Supported Ionic‐Liquid Films (SILF) as Two‐Dimensional Nanoreactors for Enantioselective Reactions: Surface‐Mediated Selectivity Modulation (SMSM)

Abstract

Supported ionic‐liquid films (SILF) of nanometric thickness containing bis(oxazoline)–copper complexes can be used as recoverable catalysts for enantioselective cyclopropanation reactions. When a thin film of ionic liquid is supported on a clay, the system behaves as a nearly two‐dimensional nanoreactor in which the restrictions in rotational mobility and the close proximity to the surface support produce variations in the stereo‐ and enantioselectivities, leading to a complete reversal of the overall selectivity of the reaction. As an example, the cyclopropanation of styrene changes from a preference for the (1S,2S)‐trans isomer in bulk solution to the (1R,2S)‐cis isomer in the SILF. This variation is strongly dependent on both the thickness of the film and the nature of the support. Only layered solids with negative charges in the layers (clays) give rise to this type of behaviour, showing that the formation of ion pairs may be a decisive factor. This unexpected effect also shows the existence of differences between bulk and supported ionic liquids.

Enantioselective catalysis with chiral complexes immobilized on nanostructured supports

“…8 The use of different supports (clays, graphite, 15 Y zeolite, silica and hydrotalcite) demonstrated the need for a layered support, as only clays showed confinement effects ( Figure 8). The close proximity of the complex to the surface was obtained by using decreasing amounts of IL, reducing in this way the thickness of the IL film.…”

Ionic Liquid Effect on the Reversal of Configuration for the Magnesium(II) and Copper(II) Bis(oxazoline)‐Catalysed Enantioselective Diels–Alder Reaction

“…SILPs have also been used to immobilise achiral homogeneous catalysts for Heck and Suzuki-Miyaura reactions [3] and Knoevenagel condensations. [4] To date, asymmetric SILP-mediated reactions have been limited to cyclopropanations [5] and the Mukaiyamaaldol [6] reactions using bis(oxazoline)-Cu(II) catalysts supported on silica. In both cases, comparable enantioselectivities and conversions as under homogeneScheme 1.…”

Recyclable Stereoselective Catalysts

“…Despite the moderate success, a totally different trans / cis ratio was disclosed when compared with the same reaction run in IL media. It was also observed that the thickness of the fi lm had a crucial infl uence on the selectivity [83] . The carbonyl -ene reaction catalyzed by copper -BOX complexes also proved to be considerably dependent on the BOX bite angle.…”