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-IC(6)H(4) (1c), p-BrC(6)H(4) (1d), p-ClC(6)H(4) (), p-FC(6)H(4) (1e)) with the pyrroline N-oxide (-)O(+)[upper bond 1 start]N[double bond, length as m-dash]CHCH(2)CH(2)C[upper bond 1 end]Me(2) in the presence of PdCl(2) proceed at room temperature for 12 h and allow the isolation of the corresponding palladium(ii) complexes trans-[PdCl(2){[upper bond 1 start]N[double bond, length as m-dash]C(R)O[lower bond 1 start]NC[upper bond 1 end](H)CH(2)CH(2)C[lower bond 1 end]Me(2)}(2)] 3a-3f containing fused bicyclic Delta(4)-1,2,4-oxadiazoline ligands, in good to excellent yields (75-90%). Treatment of trans-[PdCl(2)(EtCN)(2)] with the nitrone in acetone or propionitrile at room temperature affords, in good yield, (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 [upper bond 1 start]N[double bond, length as m-dash]C(R)O[lower bond 1 start]NC[upper bond 1 end](H)CH(2)CH(2)C[lower bond 1 end]Me(2) (R = Et (4b), p-FC(6)H(4) (4f)) are liberated upon reaction of complexes 3b,3f with a diphosphine (dppe). During the liberation of , the complex trans-[PdCl(2){[upper bond 1 start]N[double bond, length as m-dash]C(NHCOC(6)H(4)F-4)CH(2)CH(2)C[upper bond 1 end]Me(2)}(2)] 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, (1)H and (13)C 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 x 10(5) and a TOF value of 1.1 x 10(6) h(-1).