Regiocontrolled one-step synthesis of 3,3'-disubstituted 2,2'-bipyridine ligands by cobalt(I)-catalyzed cyclotrimerization

A one-step, regioselective synthesis of annelated symmetric and asymmetric 3,3'-disubstituted 2,2'-bipyridines by cobalt(I)-catalyzed [2+2+2] cycloadditions between 5-hexynenitrile and 1,3-diynes is described. In the symmetric case, the total regioselectivity of the first cycloaddition is ensured electronically by the conjugation of the triple bonds, and for aminomethylated diynes that of the second is ensured by the cobalt coordinating to the aminomethyl rather than to the hexynenitrile nitrogen. In the asymmetric case, the first cycloaddition took place chemoselectively, which in the case of bis(trimethylsilyl)-1,3,5-hexatriyne (viewed as a 1,3-diyne) is explained by semiempirical calculation of LUMO coefficients. The copper(I) complex of 6b, constituting the first reported complex of the form ML2 (L is a symmetric 3,3'-disubstituted 2,2'-bipyridine), has been prepared. It had UV/Vis and NMR spectra reflecting the 3-substituent-induced mutual torsion of the bipyridine rings in the cis conformation, as was confirmed by x-ray diffractometric determination. The bipyridine 6c forms the dinuclear complex [Cu2(6c)2(CH3CN)2]2+ in the solid state.