The First Solid-Phase Synthesis of Bis(oxazolinyl)pyridine Ligands

Abstract

Chiral Pybox (pyridine-2,6-bis(oxazoline)) ligands can be cleanly and efficiently prepared on polystyrene support via a five-step solid-phase synthetic sequence. Cu(I)-complexed polymer-bound Pybox was used as a catalyst in the first heterogeneously catalyzed asymmetric addition of alkynes to imines. Best enantioselectivity was observed with (t)Bu-substituted oxazolines.

Recyclable Stereoselective Catalysts

“…Ph Ph-PyBOX * Alternatively, O ' Leary and co -workers immobilized a Cu(I) -and Cu(II) -Ph -PyBOX trifl ate complex on silica due to electrostatic interactions, but the catalyst ' s reusability was studied only during three cycles [68] . The immobilization of the Py -BOX ligand in a Wang resin did not provide a very recyclable system for the same transformation [69] . The reactions performed in IL provided the higher levels of enantiomeric induction, but the water/stearic acid system was more effi ciently recyclable.…”

The Enantioselective Addition of Alkyne Nucleophiles to Carbonyl Groups

“…Bisia and Singh further optimized the enantioselectivity of these addition reactions by surveying other pybox ligands, cii and Weissberg et al demonstrated the viability of this process with a solid supported pybox catalyst. ciii …”

Recent Developments in Asymmetric Alkynylation of Imines

“…A series of polymer-bound pybox ligands were prepared by Portnoy [48]. The synthesis of these supported ligands required five steps starting from Wang trichloroacetimidate 107 (Scheme 44).…”