Copper(i) complexes bearing pyrrole-bridged S,N and N-donor ligands as catalysts for tandem hydroamination–alkynylation: effect of anions on product formation

Abstract

In this study, the reaction between 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)pyrrole and thiophenol under heating conditions afforded the new ligand 2-(3,5-dimethylpyrazolylmethyl)-5-(phenylthiomethyl)pyrrole 2. The reaction of 2 with meta-chloroperbenzoic acid provided sulfoxide 3 and sulfone 4 group-containing ligands. The reaction of 2 with copper(I) halides provided the binuclear complexes [Cu(μ-X){μ-C₄H₃N-2-(CH₂Me₂pz)-5-(CH₂SPh)-κ²-S,N}]₂ (X = Cl, Br and I, 5–7) in high yields. Conversely, the analogous reaction of 4 with copper(I) halides yielded two types of complexes, three coordinate bi- and mononuclear of the type [Cu(μ-Cl){C₄H₃N-2-(CH₂Me₂pz)-5-(CH₂SO₂Ph)-κ¹-N}]₂8 and [CuX{C₄H₃N-2-(CH₂Me₂pz)-5-(CH₂SO₂Ph)-κ¹-N}₂], X = Br, 9 and I, 10. When the reaction was carried out in the presence of KPF₆, the two-coordinate complex [Cu{C₄H₃N-2-(CH₂Me₂pz)-5-(CH₂SO₂Ph)-κ¹-N}₂]PF₆⁻11a was isolated, whereas its BF₄⁻ analogue 11b was synthesized by the reaction of 8 with AgBF₄. The structures of these complexes were determined using single-crystal X-ray crystallography. These copper complexes catalyzed the hydroamination–alkynylation reaction between several secondary amines and alkyl and aryl terminal alkynes. Using 1 mol% of complexes 5–10 as catalysts, both tri- and tetra-substituted propargylamines were isolated. Alternatively, phenylacetylene and different secondary amines afforded the corresponding trisubstituted propargylamines as the major products and alkyl terminal alkynes gave the tetrasubstituted products in excellent yields. In addition, the role of counter anions such as TfO⁻, PF₆⁻, BF₄⁻, PO₄³⁻ and Ph₄B⁻ on the product selectivity was studied. When fluorinated anions such as TfO⁻, PF₆⁻, and BF₄⁻ were present with the copper complexes, the hydroamination–hydrovinylation product 1-aminodiene 23 was observed, which was not formed with PO₄³⁻, Ph₄B⁻ or halide ions. Specifically, TfO⁻ and PF₆⁻ favored the formation of 23, while BF₄⁻ favored the tetrasubstituted product as the major product. This was further supported by the isolated copper(I) complexes containing PF₆⁻ and BF₄⁻ and by other specific reactions. The peaks for enamines and [LCu]⁺ species in the HRMS spectra of the reaction mixtures and the isolation of the morpholinium copper(I) salt support the proposed mechanism.