In this report, we describe a modular synthesis approach towards a new series of non-centrosymmetric, dipolar 4,4'-bipyridines bearing 2,6- and 3,5-functionalized pyridyl moieties at the peripheries. Central to our strategy is the selective substitution on only one pyridyl motif that could contain electron-donating (-CH3 ) or electron-withdrawing (-F, -Cl, -CF3 ) groups which causes electronic/steric effects on one nitrogen atom in 4,4'-bipyridines. This synthetic protocol was further applied to prepare azo-functionalized (-N=N-) asymmetric bipyridines and non-centrosymmetric 4,4'-bipyridine N-oxide scaffolds, which overcome the synthetic hurdles oxidizing 4,4'-bipyridines to N-monoxides selectively at only one pyridine. Compared to the conventional symmetrical bipyridines, the dipolar non-centrosymmetric molecular tectons pave the way for the realization of non-centrosymmetric supramolecular assemblies because of the difference in the binding energy of the pyridyl nitrogen atoms.
Keywords: Building-blocks strategy; Modular synthesis; Molecular Tectonics; Non-centrosymmetric 4,4′-bipyridines; Palladium catalysis; Pyridine N-oxidation.
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