PdII-promoted [2 + 3] cycloaddition of pyrroline N-oxide to organonitriles. Application of (Δ4-1,2,4-oxadiazoline)-PdII complexes in the Suzuki–Miyaura reaction

Abstract

[2 + 3] Cycloaddition reactions of the organonitriles RCN 1 (R = Me (1a), Et (1b), p-IC₆H₄ (1c), p-BrC₆H₄ (1d), p-ClC₆H₄ (1e), p-FC₆H₄ (1f)) with the pyrroline N-oxide⁻O⁺NCHCH₂CH₂CMe₂2 in the presence of PdCl₂ proceed at room temperature for 12 h and allow the isolation of the corresponding palladium(II) complexes trans-[PdCl₂{NC(R)ONC(H)CH₂CH₂CMe₂}₂] 3a–3f containing fused bicyclic Δ⁴-1,2,4-oxadiazoline ligands, in good to excellent yields (75–90%). Treatment of trans-[PdCl₂(EtCN)₂] with the nitrone 2 in acetone or propionitrile at room temperature affords, in good yield, 3b (R = Et), confirming that complexes 3 are formed vianitrile activation by coordination. These reactions proceed with high diastereoselectivity and afford mixtures of enantiomers, due to the rigid conformation (E) of the cyclic nitrone 2. The oxadiazolines NC(R)ONC(H)CH₂CH₂CMe₂ (R = Et (4b), p-FC₆H₄ (4f)) are liberated upon reaction of complexes 3b,3f with a diphosphine (dppe). During the liberation of 4f, the complex trans-[PdCl₂{NC(NHCOC₆H₄F-4)CH₂CH₂CMe₂}₂] 5 was formed in a minor amount upon N–O bond cleavage of the oxadiazoline ring to give a pyrrolylbenzamide species coordinated by the N-atom of the pyrrolyl moiety. The compounds were characterized by IR, ¹H and ¹³C NMR spectroscopies, ESI-MS, elemental analyses and, in the cases of 3b, 3d and 5, also by X-ray diffraction analyses. Complexes 3b and 3f show a high catalytic activity towards the microwave-assisted Suzuki–Miyaura cross-coupling reaction in aqueous medium, achieving a TON value of 2.7 × 10⁵ and a TOF value of 1.1 × 10⁶ h⁻¹.