Design and synthesis of gold monolayer cluster supported catalysts: towards asymmetric synthesis

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University of New Brunswick


Asymmetric synthesis is the comer stone of the modem pharmaceutical industry. Its success can be attributed to the development of chiral catalysts that provide a high degree of enantioselectivity. The C₂ symmetric bisoxazoline (Box) ligand metal complexes have been a dominant force in the field of asymmetric catalysis. In recent years, the quest for metal free catalysis has resulted in the emergence of organocatalysts, such as the versatile MacMillan’s catalyst, with impressive success. The push towards environmentally friendly processes has ensued in the search for suitable support materials for catalysts since immobilization enhances the recovery, recyclability, and reuse of catalysts. Nanotechnology has become an integral part of science and has opened up the possibility of incorporating gold monolayer protected clusters (Au-MPCs) in asymmetric synthesis. Due to their large surface to volume ratio, Au-MPCs are believed to be ideal candidates for catalyst support materials. In an attempt to combine green chemistry with asymmetric synthesis, hexanethiol, octanethiol, dodecanethiol, and hexadecanethiol stabilized Au-MPCs were synthesized via the Schiffrin reaction as support materials. Thiol ended 10-carbon tethered Box ligand and MacMillan’s catalyst were synthesized and immobilized on the Au-MPCs through place exchange with stabilizing alkanethiols. The supported catalysts were designed to represent homogeneous, heterogeneous and enzyme type catalytic systems. Unfortunately, after the preliminary investigations of the Michael addition reaction between trans-β-nitrostyrene and indole with the Au-MPCs supported Box-Zn(OTf)₂ complexes, further analyses were terminated due to the publication of a similar study by a Japanese research team. The catalytic activity of the Au-MPCs supported MacMillan’s catalyst was tested with the Diels-Alder reaction between trans-cinnamaldehyde and cyclopentadiene and the 1,3-dipolar cycloaddition between trans-crotonaldehyde and nitrone. In addition, catalyst recovery and recyclability were also investigated. The endo/exo selectivity of these reactions was comparable to that of reactions catalyzed by homogeneous catalysts, but the yields and enantioselectivities were lower. The enzyme type catalytic system with catalyst supported on hexadecanethiol stabilized Au-MPCs showed the best catalytic activity with enantioselectivity remaining consistent with the recycled catalyst. A comparison of the catalytic activity with the catalyst supported on Merrifield resin, which is mainly a heterogeneous catalytic system, showed that Au-MPCs are better support materials for the MacMillan’s catalyst.