Complexation of N- and C-substituted racemic aziridines with rhodium(II) tetraacylates in solution: 1H, 13C and 15N nuclear magnetic resonance spectroscopy

The complexation of five racemic N- and C-substituted aziridines with three rhodium(II) tetraacylates Rh2(CO2R)4 [R = CH3, CF3 or (S)-C(OCH3)(CF3)Ph] in CDCl3 solution has been investigated by 1H, 13C and 15N NMR spectroscopy at various temperatures. Depending on the rhodium salt to ligand L molar ratio the 1:1 and 1:2 ([Rh2(CO2R)4]L2) axial adducts were formed. The enantiopure rhodium salt formed two 1:1 and three 1:2 distereoisomeric adducts in solution. In contrast to non-complexed aziridines, that exist as either a trans-conformer or as a cis- and trans-conformer mixture, an aziridine ring in the adducts adopted only the cis conformation. Under particular conditions the formation of 2:1 adducts ([Rh2(CO2R)4]2L) was observed. Nitrogen adduct formation shifts Δδ = δadduct − δligand from −17 to −30 ppm were used as a proof of complexation via the nitrogen atom in the aziridine ring. These experimental findings were validated by DFT calculations using 6-31g(2d), 6-311G++(2d,p) and LANL2DZ basis sets and two functionals, B3LYP and B3PW91.

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