Borane-functionalized heteroscorpionate copper complexes as catalysts for azide–alkyne cycloaddition

Abstract

In this work, new copper(I) complexes of borane-functionalized bis(3,5-dimethylpyrazolyl)methane ligands were synthesized, characterized and used as catalysts for the cycloaddition of phenylacetylene and azides. Reaction of the allylated bis(3,5-dimethylpyrazolyl)methane proligand (La) with CuCl or [Cu(NCMe)₄]BF₄ gave rise to the neutral copper chloride complex [(La)CuCl] (1a) or the binuclear cationic copper complex [(La)Cu(NCMe)]₂(BF₄)₂ (2a₂), respectively. The same reactions using a borane-functionalized bis(3,5-dimethylpyrazolyl)methane proligand (Lb) led to the isolation of complexes [(Lb)CuCl] (1b) and [(Lb)Cu(NCMe)₂]BF₄ (2b). The new complexes were characterized by NMR and FTIR spectroscopies, elemental analysis, cyclic voltammetry and, in selected cases, single-crystal X-ray diffraction. Complexes 1a, 1b, 2a₂ and 2b catalyzed the cycloaddition of phenylacetylene and in situ prepared benzyl azide to afford the respective 1,2,3-triazole products. When using complex 0.33 mol% of complex 2b in the presence of 40 equivalents of diisopropylethylamine (DIPEA), a maximum yield of 97% with a maximum TOF value of 800 h⁻¹ was obtained. Complex 2b also catalyzed the cycloaddition of phenylacetylene with 7 azides prepared in situ in 87%–99% yields. The borane-functionalized complexes 1b and 2b were, on average, 4-fold more active than their respective allylated analogues, and complex 2b readily led to the formation of the η²-phenylacetylene complex [(Lb)Cu(η²-PhCCH)]BF₄ (4) and the presumed detection of borane-centered Lewis pairs with MesN₃. These experimental observations, along with computational insights based on DFT calculations, indicated that the respective borane functionalities are beneficial to catalytic activity due to a substrate-directing effect.