Ramón García-Lago, J. Alonso-Gómez, Cristina Sicre
2008
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Abstract
−We report an efficient method for preparing 2,4-dibromopyridine and 4,4’-dibromo-2,2’-bipyridine from the corresponding nitroazine N-oxide in one step via a tandem nucleophilic substitution–N-oxide reduction process. The one step preparation of 4,4’-dihalo-2,2’-bipyridines from dihalopyridines via a Stille reaction is also described. 4,4'-Dibromo-2,2’-bipyridine undergoes selective monoor disubstitution processes under palladium catalysis. This short synthetic procedure is an efficient and reliable process for preparing conjugated pyridine and 2,2’-bipyridine building blocks for applications in coordination chemistry and materials science. INTRODUCTION There has been an extensive effort to develop synthetic methods for azines, specially pyridines and 2,2’-bipyridines, due to their presence as substructures in many natural products and pharmaceuticals, and also as ligands for transition metals. The renewed interest in this chemistry has been driven, to a great extent, by their ability to act as chelating agents for almost all kinds of metal ions and the wide range of applications rising from their involvement in supramolecular chemistry and in materials science. However, synthetic access to functionalized derivatives is still difficult, and reliable preparative protocols are therefore highly desirable. Disubstituted 2,2’-bipyridines have been prepared by connecting two pyridine precursors at C-2 with the help of palladium or nickel transition metal complexes and some protocols have proven to be quite successful for the preparation of symmetrically and non–symmetrically substituted derivatives. 4, 5 However, most of the reported disubstituted pyridine building blocks are 2,6or 2,5-derivatives, whereas 2,4-derivatives are scarce, because 2,4-dihalopyridines are not readily available. The growing interest in this field and our interest in the synthesis of complex pyridine-containing HETEROCYCLES, Vol. 75, No. 1, 2008 57