Synthesis of Copper(I) Complexes Containing Enantiopure Pybox Ligands. First Assays on Enantioselective Synthesis of Propargylamines Catalyzed by Isolated Copper(I) Complexes

Dinuclear [Cu₂(R-pybox)₂][X]₂ [X = PF₆, R-pybox = 2,6-bis[4′-(S)-isopropyloxazolin-2′-yl]pyridine (S,S)-ⁱPr-pybox (1), 2,6-bis[4′-(R)-phenyloxazolin-2′-yl]pyridine) (R,R)-Ph-pybox (2), 2,6-bis[4′-(S)-isopropyl-5′,5′-difeniloxazolin-2′-yl]pyridine (S,S)-ⁱPr-pybox-diPh (3); X = OTf, (R,R)-Ph-pybox (4)] and mononuclear complexes [Cu(R-pybox)₂][PF₆] [R-pybox = (S,S)-ⁱPr-pybox (5), (R,R)-Ph-pybox (6)] have been diastereoselectively prepared by reaction of [Cu(MeCN)₄][PF₆] or CuOTf·0.5C₆H₆ and the corresponding pybox ligand. The reaction of CuX (X = I, Br, Cl) with substituted pybox in a 2:1 molar ratio allows us to synthesize tetranuclear complexes [Cu₄X₄(R-pybox}₂] [R-pybox = (S,S)-ⁱPr-pybox, X = I (7), Br (8); (R,R)-Ph-pybox, X = I (9), Br (10), Cl (11); (S,S)-ⁱPr-pybox-diPh, X = I (12), Cl (13)]. Dinuclear complexes [Cu₂(μ-Cl)(R-pybox)₂][CuCl₂] [R-pybox = (S,S)-ⁱPr-pybox (15), (R,R)-Ph-pybox (16)] have been prepared by reaction of CuCl with ⁱPr-pybox or Ph-pybox in 3:2 molar ratio. The structures of the complexes 1, 2, 7, 15, 16 and that of the polymeric species [Cu₄(μ₃-Br)₃(μ-Br)(ⁱPr-pybox)]_n (14) have been determined by single-crystal X-ray diffraction analysis. Diffusion studies using ¹H and ¹⁹F-DOSY experiments provide evidence that the different nuclearity of compounds 1 and 5 is maintained in the solution state and confirm that these ionic compounds exist in solution as stable, discrete, cationic complexes. The complexes 1−4, 6, 9, 10, and 16 as well as the previously reported [Cu₂{(S,S)-ⁱPr-pybox}₂][OTf]₂ have been assayed as catalysts in the enantioselective synthesis of propargylamines. The dinuclear complexes [Cu₂{(R,R)-Ph-pybox}₂][X]₂ (2, 4) were found to be the most efficient catalysts (up to 89% e.e.).