Efficient Catalytic Systems for Synthesis of 5,5 -Dibromo-2,2':6',2 -terpyridine and 5,5'-Dibromo-2,2'-bipyridine via Coupling of Dihalogenopyridines with 5-Bromo-2-trialkylstannylpyridines

The results of the studies on the synthesis of5,5'-dibromo-2,2'-bipyridine and 5,5"-dibromo-2,2':6',2"-terpyridine via coupling of 5-bromo-2-iodopyridine and 2,6-dihalogenopyridines with 5-bromo-2-trialkylstannylpyridines mediated by palladium catalysts have been presented. The catalytic activity of the Pd(II) and Pd(0) complexes (e.g. [PdCl 2 (PPh 3 ) 2 ], [PdCl 2 (COD)], [Pd(dba) 2 ]), and catalytic systems generated in situ from a stable precursor (e.g. [PdCl 2 ], [Pd(acac) 2 ]) and an external ligand (APh 3 , where A = P, As, Sb; phosphines, phosphites) in the coupling ofdiiodopyridine with 5-bromo2-tributylstannylpyridine was investigated. The most active system was that generated from [Pd(acac) 2 ] and P(OPh) 3 , while the highest coupling selectivity was achieved with [Pd(acac) 2 ] and PPh 3 . The catalytic activity of systems containing chelating ligands BINAP or dppf was slightly inferior. In all reactions the formation of 5,5'-dibromo2,2'-bipyridine, the product of homocoupling of 5-bromo-2-tributylstannylpyridine, was observed. An increase of the L/Pd ratio for catalytic system generated from [Pd(acac) 2 ] and P(OPh) 3 resulted in improved selectivity ofdbtpy formation (the yield of dbbpy, the product of homocoupling, decreased) and in an increased stability of the catalytic system (without decreasing the reaction rate even for Pd/L = 1:32). On the other hand, for the systems containing phosphines the increase of L/Pd above 4:1 for monodentate phosphines and above 2:1 for bidentate phosphines resulted in a fast decrease of the reaction rate. The coupling is faster for 2,6-diiodopyridine and slower for 2,6-dibromopyridine, while 2,6-dichloropyridine is nonreactive. The yield of coupling fortrimethyl- and tributylstannyl derivatives is practically identical. Particularly advantageous solvents for the studied coupling reaction are xylene, toluene, and 1,2-diethoxyethane.