Chiral bis(trialkylsiloxymethyloxazolinyl)pyridine ligands. Highly enantioselective 1,3-dipolar cycloaddition reactions

A series of trialkylsilyl groups containing ligands have been prepared from C-2 symmetric 2,6-bis(oxazolinyl)pyridine (pybox-hm) (2a) bearing a hydroxymethyl group on the oxazoline ring. 1, 3-Dipolar cycloaddition reaction of nitrones and oxazolidinone derivatives in the presence of sterically tuned bis(oxazolinyl)pyridine (pybox-tbdmsom 2b, -tipsom 2c and -tbdpsom 2d) and Ni(II) complexes as a Lewis acid catalyst proceeded smoothly to give the corresponding cycloadducts in 97:3 to >99:1 of endo/exo ratio and 97 to >99% ee for the endo adduct. Steric tuning of the chiral environment of pybox ligands was simply achieved by using silyl protecting groups of the hydroxyl groups. The solubility of the new pybox series 2b-d in organic solvent is dramatically increased; hence, the efficiency of these catalysts enhanced the rate of nitrone 1,3-dipolar cycloaddition. Furthermore, these ligands can make a single-coordinated Ni(pybox) as a catalyst, since clear linear effect between the enantiopurity of the ligand 2c and product 6a was observed. (C) 2002 Elsevier Science Ltd. All rights reserved.