General Method for the Preparation of Alkyne-Functionalized Oligopyridine Building Blocks

A large series of alkyne-substituted oligopyridines based on 2,2'-bipyridine, 1,10-phenanthroline, 2,2':6',2"-terpyridine, or 1,8-naphthyridine substrates has been synthesized and fully characterized. The palladium(0)-catalyzed coupling of bromo- or chloro-substituted derivatives with (trimethylsilyl)acetylene proceeds readily in diisopropylamine under ambient conditions giving good yields of the corresponding alkyne-substituted substrates oligoPy(CC)SiMe(3). The terminal monoynes oligoPyCCH become available upon treatment with K(2)CO(3) in methanol. Stepwise homologation of the acetylene function by Cadiot-Chodkiewicz coupling of oligoPyCCH with (bromoethynyl)triethylsilane (BrCCSiEt(3)) affords, in good yield, the silylated diynes oligoPy(CC)(2)SiEt(3), from which the terminal diynes oligoPy(CC)(2)H are formed by treatment with aqueous methanolic alkali. Reaction of oligoPy(CC)(2)H with BrCCSiEt(3) yields the silylated triynes oligoPy(CC)(3)SiEt(3) in modest yield. Further homologation is limited by nucleophilic attack of n-propylamine at the C-2 carbon of the alkyne chain, giving rise to a mixture of cis/cis (48%), cis/trans (33%), and trans/trans (19%) enaminediyne compounds 21a-c. Glaser oxidative self-coupling of the terminal diynes provides access to ditopic bipyridine or terpyridine ligands oligoPy(CC)(4)oligoPy comprising a tetrayne spacer. Quantitative formation of air-stable copper(I) complexes is described for the 6,6'-substituted ligands. A single crystal X-ray structure of complex 22a shows that the two ligands are interlocked around the copper(I) center in a pseudotetrahedral arrangement, similar to the structure deduced from NMR and FAB(+) data. The synthetic methods reported herein represent a valuable approach to the large-scale preparation of alkyne-functionalized oligopyridines.