Chiral Chromium Salen@rGO as Multipurpose and Recyclable Heterogeneous Catalyst

Abstract

The first immobilization of a pyrene-tagged chromium salen complex through π-π noncovalent interactions on reduced graphene oxide (rGO) is described. A very robust supported catalytic system is obtained to promote asymmetric catalysis in repeated cycles, without loss of activity or enantioselectivity. This specific behavior was demonstrated in two different catalytic reactions (up to ten reuses) promoted by chromium salen complexes, the cyclohexene oxide ringopening reaction and the hetero-Diels-Alder cycloaddition between various aldehydes and Danishefsky's diene. Furthermore, the chiral chromium salen@rGO has been found to be compatible with a multi-substrate type use, in which the structure of the substrate involved is modified each time the catalyst is reused.

Post-Modification of Copolymers Obtained by ATRP for an Application in Heterogeneous Asymmetric Salen Catalysis

“…The reaction of choice to study the influence of the controlled ATRP on a catalytic transformation is the ring-opening reaction of meso -epoxides with trimethylsilylazide [ 14 , 15 ], a reaction for which dual bimetallic activation of each reactive partner is known to be of high importance to reach excellent activity and enantioselectivity. This demanding transformation leads indeed interestingly to precursors of cyclic 1,2-amino alcohols and has been efficiently promoted by chiral chromium salen complexes [ 16 ] both in homogeneous and heterogeneous processes [ 17 , 18 , 19 , 20 , 21 , 22 ]. Such a strategy of using polymer methodologies to target the best composition and location of the catalytic entities has been investigated by the group of Jones for the preparation of polystyrene brushes containing cobalt salen complexes grafted either on silica or on superparamagnetic iron oxide nanoparticles, to evaluate the hydrolytic kinetic resolution of epibromohydrin [ 23 , 24 ].…”

“…Subsequent azide–alkyne Huisgen cycloadditions promoted by Cu (I) salts were conducted to react the propargyl-tagged chromium salen complex with the azide groups of all three copolymers [ 22 , 25 ], delivering in high yield the corresponding supported salen catalysts, as brown powders. Completion of this reaction was verified by FTIR spectroscopy, since the typical strong azide band at 2090 cm −1 fully disappeared, at least for Copo1 and Copo2, after 36 h of stirring at 40 °C (see Figure 7 as illustration for Copo1, and Figures S6 and S7 for Copo2 and Copo3 in Supplementary Materials ).…”

Enantiopure isothiourea@carbon-based support: stacking interactions for recycling a lewis base in asymmetric catalysis

“…Immobilization of the pyrene-tagged hyperBTM onto the rGO surface was achieved by impregnation of the carbon suspension with a solution of pyr-hyperBTM in DCM with a mass ratio of 4/1. 16 After sonication for a few minutes and 24 h of stirring at room temperature, the solid was filtered off, washed with DCM and vacuum-dried. The efficiency of the immobilization was monitored by UV-Vis analyses (see Figure 1 and Supporting Information for details).…”