Pyridines 2,2 -bipyridine from

In 1956 it was found that when pyridine is refluxed with a modified Raney-nickel catalyst, 2,2 -bipyridine (1) is formed in satisfactory yield. The isomeric bipyridines could not be detected, and the product was readily purified. Similar heterocyclic biaryls have been formed in the same way from substituted pyridines and from some related compounds, the yield being dependent on the nature of the compound. The reaction has become the method of choice for the preparation of 2,2 -bipyridine, and it is now used on an industrial scale. Bipyridyls are of particular importance as chelating agents. 

The formation of trace amounts of 2,2 -bipyridine following reaction between pyridine and ammonia in the presence of a variety of catalysts led Wibaut and Willink to develop a method for the preparation of 2,2 -bipyridine from pyridine under the influence of a nickel-alumina catalyst. Using a pyridine-to-catalyst ratio of 10 1, temperatures between 320° and 325°C, and pressures between 42 and 44 atm, 2,2 -bipyridine was formed in yields of 0.30-0.67 gm per gram of catalyst. This method w as later applied to -picoline, to quino-line, - and to some of its derivatives, ... 

The Preparation of Substituted 2,2 -Bipyridines from Substituted Pyridines and Degassed Raney Nickel -i ... 

In the absence of an added solvent, 3-alkyIpyridines, 4-alkyl-pyridines, and 3,4-dialkylpyridines all gave yields of substituted 2,2 -bipyridines that were up to three times greater than that of 2,2 -bipyridine from pyridine under similar conditions. With 3-ethyl-4-methylpyridine a marked improvement in yield was ob.served when the reaction was carried out at about 150°C in a vacuum, rather than at the atmospheric boiling point (195°C) of this base. This effect has also been observed with some other bases but the amount of 3,3, 5,5 -tetramethy 1-2,2 -bipyridine from 3,5-lutidine could not be increased in this way, and this pyridine was as unreactive as the 2-substituted pyridines. This finding is undoubtedly related to the reluctance of 3-substituted pyridines to form 3,3 -disubstituted 2,2 -bipyridines. 

The three catalysts which have been used for the preparation of 2,2 -bipyridines from pyridines have also been employed for the preparation of 2,2 -biquinolines from quinolines. Tlie results have... 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

The outstanding feature of the preparation of 2,2 -bipyridine from pyridine under the influence of metal catalysts is the absence of isomeric bipyridines among the products. In this respect reactions using metal catalysts in a heterogeneous system differ from methods which have been used to prepare bipyridines in homogeneous sys-tems. ... 

Several lithium reagents have been found to convert pyridine to 2,2 -bipyr-idines in satisfactory yield. Following from the formation of 2,2 -bipyridines from 2-lithiopyridines, it has been found that 2-benzyl-2-lithio-1,3-dithiane forms 2,2 -bipyridine in 30-75% yield when refluxed with pyridine in tetrahydrofuran, " and related compounds behave similarly. Lithium diisopropylamide with pyridine in ether affords up to 50% 2,2 -... 

As in the case of 2,2 -bipyridine, the most important synthetic routes to 4,4 -bipyridine use pyridine as starting material. One method of synthesizing 4,4 -bipyridine from pyridine was discovered by Dimroth in 1921. If pyridine in acetic anhydride is treated with zinc dust, l,l -diacetyl-l,r,4,4 -tetrahydro-4,4 -bipyridine is formed. This compound is readily oxidized and hydrolyzed by moist air to 4,4 -bipyridine. Various oxidizing agents assist in the conversion to 4,4 -bipyridine. By-products from the reaction include 1,1 -diacetyl-l,T-dihydro-4,4 -bipyridine. This method of synthesizing 4,4 -bi-pyridine has frequently been used. ° The reduction of pyridine in acetic anhydride by catalytic hydrogenation instead of by zinc dust is less satisfactory because of the formation of other reduction products. Several variations and improvements in the Dimroth reaction have subsequently... 

Apart from the aqua ions and partially substituted species such as [GaCl(H20)s]2+ noted previously, complexes with pyridine, bipyridine, and phenanthroline are known, for example, [GaCl2(phen)2]+ and [Ga(phen)3]3+. For thallium(III) com-... 

Cationic Compiexes. Apart from the aqua ions and partially substituted species such as [GaCl(H20)5] +, cationic complexes of gallium halides with pyridine, bipyridine, or phenanthroline are known, for example, [GaCl2(phen)2]+ and [Ga(phen)3] +. Furthermore, given the steric hindrance about the gallium center in complexes formulated as GaX3(NH3) (X = Cl, n = 1, 3, 5, 6, 7, 14 X = Br, n = 1, 5, 7, 9, 14 X = I, n = 1, 5, 6, 7, 9, 13, 20), it is likely that where n is greater than 3 these are also cationic solvates. 

---