Synthesis of 1,4-Diamino-2,3-di(2-pyridyl)butane and its Dinuclear Zinc(II) Chloride Complex

In a Henry-type reaction nitromethane reacts with N-(2-pyridylmethylidene)-methylamine (1) yielding 1-methylamino-1-(2-pyridyl)-2-nitromethane (2) in nearly quantitative yield. This orange compound decomposes slowly in an inert gas atmosphere and fast in contact with air. Therefore 2 has to be stored at −78 ◦C as a methanol solution. Reduction of 2 with hydrogen in the presence of a Pd/C catalyst leads to the formation of 1,4-dinitro-2,3-di(2-pyridyl)butane (3) in the rather poor yield of 12%. The major product is the meso-isomer, meso-3, whereas only traces of (R,R)and (S,S)isomers of 3 are formed. A conversion of the nitro groups into amino functionalities succeeds with hydrazine hydrate in the presence of a Pd/C catalyst yielding meso-1,4-diamino-2,3-di(2-pyridyl)butane (4). Recrystallization from an aqueous solution gives 4·2H2O. The zinc(II) chloride complex 5 with the metal atoms in distorted tetrahedral environments can be isolated after addition of two equivalents of ZnCl2 to 4. The molecular structures of trimeric 1, meso-3, (R,R)/(S,S)-3, meso-4, and 5 have been determined and are discussed.