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Macrocyclic vs. acyclic derivatives of chiral bis(oxazolines) ; ligand distortion and enantioselectivity of Pd(II) complexes in catalytic allylic alkylation

Pd(II) complexes of acyclic (1, 2 ; 4, 5) and macrocyclic (3, 6-10) derivatives of 1, 5-bis(oxazolines), are tested in the enantioselective allylic alkylation of racemic 1, 3-diphenyl-3-acetoxyprop-2-ene (14) by dimethylmalonate anion to allyl malonate derivative 15. Conformation in solution of representative allyl Pd(H) complexes 12 and 13 is studied by 2D NMR and CD spectroscopy. 2D NMR data reveal loss of C-2 symmetry of the ligands in Pd(II)allyl-bis(oxazoline) complexes. CD spectra indicate distortion of the bidentate ligand in the complex and a conformationally forced larger twist between two chromophores in the macrocyclic complex. Only moderate variation of enantioselectivity with the length and ring size of the ligand is observed, and a rationale offered.

enantioselectivity; allylic alkylation; Pd(II) complex; 1; 5-(bis)oxazoline

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