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(C=C)SiMe(3). The terminal monoynes oligoPyC=CH become available upon treatment with K2CO3 in methanol. Stepwise homologation of the acetylene function by Cadiot-Chodkiewicz coupling of oligoPyC=CH with (bromoethynyl)triethylsilane (BrC=CSiEt(3)) affords, in good yield, the silylated diynes oligoPy(C=C)(2)SiEt(3), from which the terminal diynes oligoPy(C=C)(2)H are formed by treatment with aqueous methanolic alkali. Reaction of oligoPy(C=C)(2)H with BrC=CSiEt(3) yields the silylated triynes oligoPy(C=C)(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(C=C)(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.