Unexpectedly efficient activation of push–pull nitriles by a PtII center toward dipolar cycloaddition of Z-nitrones

Abstract

Pt^II-coordinated NCNR′₂ species are so highly activated towards 1,3-dipolar cycloaddition (DCA) that they react smoothly with the acyclic nitrones ArCHN⁺(O⁻)R′′ (Ar/R′′ = C₆H₄Me-p/Me; C₆H₄OMe-p/CH₂Ph) in the Z-form. Competitive reactivity study of DCA between trans-[PtCl₂(NCR)₂] (R = Ph and NR′₂) species and the acyclic nitrone 4-MeC₆H₄CHN⁺(O⁻)Me demonstrates comparable reactivity of the coordinated NCPh and NCNR′₂, while alkylnitrile ligands do not react with the dipole. The reaction between trans-[PtCl₂(NCNR′₂)₂] (R′₂ = Me₂, Et₂, C₅H₁₀) and the nitrones proceed as consecutive two-step intermolecular cycloaddition to give mono-(1a–d) and bis-2,3-dihydro-1,2,4-oxadiazole (2a–d) complexes (Ar/R′′ = p-tol/Me: R′₂ = Me₂a, R′₂ = Et₂b, R′₂ = C₅H₁₀c; Ar/R′′ = p-MeOC₆H₄/CH₂Ph: R′₂ = Me₂d). All complexes were characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, ¹H and ¹³C{¹H} NMR spectroscopy. The structures of trans-1b, trans-2a, trans-2c, and trans-2d were determined by single-crystal X-ray diffraction. Metal-free 5-NR′₂-2,3-dihydro-1,2,4-oxadiazoles 3a–3d were liberated from the corresponding (dihydrooxadiazole)₂Pt^II complexes by treatment with excess NaCN and the heterocycles were characterized by high resolution ESI⁺-MS, ¹H and ¹³C{¹H} spectroscopy.