2-addition has been extensively studied and various diene ligands have been developed. [27][28][29] However, until now the influence of a solid confinement provided by tailored mesoporous silica on the catalytic activity and enantioselectivity of Rh diene complexes has not been investigated. Thus, the aim of the present study was to show that the chiral Rh-containing complexes are located exclusively in the mesopores of SBA-15 and can effectively control the enantioselectivity in the 1,2addition of triphenylboroxine to N-tosylimines.…”

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

“…The synthesis of chiral bicyclo[3.3.0]octadienes L1 and L2 started from the known bis-enoltriflate [70] 7 with a previously published Suzuki coupling [71] using an equimolar mixture of phenylboronic acid and 4-hydroxyphenylboronic acid in the presence of 10 mol % Pd(PPh 3 ) 4 to yield a stochiometric mixture of diphenyldiene L3 (23 %), di(4-hydroxyphenyl)diene 9 (15 %) and mono(4-hydroxyphenyl)diene 8 (41 %) (Scheme 2). Notably, no waste is generated since both phenolic dienes 8 and 9 could be functionalized in a second run, while the diphenyldiene L3 serves perfectly as a benchmark ligand with broad applications.…”

Adventures and Detours in the Synthesis of Hydropentalenes

“…101 More recently, bicyclo[3.3.0]octadienes and the resulting Rh complexes were probed in microemulsions (MEs) as nonconventional solvents. 102 MEs are stable single-phase emulsions of a hydrophobic solvent (oil), a polar solvent (e.g., H 2 O), and a surfactant, such as a sugar amphiphile C 8 G 1 . 103 These MEs have a sponge-like nanostructure with channels measuring 55 Å that provide excellent liquid confinement, in particular for multicomponent reactions involving substrates, catalysts, additives, and products with various polarities.…”

“…Furthermore, preliminary kinetic experiments revealed a rate increase in the ME as compared with conventional solvent mixtures such as 1,4-dioxane-H 2 O. 102 The promising results of the roof-shaped bicyclo[3.3.0]octadienes in asymmetric catalysis motivated several groups worldwide to step into this area and to develop novel hydropentalene-derived ligands. [104][105][106] A brief overview is given in Scheme 41.…”