Covalent and Noncovalent Immobilization of Arylid-BOX Ligands and Their Derivatives: Evaluation in the Catalytic Asymmetric Cyclopropanation of Styrenes

Abstract

The aim of this work was to immobilize an Arylid-BOX ligand on solid supports either by covalent attachment (to Wang resin) or by noncovalent attachment (on MK10 and SiO2) and to evaluate the products in the catalytic asymmetric cyclopropanation (CACP) of styrene in the presence of Cu-I and Cu-II. The synthetic strategy used to obtain the target functionalized Arylid-BOX furnished a new, partially hydrogenated, Arylid-BOX derivative. Evaluation of this ligand in the catalytic asymmetric cyclopropanation of styrene in the presence of Cu-I gave a highest ee of 68%, a result better than that obtained for the nonhydrogenated Arylid-BOX ligand. The new ligand was grafted to Wang resin; a loading of 0.321 mmol of ligand per gram of polymer could be obtained. [Cu(CH3CN)(4)]PF6 or Cu(OTf)(2) were used in the CACP of styrene with CH2Cl2 or toluene as solvents. A highest ee of 71% was obtained and the catalyst could be recycled and reused up to four times. In the case of the heterogeneous CACP in the presence of [Cu(CH3CN)(4)] PF6 and toluene, the yields, ees, des, and selectivities were maintained over the four cycles. This heterogeneous catalyst was shown to be very similar to the homogeneous catalyst with regard to selectivity and activity. In the case of noncovalent immobilization, both Cu-I and Cu-II Arylid-BOX complexes were immobilized on Montmorillonite K10 (MK10) and silica gel through noncovalent interactions. The obtained results depended on the natures both of the chiral ligand and of the support. In the case of MK10, enantioselectivities of 62% ee (trans-cyclopropane) and 65% ee (cis-cyclopropane) could be achieved, whereas in the case of the silica-gel-supported catalyst, enantioselectivities of 50% ee (trans-cyclopropane) and 43% ee (cis-cyclopropane) were obtained.