PdII-promoted [2 + 3] cycloaddition of pyrroline N-oxide to organonitriles. Application of (Δ4-1,2,4-oxadiazoline)-Pd II 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+NCHCH2CH 2CMe22 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)CH2CH 2CMe2}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 via nitrile 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)CH 2CH2CMe2 (R = Et (4b), p-FC6H 4 (4f)) are liberated upon reaction of complexes 3b,3f with a diphosphine (dppe). During the liberation of 4f, the complex trans-[PdCl 2{NC(NHCOC6H4F-4)CH2CH 2CMe2}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. © 2009 The Royal Society of Chemistry.