Abstract
A chiral bis(oxazoline) bearing CH2OH groups was synthesized from a commercial bis(oxazoline) and characterized by 1H- and 13C-NMR, high resolution ESI-mass spectrometry and FTIR. The corresponding copper(II) complex was immobilized onto the surface of a mesoporous carbonaceous material (Starbon® 700) in which the double bonds had been activated via conventional bromination. The materials were characterized by elemental analysis, ICP-OES, XPS, thermogravimetry and nitrogen adsorption at 77 K. The new copper(II) bis(oxazoline) was tested both in the homogeneous phase and once immobilized onto a carbonaceous support for the kinetic resolution of hydrobenzoin. Both were active, enantioselective and selective in the mono-benzoylation of hydrobenzoin, but better enantioselectivities were obtained in the homogeneous phase. The heterogeneous catalyst could be separated from the reaction media at the end of the reaction and reused in another catalytic cycle, but with loss of product yield and enantioselectivity.
Highlights
Bis(oxazoline) ligands are chiral, privileged ligands that when coordinated to for example copper act as very efficient and enantioselective homogeneous catalysts in several organic transformations, such as cyclopropanation of alkenes, aziridination of alkenes, Diels-Alder reactions, etc. [1]
The copper(II) complex with the PhBox bis(oxazoline) was anchored onto the surface of the porous carbonaceous material, Starbon® 700, in a three step procedure according to Scheme 1
PhBox ligand bridge was functionalized with CH2OH groups (Scheme 2), by adapting procedures described in the literature [20]
Summary
Bis(oxazoline) ligands are chiral, privileged ligands that when coordinated to for example copper act as very efficient and enantioselective homogeneous catalysts in several organic transformations, such as cyclopropanation of alkenes, aziridination of alkenes, Diels-Alder reactions, etc. [1]. Several immobilization strategies have been used; covalent attachment of the homogeneous catalysts to the surface of the material increases the resistance of the catalyst to leaching [3,4,5,6] Due to their availability and low cost, organic polymers have been widely explored as supports for homogeneous catalysts and in particular for copper bis(oxazoline) ligands [3,4,5,6]. This material, as well as other ordered mesoporous carbons prepared in a one-pot synthesis together with the silica precursor, were used as supports for the immobilization of aza-bis(oxazoline) and commercial bis(oxazoline) via organosilane reagents [8,9,10] These heterogeneous catalysts were active and enantioselective in several organic transformations, but with inferior performance compared to the corresponding ordered mesoporous silicas supports [8,9,10].
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