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)SiMe3. 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⋮CSiEt3) affords, in good yield, the silylated diynes oligoPy(C⋮C)2SiEt3, from which the terminal diynes oligoPy(C⋮C)2H are formed by treatment with aqueous methanolic alkali. Reaction of oligoPy(C⋮C)2H with BrC⋮CSiEt3 yields the silylated triynes oligoPy(C⋮C)3SiEt3 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)4oligoPy 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.