Facile and rapid immobilization of copper(II) bis(oxazoline) catalysts on silica: application to Diels-Alder reactions, recycling, and unexpected effects on enantioselectivity |
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Authors: | Patrick O'Leary Krijn P De Jong Robertus JM Klein Gebbink |
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Institution: | a Utrecht University, Debye Institute, Department of Metal Mediated Synthesis, Padualaan 8, 3584 CH Utrecht, The Netherlands b Utrecht University, Department of Inorganic Chemistry and Catalysis, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands |
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Abstract: | Two chiral copper(II) bis(oxazoline) complexes have been immobilized on silica via electrostatic interactions using a remarkably straightforward procedure. The immobilized catalysts were tested in a standard Diels-Alder reaction and gave surprising results. Where the immobilized Cu((S,S)-phenyl-box)(OTf)2 catalyst was used, the predominant enantiomer formed was the opposite of that produced using the same catalyst in a homogeneous reaction. This is a startling result given that the only difference is the electrostatic immobilization of the catalyst on amorphous silica. The activity of the catalyst in a hetero Diels-Alder reaction was also tested. This catalyst was also recycled, successfully maintaining a similar activity to the homogeneous analogue through a number of cycles. |
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Keywords: | Copper(II) bis(oxazoline) Electrostatic immobilization Diels-Alder Silica Enantioselectivity reversal |
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